linux administration with scripting.pdf
TRANSCRIPT
1
Linux Administration
with
Scripting
2
Disclaimer
The copyright of the content used in the courseware will remain with Principle
Company.
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S.No. Topic Page No.
1 Brief History of Linux 5
2 Installation of RHEL 6 6
3 Linux File System 13
4 Getting Started On Your Linux Desktop 19
5 Linux directory structure 36
6 The Linux Console 38
7 Absolute Path/Relative Path 39
8 Beginning with command line utility 41
9 VI Editor 64
10 Linux files and File Permissions 70
11 Hard link and Soft link 78
12 Users and Groups 81
13 Using the bash shell 88
14 Filter command and Text processing tools 92
15 Investigating and managing Process 104
16 Standard Input/Output 111
17 Basic Networking 113
18 Advance topics in user’s, groups and permissions 119
19 Inode Numbers 123
20 Redhat Package Management 124
21 Secure Shell 126
22 Sudo 130
23 YUM 134
24 Printing in Linux 135
25 System Monitoring 137
26 Monitoring Devices 144
27 Monitoring Users 146
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28 Kernel Monitoring 147
29 Mounting External Devices 149
30 Creating New Partition 150
31 Creating Swap Partition 156
32 Logical Volume Manager 157
33 RAID 159
34 User Quota 161
35 User Access Control List 163
36 Run Level Management 165
37 NFS Server 167
38 Samba Server 169
39 Squid Server 171
40 DNS Server 175
41 Apache Server 193
42 FTP Server 195
43 DHCP Server 197
44 Telnet 199
45 NIS Server 200
46 NTP Server 203
47 Syslog Server 205
48 KVM Virtualization 209
49 TCP Wrappers 215
50 Shell Scripting 221
51 Advance topics in shell scripting 251
52 Troubleshooting 267
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A Brief History of Red Hat Linux
Arguably one of the most highly regarded and widely used enterprise Linux distributions
available today is Red Hat Enterprise Linux (RHEL). It is considered to be amongst the most
stable and reliable operating systems and is backed by the considerable resources and technical
skills of Red Hat, Inc.
RHEL 6 Essentials is designed to provide detailed information on the use and administration of
the Red Hat Enterprise Linux 6 distribution. For beginners, the book covers the basics of
configuring the desktop environment, resolving screen resolution issues and
configuring email and web servers. Installation topics such as network installation and dual
booting with Microsoft Windows are covered together with all important security topics such as
configuring a firewall and user and group administration.
For the experienced user, topics such as remote access, logical volume management (LVM), disk
partitioning, swap management, KVM virtualization, Secure Shell (SSH) and file sharing using
both Samba and NFS are covered in detail to provide a thorough overview of this enterprise class
operating system.
Red Hat Enterprise Linux is the product of a U.S. company called Red Hat, Inc., based in
Raleigh, North Carolina near the campus of NC State University. The company was founded in
the mid-1990s when Marc Ewing and Bob Young merged two companies.
In 1993 Bob Young created a company called ACC Corporation that he says he ran from his
wife‘s ―sewing closet‖. The name ACC was intended to represent a catalog business but was also
an abbreviation of a small business his wife ran called Antiques and Collectables of Connecticut.
Amongst the items sold through the ACC catalog business were Linux CDs and
related open source software.
Around the same time Marc Ewing had created his own Linux distribution which he named Red
Hat Linux (after his propensity to wear a red hat).
In 1995, ACC acquired Red Hat and adopted the name Red Hat, Inc. Red Hat went public in
1999 making both owners fabulously wealthy.
Early releases of Red Hat Linux were shipped to customers on floppy disks and CDs (this, of
course, predated the widespread availability of broadband internet connections). When users
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encountered problems with the software they could contact Red Hat only by email. In fact, Bob
Young often jokes that this was effective in limiting support requests since by the time a
customer realized they needed help, their computer was usually inoperative and therefore
unavailable to be used to send an email message seeking assistance from Red Hat‘s support
team.
In later years Red Hat provided better levels of support tied to paid subscriptions (and you can
now get phone support 24 hours a day).
Red Hat Enterprise Linux 6 is the latest commercial offering from Red Hat and is targeted at
corporate, mission critical installations. It is an open source product in that you can download the
source code free of charge and build the software yourself if you wish to do so (a task not to be
undertaken lightly) but if you wish to download a pre-built binary version of the software you
have to buy a maintenance subscription which also provides you with support and updates.
Red Hat also sponsors the Fedora Project. The goal of this project is to provide access to a
free Linux operating system (in both source and binary distributions) in the form
of Fedora Linux. Fedora also acts as a proving ground for new features that eventually are
adopted into the Red Hat Enterprise Linux operating system family.
For users that cannot afford a Red Hat Enterprise Linux subscription, another option is provided
in the form of the CentOS operating system. CentOS is a community driven project that takes the
source code to Red Hat Enterprise Linux, removes the Red Hat branding and subscription
requirements, compiles it and provides the distribution for download.
Installing Red Hat Enterprise Linux 6
Insert the RHEL 6 DVD into the appropriate drive and power on the system. If the system tries
to boot from the hard disk drive you will need to enter the BIOS set up for your computer and
change the boot order so that it boots from the DVD drive first. Once the system has booted you
will be presented with the following screen:
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Figure 1 RHEL-6 Booting from DVD
To install using the graphical installer, simply select the first installation option and press the
<Enter> key. If you encounter problems with the display when the graphical installer begins,
reboot and try again with the basic video driver option. Note also that if the host system has
insufficient memory or a graphics card is not detected, the installer will run in text mode.
Options are also available to boot from the current operating system on the local drive (if one is
installed), test the system memory, or rescue an installed RHEL 6 system. The last option alone
is reason enough to keep the installation DVD in a safe place in case you need to perform a
rescue at some future date.
The RHEL installer will then provide the option to test the installation media for errors. Use the
arrow keys to navigate between the options and make a selection with the <Enter> key. After a
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short delay the first screen of the graphical installer will appear. Navigate through the next few
pages to configure your preferred language, keyboard type and storage devices (unless you plan
to use a Storage Area Network device, the Basic option is recommended). If the installer detects
that the target disk needs to be initialized a dialog will appear seeking confirmation.
Timezone and the Root Password
Subsequent screens will request information about Timezone and the root password of the
system.
On the Timezone screen, make a selection corresponding to your geographical location. The
option is also provided to use UTC which automatically adjusts the time to account for daylight
savings time. If the computer on which RHEL is being installed also runs another operating
system which already uses UTC (such as Windows), leave this option unselected.
On the next screen, enter a password for the root account on the system. The root, or super-user
account is a special user that has administrative privileges on the system. Whilst you will
generally use your own account to log into the system, you will need to gain root privileges in
order to configure the system and to perform other administrative tasks.
The installer will subsequently move on to the disk partitioning screen.
Partitioning a Disk for RHEL 6
When you reach the disk partitioning phase of the installation, the installer will present a screen
similar to the one illustrated in the following figure:
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Figure 2 Partitioning while installing
A number of options are provided for allocating space for the installation of RHEL 6:
Use All Space - The entire disk drive will be assigned to the RHEL 6 operating system
installation. Any pre-existing partitions, together with any existing operating systems and
associated data files contained therein will be deleted to make room for RHEL. This option
should only be used if you are absolutely sure you no longer need anything that is currently
stored on that disk, or have already backed up all user files.
Replace existing Linux System(s) - If the drive was previously configured to support a
Windows/Linux dual boot environment or was devoted entirely to another Linuxinstallation,
this option may be selected to instruct the installer to delete the pre-existing Linux partition
and replace it with RHEL 6. Once again, it is important to backup any user data that may still
be needed.
Shrink Current system - Allows an existing partition to be reduced in size to make room on
the drive for the RHEL 6 installation. More details on this option are provided in a later
chapter entitled Installing RHEL 6 with Windows in Dual Boot Environment.
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Use Free Space - If the current partitions on the drive do not take up the entire disk space
available, any unallocated space may be assigned to the RHEL installation using this option.
Create Custom Layout - When selected, this option displays the disk partitioning tool
allowing each partition on the disk to be manually configured. Unless you have experience
with low level disk partitioning this option is not recommended.
On this screen, make a selection based on your requirements. If, for example, the entire disk is to
be used for RHEL 6, select the Use All Space option. In order to implement a dual boot
configuration, refer to Installing RHEL 6 with Windows in Dual Boot Environment.
For the purposes of this chapter we are assuming the entire disk is available to accommodate the
RHEL 6 installation so select the Use All Space option.
Beneath the partition menu is the option to encrypt the system. The choice of whether to use
encryption will depend on the purpose for which the system is being used, its physical location
and type of data it is going to store. Keep in mind that as with any form of encryption there are
performance overheads associated with selecting this option.
Having made the partitioning selection click Next to begin the partitioning process.
Package Selection
Linux is a modular operating system in that it provides a basic operating system kernel and
infrastructure upon which a range of different packages may be installed depending on your
specific requirements for the system. If, for example, you plan on using the system as a web
server you would need to install the Apache web server package.
At this point in the installation the installer needs us to decide which packages should be
installed along with the base operating system and displays the screen shown in the following
figure:
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Figure 3 customize menu
This screen allows you to make general choices about the type of functions you need the system
to perform. Ideally, you should select the option that most closely resembles the intended
purpose of the system.
To view or modify the specific package selections, make sure that the Customize Now option is
selected before proceeding. You will then be provided a complete overview of which packages
are selected for installation and which are not together with the ability to make changes to these
selections. Don‘t worry too much about getting this exactly right at this stage. Packages can be
added and removed at any time after the installation is complete by selecting the desktop System
-> Administration -> Add/Remove Software menu option.
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The Physical Installation
Having made the appropriate package selections, clicking next will initiate the installation
process. During the installation process, the installer will provide a running commentary of the
selected packages as they are installed and a progress bar. If you are using the DVD the
installation will complete without further interaction. Once the installation process is complete a
screen will appear containing a button to reboot the system. Remove the installation media and
click the button.
Final Configuration Steps
After the system has started for the first time, the RHEL 6 Setup Agent will appear with a
welcome message. Click on the Forward button to display licensing terms and conditions and
select the option to accept the terms of the agreement (assuming of course that you do agree to
them).
On the Setup Software Updates screen, register the system with the Red Hat Network (RHN).
This enables additional packages to be installed from the RHN repositories and also allows the
system to receive software updates. In order to register the system, enter your RHN user login
and password when prompted. If you would prefer to register the system at a later time, do so
simply by running the rhn_register command from a terminal window or selecting the System ->
Administration -> RHN Registration menu option.
Choose whether or not to enable Kdump support and then work through the remaining screens to
create a user account for yourself and verify the date and time. If you would like the date and
time of your RHEL system to be synchronized with an external Network Time Protocol server,
select the Synchronize date and time over network option before proceeding.
Having worked through all the set up pages, click Finish to exit the setup agent and log in using
your newly created account credentials.
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Linux File System
The ext4 or fourth extended filesystem is a journaling file system for Linux, developed as the
successor to ext3.
It was born as a series of backward compatible extensions to ext3, many of them originally
developed by Cluster File Systems for the Lustre file system between 2003 and 2006, meant to
extend storage limits and add other performance improvements. However, other Linux
kernel developers opposed accepting extensions to ext3 for stability reasons, and proposed
to fork the source code of ext3, rename it as ext4, and do all the development there, without
affecting the current ext3 users. This proposal was accepted, and on 28 June 2006, Theodore
Ts'o, the ext3 maintainer, announced the new plan of development for ext4.
A preliminary development version of ext4 was included in version 2.6.19 of the Linux kernel.
On 11 October 2008, the patches that mark ext4 as stable code were merged in the Linux 2.6.28
source code repositories, denoting the end of the development phase and recommending ext4
adoption. Kernel 2.6.28, containing the ext4 filesystem, was finally released on 25 December
2008. On 15 January 2010, Google announced that it would upgrade its storage infrastructure
from ext2 to ext4. On December 14, 2010, they also announced they would use ext4, instead
of YAFFS, on Android 2.3.
Features
Large file system
The ext4 filesystem can support volumes with sizes up to 1 exbibyte (EiB) and files with
sizes up to 16 tebibytes (TiB). The current e2fsprogs can only handle a filesystem of
16 TiB, but support for larger filesystems is under development.
Extents
Extents replace the traditional block mapping scheme used by ext2/3 filesystems. An
extent is a range of contiguous physical blocks, improving large file performance and
reducing fragmentation. A single extent in ext4 can map up to 128 MiB of contiguous
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space with a 4 KiB block size. There can be 4 extents stored in the inode. When there are
more than 4 extents to a file, the rest of the extents are indexed in an Htree.
Backward compatibility
The ext4 filesystem is backward compatible with ext3 and ext2, making it possible to
mount ext3 and ext2 filesystems as ext4. This will slightly improve performance, because
certain new features of ext4 can also be used with ext3 and ext2, such as the new block
allocation algorithm.
The ext3 file system is partially forward compatible with ext4, that is, an ext4 filesystem
can be mounted as an ext3 partition (using "ext3" as the filesystem type when mounting).
However, if the ext4 partition uses extents (a major new feature of ext4), then the ability
to mount the file system as ext3 is lost.
Persistent pre-allocation
The ext4 filesystem allows for pre-allocation of on-disk space for a file. The current
method for this on most file systems is to write the file full of 0s to reserve the space
when the file is created. This method is no longer required for ext4; instead, a new
fallocate() system call was added to the Linux kernel for use by filesystems, including
ext4 and XFS, that have this capability. The space allocated for files such as these would
be guaranteed and would likely be contiguous. This has applications for media streaming
and databases.
Delayed allocation
Ext4 uses a filesystem performance technique called allocate-on-flush, also known
as delayed allocation. It consists of delaying block allocation until the data is going to be
written to the disk, unlike some other file systems, which may allocate the necessary
blocks before that step. This improves performance and reduces fragmentation by
improving block allocation decisions based on the actual file size.
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Break 32,000 subdirectory limit
In ext3 the number of subdirectories that a directory can contain is limited to 32,000. This
limit has been raised to 64,000 in ext4, and with the "dir_nlink" feature it can go beyond
this (although it will stop increasing the link count on the parent). To allow for continued
performance given the possibility of much larger directories, Htree indexes (a specialized
version of a B-tree) are turned on by default in ext4. This feature is implemented in Linux
kernel 2.6.23. Htree is also available in ext3 when the dir_index feature is enabled.
Journal checksumming
Ext4 uses checksums in the journal to improve reliability, since the journal is one of the
most used files of the disk. This feature has a side benefit; it can safely avoid a disk I/O
wait during the journaling process, improving performance slightly. The technique of
journal checksumming was inspired by a research paper from the University of
Wisconsin titled IRON File Systems (specifically, section 6, called "transaction
checksums") with modifications within the implementation of compound transactions
performed by the IRON file system (originally proposed by Sam Naghshineh in the
RedHat summit).
Figure 4 fsck time/Inode Count(ext3 vs. ext4)
Faster file system checking
In ext4, unallocated block groups and sections of the inode table are marked as such. This
enables e2fsck to skip them entirely on a check and greatly reduces the time it takes to
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check a file system of the size ext4 is built to support. This feature is implemented in
version 2.6.24 of the Linux kernel.
Multiblock allocator
When a file is being appended to, ext3 calls the block allocator once for each block
individually; with multiple concurrent writers, files can easily becomefragmented on
disk. With delayed allocation, however, ext4 buffers up a larger amount of data, and then
allocates a group of blocks in a batch. This means that the allocator has more information
about what's being written and can make better choices for allocating
files contiguously on disk. The multiblock allocator is used when delayed allocation is
enabled for a file system, or when files are opened in O_DIRECT mode. This feature
does not affect the disk format.
Improved timestamps
As computers become faster in general and as Linux becomes used more for mission-
critical applications, the granularity of second-based timestamps becomes insufficient. To
solve this, ext4 provides timestamps measured in nanoseconds. In addition, 2 bits of the
expanded timestamp field are added to the most significant bits of the seconds field of the
timestamps to defer the year 2038 problem for an additional 204 years.
Ext4 also adds support for date-created timestamps. But, as Theodore Ts'o points out,
while it is easy to add an extra creation-date field in the inode (thus technically enabling
support for date-created timestamps in ext4), it is more difficult to modify or add the
necessary system calls, like stat() (which would probably require a new version), and the
various libraries that depend on them (like glibc). These changes would require
coordination of many projects. So, even if ext4 developers implement initial support for
creation-date timestamps, this feature will not be available to user programs for now.
Delayed allocation and potential data loss
Because delayed allocation changes the behavior that programmers have been relying on
with ext3, the feature poses some additional risk of data loss in cases where the system crashes or
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loses power before all of the data has been written to disk. Due to this, ext4 in kernel versions
2.6.30 and later automatically detects these cases and reverts to the old behavior.
The typical scenario in which this might occur is a program replacing the contents of a file
without forcing a write to the disk with fsync. There are two common ways of replacing the
contents of a file on Unix systems:
fd=open("file", O_TRUNC); write(fd, data); close(fd);
In this case, an existing file is truncated at the time of open (due to O_TRUNC flag), then new
data is written out. Since the write can take some time, there is an opportunity of losing contents
even with ext3, but usually very small. However, because ext4 can delay allocating file data for a
long time, this opportunity is much greater.
There are several problems with this approach:
If the write does not succeed (which may be due to error conditions in the writing program, or
due to external conditions such as a full disk), then both the original version and the new version
of the file will be lost, and the file may be corrupted because only a part of it has been written.
If other processes access the file while it is being written, they see a corrupted version.
If other processes have the file open and do not expect its contents to change, those processes
may crash. One notable example is a shared library file which is mapped into running programs.
Because of these issues, often the following idiom is preferred over the above one:
fd=open("file.new"); write(fd, data); close(fd); rename("file.new", "file");
A new temporary file ("file.new") is created, which initially contains the new contents. Then the
new file is renamed over the old one. Replacing files by the "rename" call is guaranteed to be
atomic by POSIX standards – i.e. either the old file remains, or it's overwritten with the new one.
Because the ext3 default "ordered" journalling mode guarantees file data is written out on disk
before metadata, this technique guarantees that either the old or the new file contents will persist
on disk. ext4's delayed allocation breaks this expectation, because the file write can be delayed
for a long time, and the rename is usually carried out before new file contents reach the disk.
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Using fsync more often to reduce the risk for ext4 could lead to performance penalties on ext3
filesystems mounted with the data=ordered flag (the default on most Linux distributions). Given
that both file systems will be in use for some time, this complicates matters for end-user
application developers. In response, ext4 in Linux kernels 2.6.30 and newer detect the
occurrence of these common cases and force the files to be allocated immediately. For a small
cost in performance, this provides semantics similar to ext3 ordered mode and increases the
chance that either version of the file will survive the crash. This new behavior is enabled by
default, but can be disabled with the "noauto_da_alloc" mount option.
The new patches have become part of the mainline kernel 2.6.30, but various distributions chose
to backport them to 2.6.28 or 2.6.29. For instance Ubuntu made them part of the 2.6.28 kernel in
version 9.04 ("Jaunty Jackalope").
These patches don't completely prevent potential data loss or help at all with new files. No other
filesystem is perfect in terms of data loss either, although the probability of data loss is lower on
ext3. The only way to be safe is to write and use software that does fsync when it needs to.
Performance problems can be minimized by limiting crucial disk writes that need fsync to occur
less frequently.
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Getting Started On Your Linux Desktop
If you have logged in from the graphical login screen, the graphical desktop will be started
automatically for you. The graphical desktop presents a Graphical User Interface (GUI) for the
user to interact with the system and run applications. If you have used the text-based screen
login, you will have to start the graphical desktop manually by entering the command startx
followed by the ENTER key.
The graphical desktop that we will be using throughout most of this guide is called the GNOME
Desktop. There is another desktop environment in popular use on Linux systems - the KDE
Desktop.
In order to start using your system you will usually have to perform what is known as a user
login. This procedure is necessary to identify yourself to the system. The system allows multiple
users to use it concurrently and so it has to be able to identify a user in order to grant them the
necessary privileges and rights to use the system and applications. Each user upon successful
login will be assigned to his home directory (folder).
Depending on how you have set up your system, you will either have a graphical login screen or
a text-based login prompt for you to perform the login process.
To login, enter the username followed by the ENTER key and when the password prompt
appears, enter the password followed by the ENTER key.
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Some systems may have been set up so that upon power-on, a default user is logged in
automatically. If this is so, then you will not be presented with a login screen
The Desktop Background
The desktop background constitutes the swirling graphics in the above figure. If you don't like
the default desktop background you can change it to either a selection of pre-installed images, or
to a digital picture or image of your choosing.
The desktop also provides a location for folders and applications for which quick access is
required. A link to the user‘s home folder is added to the desktop by default, as are the Computer
and Trash icons. The former provides access to storage devices and the network. The latter
contains any files that have been placed in the trash can.
Clicking with the right mouse button on any area of the desktop background presents the
following popup menu containing a number of useful options for changing or adding items to the
RHEL GNOME desktop:
Figure 5 The Desktop
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Figure 6 Right click dialog box
Create Folder - Creates a new folder on the desktop. A new folder icon appears on the desktop
with a field provided for the user to enter a folder name. The folder is physically located in the
Desktop folder of the user's home directory.
Create Launcher - Allows an icon to be placed on the desktop which, when double clicked,
launches a specified application. Another way to add application launchers to the desktop is to
find them in the menu systems at the top of the screen, right click on them and select Add this
launcher to desktop.
Create Document - Creates a new document on the desktop (the resulting file is physically
located in the Desktop folder of the current user's home directory).
Open in Terminal – Launches a terminal window containing a shell command prompt where
commands may be entered and executed. The current working directory of the shell within the
terminal is set to the Desktop subdirectory of the user‘s home folder.
Clean Up by Name - Sorts and organizes the desktop icons in alphabetical order.
Keep Aligned - A toggle setting which dictates whether icons should be neatly aligned on the
desktop or allowed to be placed arbitrarily around the desktop.
Change Desktop Background - Allows a different background image to be specified.
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Right clicking on an icon produces the following menu allowing tasks to be performed on the
selected item:
Figure 7 Right click dialog box on a folder
The Desktop Panels
The bars across the top and bottom of the desktop are called panels. Both the content and
position of these panels can be configured
By default the top panel appears as follows:
The Application menu provides access to the applications installed in the system. The Places
menu provides a list of locations which, when selected, are opened in file browser windows.
Available locations include devices (such as disk or CD/DVD Drives), the current user's home
Figure 8 Desktop Panel
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folder and other systems on the network. The System menu provides options for configuring the
system and desktop environment (including factors such as desktop theme and screen
resolution).
The icons next to the system menu provide quick access to common applications and tools. The
default icons will depend on the packages installed (such as Firefox, the Evolution email client
and the OpenOffice office productivity tools) but may be configured to add additional launchers
for tools or applications you find you use regularly.
The right hand side of the panel includes the current time, a volume control and a network status
indicator. To establish a network connection, access a variety of configuration options by
clicking with the left and right mouse buttons on the icon.
Also present on laptop based installations is an icon indicating current power status. A variety of
other status icons will appear in this section from time to time. Typically hovering over or
clicking on the status icon will display a brief description of the icon‘s purpose. For example,
two additional icons are present in the above figure indicating that updates are available for the
system (represented by the bug icon) and that the package manager has messages that are need to
be read (the cardboard box icon).
As you perform administrative tasks on the system that require authentication using the root
password, you will see a set of keys appear in the top panel. This indicates that elevated root
(administrative) privileges have been assigned to the desktop session allowing sequential tasks to
be performed without repeatedly entering the root password. To de-authorize these elevated
privileges simply click on the icon.
The bottom desktop panel contains three items by default but may similarly be configured as By
default the panel appears as follows:
Figure 9 Bottom Desktop Panel
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The area containing squares controls the currently displayed virtual workspace. The RHEL
GNOME desktop allows multiple workspaces to be active at any one time with different
applications and windows visible on each workspace. To switch to a different desktop, simply
click on one of the squares in the bottom toolbar. If your mouse has a scroll wheel, click on a
workspace in the panel and then scroll back and forth through the workspaces. By default two
workspaces are configured, though this number can be increased or decreased. To move an
application window from one workspace to another, right click on the window toolbar and select
the destination workspace from the Move to Another Workspace sub-menu.
When applications are running on the desktop, a button will appear for each application located
on the currently selected workspace. If the application‘s windows are currently visible on the
desktop, clicking the corresponding button in the panel will minimize the windows so that they
are no longer visible. Clicking the button again will re-display the windows once again. The final
item on the bottom panel is the trash can. Items may be dragged and dropped into the trash can.
Right click on the icon to access options to open or empty the trash can.
Now that we have covered the basics of the desktop we can begin looking at customizing the
desktop. The first step involves changing the RHEL GNOME desktop background.
The RHEL 6 GNOME Slideshow Background
The default background for Red Hat Enterprise Linux 6 is actually made of multiple images that
are shown in sequence in the form of a slide show. The images are located in
the/usr/share/background directory and the order in which the images appear and the duration for
which each is displayed is configured in the /usr/share/background/default.xml file. Use this file
as a template to create your own slide shows if desired.
Changing the RHEL 6 GNOME Desktop Background
There are three methods for changing the background, either by clicking with the right hand
mouse button on the current desktop background and selecting Change Desktop Background,
selecting the Administration -> Preferences -> Appearance menu option and selecting
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the Background tab, or clicking with the right hand-mouse button on an image in the web
browser and selecting Set As Desktop Background from the resulting popup menu. The latter
method allows you find images from one of the many web sites that provide them and quickly
make one of them your background.
Regardless of which method you use, the Desktop Background Preferences dialog will appear as
illustrated in the following figure:
Figure 10 Background Selection
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Selecting a Background from the Wallpaper List
The background preferences dialog provides a list of pre-installed wallpaper background images
from which to choose. Those images that are shown stacked are slideshows.
As each example is selected the background will automatically change to reflect the new
selection. Additional images may be added to the gallery by clicking on the Add... button and
navigating to the image file. The Style menu provides a range of options for handling images that
do not precisely match the screen dimensions. Choices include zooming, tiling, scaling and
fitting.
Creating a Solid or Graded Background
If you prefer a background consisting of a solid color simply select the blank option option
located at the top of the gallery list, make sure that Solid Color is selected next to Colors and
click on the color box to select a color.
The background can also be created by specifying a color gradient transition from one color to
another. The gradient may be horizontal or vertical. With the blank wallpaper option selected
Figure 11 Desktop Background with shades
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change the Desktop Color menu to Vertical or Horizontal Gradient. Using the two color boxes
located to the right of the menu, specify the start and end colors. The desktop will then create a
background image based on these settings. For example, the following figure shows a horizontal
gradient from blue to red:
Specifying a Background Image
As mentioned previously, there are two ways to specify an image file for the RHEL desktop
background. One way is to open an image (for example in a web browser) clicking with the right
mouse button and selecting Set as Desktop Background from the resulting menu. This will cause
the following dialog to appear asking how the image is to be positioned on the desktop:
Figure 12 Set Desktop Background
From this dialog select whether the image should be tiled, stretched or centered. If space is going
to be visible around the edges of the image be sure to select a suitable color for the border area
using the color box. Click on Set Desktop Background to apply the settings
28
Configuring the RHEL 6 GNOME Desktop Panels
The Red Hat Enterprise Linux 6 GNOME Desktop Panels are one of the most useful aspects of
the desktop in terms of providing information, ease of use and convenience to the user. As with
just about every other aspect of the GNOME desktop, these panels are accompanied by
extensive configuration options. In this chapter we will look at each of these configuration
options in detail.
What are Desktop Panels?
On a newly installed RHEL 6 system there are two panels configured by default. These appear at
the top and bottom of the desktop respectively. The top panel contains the main desktop menus
and a number of status areas with information such as the system time and audio settings:
The bottom panel contains a button to hide and show all windows on the screen, the trash can,
access to the various virtual workspaces and also has a button representing each application
currently running in the current workspace.
These panels can be configured in a variety of ways including:
Adding additional panels to the desktop
Moving the panels to different locations on the desktop
Adding mini applications (such as stock ticker, system monitor or weather applet) to a panel
Size, background, visibility and transparency of each panel
In the remainder of this chapter we will look at each of these customization options in detail.
Changing the Position of a Panel
Figure 13 Desktop Panels
Figure 14 Desktop Panels
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A panel can be placed in one of four different locations on the desktop: the top, bottom, far left
or far right of the desktop. The location of the panel can be adjusted using the Properties dialog
of the panel. This dialog is accessed by clicking with the right hand mouse button on any blank
area of the panel and selecting Properties from the popup menu:
Figure 15 Panel Properties
From the Panel Properties dialog, change the Orientation setting to the desired value to position
the panel as required.
Adding and Deleting Desktop Panels
New panels may be added to the desktop by right clicking on any existing panel and selecting
New Panel from the resulting menu. The new panel will be placed in an orientation where no
panel currently exists. If you already have panels on all four sides of your desktop, the new panel
will be placed next to a pre-existing panel. It is possible to fill your entire desktop with panels if
you wish, though it is hard to imagine a situation where this would be desirable.
30
An existing panel may be removed from the desktop by right clicking on the panel and
selecting Delete This Panel from the popup menu.
Changing the Appearance of a Desktop Panel
A number of configuration options are available for changing the appearance of each desktop
panel. These changes are implemented using the Panel Properties dialog (accessed by right
clicking on the panel to be changed and selecting Properties from the popup menu).
The Size property controls the height (when the panel is in a horizontal orientation) or width
(when in vertical orientation). It is not possible to reduce the size to the extent where items will
no longer be visible (the minimum value is typically around 23 pixels).
A different color or even an image may be specified for the panel background. These settings are
available from the Background page of the panel properties dialog:
Figure 16 Panel Background properties
The level of transparency can be specified by selecting the Solid color option and adjusting the
Style slider.
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The panel may be further configured so that it remains hidden until the mouse pointer moves to
the side of the desktop where the panel is located. To activate this feature, select
the Autohide option on the General panel of the properties window.
Finally, if you do not wish to have the panel occupy the full width or height of the desktop
(depending on the orientation of the selected panel), unset the Expand option. The panel will
then only appear as wide or as tall as it needs to be to accommodate the items it is configured to
display.
Adding Items to a RHEL 6 GNOME Desktop Panel
When RHEL 6 is first installed, a number of items are added by default to the two panels.
Additional items, however, may easily be added to any panel. There are a couple of ways to add
items to the panel.
If there is an application you frequently launch from the Applications menu there is a quick way
to add a launch icon to the panel. Simply open the Applications menu and navigate to the menu
option for the desired application. Rather than clicking on this menu item with the left mouse
button as you normally would, click instead with the right mouse button and select the Add this
launcher to panel option. An icon representing the application will subsequently appear on the
panel which, when clicked, will launch the corresponding application.
Alternatively, launch the Add to Panel dialog by right clicking on a blank area of the desired
panel and selecting Add to Panel… from the resulting menu. In the resulting dialog
select Application Launcher... and click Forward. The dialog will list each menu item and all
sub-menu items. If, for example, you wanted to be able to launch the Terminal window from the
panel simply click on the arrow next to System Tools in the Add to Panel dialog to unfold the list
of accessory applications. Scroll down the list of accessories, select Terminal and click on
the Add button:
32
In addition to applications from the menu, it is also possible to add a range of items specifically
designed to appear on panels. To add such items to a panel click with the right mouse button
over the panel to be configured and select the Add to Panel menu option from the resulting
popup menu. The Add to Panel dialog will then appear as follows:
Figure 17 Adding Item to panels
33
Figure 18 Adding Item to panels
To add an item to the panel, simply scroll through the list provided, select the required item and
press the Add button. The following RHEL 6 desktop panel includes the System Monitor and
Weather Report items:
To edit the properties or preferences for a panel item, move the mouse pointer over the item in
the panel and click the right mouse button. Select either Properties or Preferences from the menu
and the appropriate dialog will appear. For example, the Preferences panel for the Weather
Figure 19 Preview after adding items to panel
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Report item allows various settings to be changed, such as the geographical location and units for
displaying temperature (Celsius or Fahrenheit). The system monitor can similarly be configured
to also display metrics such as system load, memory and network usage.
To delete an item from a panel right click with the mouse on the item and select Remove from
Panel.
To move an item to a different location in a panel right click with the mouse on the item and
select Move. Then use the mouse to drag and drop the item to a new location. Alternatively, click
with either the middle mouse button, or the left and right mouse buttons simultaneously and drag
and drop the panel item to the new location.
To add an application not currently available from the desktop menus simply select the Custom
Application Launcher, click Add and specify the path to the application executable and provide a
name. Click on the No icon button to select an icon to represent the item on the panel.
Adding Menus to a Panel
Any of the sub-menus in the Applications, Places or System menus may be added as icons to the
main panel. To add, for example, an icon to access the Application -> Accessories menu to the
panel, open the Applications menu and right click on any item in the Accessories menu item.
From the popup menu, select the Entire menu... followed by Add this as menu to panel. An icon
will then appear on the panel which, when clicked, drops down the Accessories menu:
Alternatively, the menu may be added to the panel as a Drawer by selecting Add this as drawer
to panel. A drawer is similar to the menu with the exception that only the icons are displayed and
an Up arrow
Figure 20 Preview of item added to panel
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Changing the Number of RHEL Desktop Workspaces
The RHEL GNOME desktop supports multiple desktop workspaces (these are essentially
individual screens which the user can switch between to avoid having to have all applications
cluttered onto one screen). Switching between workspaces is achieved by clicking on the
workspaces in the bottom desktop panel. The image below shows a section of the panel from a
desktop with four workspaces:
Figure 22 Workspaces
By default, RHEL 6 configures two workspaces. To increase the number of available
workspaces, right click with the mouse on the workspace control (as shown above) in the panel
and selectPreferences. Simply increase the Number of workspaces value to the desired number.
The name of each workspace may be changed by double clicking on the default name in the list
(Workspace 1, Workspace 2 etc) and typing in a new name.
Figure 21
36
Linux Directory Structure
/ -
This is referred to as the root directory (not to be confused with the /root directory, which
is the root user's directory!). It's the baseline for the directory structure, which is
important when navigating from one directory tree to another.
/bin -
Contains essential programs for the operating system in executable form.
/boot -
This directory, as the name suggests, contains the boot information for Linux, including
the kernel.
/dev -
In Linux, devices are treated in the same way as files, so they can be read from and
written to in the same way. So, when a device is attached, it will show up in this folder.
Bear in mind that this includes every device in the system, including internal
motherboard devices and more.
/etc -
The default configuration files for Linux applications are stored in this directory. These
are text files can be easily edited, but bear in mind that they can sometimes be overridden
by copies of the files elsewhere on the system.
/home -
This is where the computer's users files are kept, so in a way it is Linux's equivalent to
Windows' "My Documents". Each user gets their own named directory, and security
permissions can be set so that users can view and edit, view only, or if required not even
see the contents of other users' home directories.
/lib -
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This directory contains shared libraries for use by applications running on Linux, similar
to Windows' DLL files.
/lost+found -
This is Linux's rescue directory, so that in the event of a crash or other serious event files
are stashed here to enable recovery.
/mnt -
In Linux, every storage device is treated as just another directory. This includes floppy
disks, hard drives, CD/DVD ROMs and USB card devices. Since it is very unlikely to
ever concern you with a dedicated server it is not covered here, but just know that this is
the directory in which storage devices are "mounted."
/proc -
This "virtual" directory contains a lot of fluid data about the status of the kernel and its
running environment. Since Linux treats everything as files, you can view this data using
text viewing software, and though even editing these files is sometimes possible, doing so
is not advised unless you really know what you're doing.
/root -
Rather than being part of the /home directory, the superuser (or root user)'s directory is
placed here. Remember that this is not the same thing as the root directory of the system
(/).
/sbin -
This is where system administration software is stored. Unlike applications in the /bin
folder, the root user is usually the only user who can run these.
/tmp -
Applications store their temporary files in this directory.
38
/usr -
This directory is where users' applications are stored, including the executables, and also
sometimes the source code, along with any images and documentation.
/var -
Similar to /proc, this directory contains data concerning the status of running application,
including many log files. This is worth knowing, because these can be viewed in the
event of a system error to help in diagnosing the problem.
The Linux Console
In the early days of Linux, when you booted up your system you would see a login prompt on
your monitor, and that‘s all. As mentioned earlier, this is called the Linux console. It was the
only place you could enter commands for the system.
With modern Linux systems, when the Linux system starts it automatically creates several
virtual consoles. A virtual console is a terminal session that runs in memory on the Linux
system. Instead of having six dumb terminals connected to the PC, the Linux system starts six (or
sometimes even more) virtual consoles that you can access from the single PC keyboard and
monitor.
In most Linux distributions, you can access the virtual consoles using a simple keystroke
combination. Usually you must hold down the Ctl +Alt key combination, then press a function
key (F2 through F6) for the virtual console you want to use. Function key F2 produces virtual
console 1, key F3 produces virtual console 2, and so on.
The first five virtual consoles use a full-screen text terminal emulator to display a text login
screen,
39
After logging in with your user ID and password, you are taken to the Linux bash shell CLI. In
the Linux console you do not have the ability to run any graphical programs. You can only use
text programs to display on the Linux text consoles.
After logging in to a virtual console, you can keep it active and switch to another virtual console
without losing your active session. You can switch between all of the virtual consoles, with
multiple active sessions running.
The first virtual console is normally reserved for X Windows graphical desktops.
Absolute filepaths
You can reference a directory name within the virtual directory using an absolute filepath. The
absolute filepath defines exactly where the directory is in the virtual directory structure, starting
at the root of the virtual directory. Sort of like a full name for a directory.
Thus, to reference the apache directory, that‘s contained within the lib directory, which in turn is
contained within the usr directory, you would use the absolute filepath:
Figure 23 Linux Console
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/usr/lib/apache
With the absolute filepath there‘s no doubt as to exactly where you want to go. To move to a
specific location in the filesystem using the absolute filepath, you just specify the full pathname
in the cd command:
root@1[~]#cd /etc
root@1[etc]#
The prompt shows that the new directory for the shell after the cd command is now /etc. You can
move to any level within the entire Linux virtual directory structure using the absolute filepath:
root@1[~]# cd /usr/lib/apache
root@1[apache]#
However, if you‘re just working within your own home directory structure, often using absolute
filepaths can get tedious. For demo, if you‘re already in the directory /home/root, it seems
somewhat cumbersome to have to type the command:
cd /home/root/Documents
just to get to your Documents directory. Fortunately, there‘s a simpler solution.
Relative filepaths Relative filepaths allow you to specify a destination filepath relative to your current location,
without having to start at the root. A relative filepath doesn‘t start with a forward slash,
indicating the root directory.
41
Instead, a relative filepath starts with either a directory name (if you‘re traversing to a directory
under your current directory), or a special character indicating a relative location to your current
directory location. The two special characters used for this are:
■ The dot (.) to represent the current directory
■ The double dot (..) to represent the parent directory
The double dot character is extremely handy when trying to traverse a directory hierarchy. For
demo, if you are in the Documents directory under your home directory and need to go to your
Desktop directory, also under your home directory, you can do this:
root@1[Documents]# cd ../Desktop
root@1[Desktop]#
The double dot character takes you back up one level to your home directory, then the /Desktop
portion then takes you back down into the Desktop directory. You can use as many double dot
characters as necessary to move around. For demo, if you are in your home directory
(/home/root) and want to go to the /etc directory, you could type:
root@1[~]# cd ../../etc
root@1[etc]#
Of course, in a case like this, you actually have to do more typing to use the relative filepath
rather than just typing the absolute filepath, /etc!
Beginning with command line utility
File and Directory Listing
The most basic feature of the shell is the ability to see what files are available on the system. The
list command (ls) is the tool that helps do that. This section describes the ls command, and all of
the options available to format the information it can provide.
42
Basic listing
The ls command at its most basic form displays the files and directories located in your current
directory:
# ls
4root Desktop Download Music Pictures store store.zip test
backup Documents Drivers myfile Public store.sql Templates Videos
#
Notice that the ls command produces the listing in alphabetical order (in columns rather than
rows). If you‘re using a terminal emulator that supports color, the ls command may also show
different types of entries in different colors. The LS COLORS environment variable controls this
feature. Different Linux distributions set this environment variable depending on the capabilities
of the terminal emulator.
If you don‘t have a color terminal emulator, you can use the -F parameter with the ls command
to easily distinguish files from directories. Using the -F parameter produces the following output:
# ls -F
4root/ Documents/ Music/ Public/ store.zip Videos/
backup.zip Download/ myfile* store/ Templates/
Desktop/ Drivers/ Pictures/ store.sql test
#
The -F parameter now flags the directories with a forward slash, to help identify them in the
listing. Similarly, it flags executable files (like the myfile file above) with an asterisk, to help you
find the files that can be run on the system easier.
The basic ls command can be somewhat misleading. It shows the files and directories contained
in the current directory, but not necessarily all of them. Linux often uses hidden files to store
configuration information. In Linux, hidden files are files with filenames that start with a period.
43
These files don‘t appear in the default ls listing (thus they are called hidden). To display hidden
files along with normal files and directories, use the -a parameter.
Wow, that‘s quite a difference. In a home directory for a user who has logged in to the system
from a graphical desktop, you‘ll see lots of hidden configuration files. This particular demo is
from a user logged in to a GNOME desktop session. Also notice that there are three files that
begin with .bash. These files are hidden files that are used by the bash shell environment.
The -R parameter is another command ls parameter to use. It shows files that are contained
within directories in the current directory. If you have lots of directories, this can be quite a long
listing. Here‘s a simple demo of what the -R parameter produces:
# ls -F -R
.:
file1 test1/ test2/
./test1:
myfile1* myfile2*
./test2:
#
Notice that first, the -R parameter shows the contents of the current directory, which is a file
(file1) and two directories (test1 and test2). Following that, it traverses each of the two
directories, showing if any files are contained within each directory. The test1 directory shows
two files (myfile1 and myfile2), while the test2 directory doesn‘t contain any files. If there had
been further subdirectories within the test1 or test2 directories, the -R parameter would have
continued to traverse those as well. As you can see, for large directory structures this can become
quite a large output listing.
Modifying the information presented
As you can see in the basic listings, the ls command doesn‘t produce a whole lot of information
about each file. For listing additional information, another popular parameter is -l. The -l
44
parameter produces a long listing format, providing more information about each file in the
directory:
# ls -l
total 2064
drwxrwxr-x 2 root root 4096 2007-08-24 22:04 4root
-rw-r--r-- 1 root root 1766205 2007-08-24 15:34 backup.zip
drwxr-xr-x 3 root root 4096 2007-08-31 22:24 Desktop
drwxr-xr-x 2 root root 4096 2001-11-01 04:06 Documents
drwxr-xr-x 2 root root 4096 2001-11-01 04:06 Download
drwxrwxr-x 2 root root 4096 2007-07-26 18:25 Drivers
drwxr-xr-x 2 root root 4096 2001-11-01 04:06 Music
-rwxr--r-- 1 root root 30 2007-08-23 21:42 myfile
drwxr-xr-x 2 root root 4096 2001-11-01 04:06 Pictures
drwxr-xr-x 2 root root 4096 2001-11-01 04:06 Public
drwxrwxr-x 5 root root 4096 2007-08-24 22:04 store
-rw-rw-r-- 1 root root 98772 2007-08-24 15:30 store.sql
-rw-r--r-- 1 root root 107507 2007-08-13 15:45 store.zip
drwxr-xr-x 2 root root 4096 2001-11-01 04:06 Templates
drwxr-xr-x 2 root root 4096 2001-11-01 04:06 Videos
[root@testbox ~]#
The long listing format lists each file and directory contained in the directory on a single line.
Besides the filename, it shows additional useful information. The first line in the output shows
the total number of blocks contained within the directory. Following that, each line contains the
following information about each file (or directory):
■ The file type (such as directory (d), file (-), character device (c), or block device (b)
■ The permissions for the file
■ The number of hard links to the file
■ The username of the owner of the file
45
■ The group name of the group the file belongs to
■ The size of the file in bytes
■ The time the file was modified last
■ The file or directory name
The -l parameter is a powerful tool to have. Armed with this information you can see just about
any information you need to for any file or directory on the system.
There are lots of parameters for the ls command that can come in handy as you do file
management. If you use the man command for ls, you‘ll see several pages of available
parameters for you to use to modify the output of the ls command.
Figure 24 ls parameters
46
Some Popular ls Command Parameters
File Handling
The bash shell provides lots of commands for manipulating files on the Linux filesystem. This
section walks you through the basic commands you will need to work with files from the CLI for
all your file-handling needs.
Figure 25 ls Parameters
47
Creating files
Every once in a while you will run into a situation where you need to create an empty file.
Sometimes applications expect a log file to be present before they can write to it. In these
situations, you can use the touch command to easily create an empty file:
# touch test1
# ls -il test1
1954793 -rw-r--r-- 1 root root 0 Sep 1 09:35 test1
#
The touch command creates the new file you specify, and assigns your username as the file
owner. Since I used the -il parameters for the ls command, the first entry in the listing shows the
inode number assigned to the file. Every file on the Linux system has a unique inode number.
Notice that the file size is zero, since the touch command just created an empty file. The touch
command can also be used to change the access and modification times on an existing file
without
changing the file contents:
# touch test1
# ls -l test1
-rw-r--r-- 1 root root 0 Sep 1 09:37 test1
#
The modification time of test1 is now updated from the original time. If you want to change only
the access time, use the -a parameter. To change only the modification time, use the –m
parameter. By default touch uses the current time. You can specify the time by using the –t
parameter with a specific timestamp:
48
# touch -t 200812251200 test1
# ls -l test1
-rw-r--r-- 1 root root 0 Dec 25 2008 test1
#
Now the modification time for the file is set to a date significantly in the future from the current
time.
Copying files
Copying files and directories from one location in the filesystem to another is a common practice
for system administrators. The cp command provides this feature. In it‘s most basic form, the cp
command uses two parameters: the source object and the destination object:
cp source destination
When both the source and destination parameters are filenames, the cp command copies the
source file to a new file with the filename specified as the destination. The new file acts like a
brand new file, with an updated file creation and last modified times:
# cp test1 test2
# ls -il
total 0
1954793 -rw-r--r-- 1 root root 0 Dec 25 2008 test1
1954794 -rw-r--r-- 1 root root 0 Sep 1 09:39 test2
#
The new file test2 shows a different inode number, indicating that it‘s a completely new file.
You‘ll also notice that the modification time for the test2 file shows the time that it was created.
If the destination file already exists, the cp command will prompt you to answer whether or not
you want to overwrite it:
49
# cp test1 test2
cp: overwrite `test2‘? y
#
If you don‘t answer y, the file copy will not proceed. You can also copy a file to an existing
directory:
# cp test1 dir1
# ls -il dir1
total 0
1954887 -rw-r--r-- 1 root root 0 Sep 6 09:42 test1
#
The new file is now under the dir1 directory, using the same filename as the original. These
demos all used relative pathnames, but you can just as easily use the absolute pathname for both
the source and destination objects.
To copy a file to the current directory you‘re in, you can use the dot symbol:
# cp /home/root/dir1/test1 .
cp: overwrite `./test1‘?
As with most commands, the cp command has a few command line parameters to help you out.
Use the -p parameter to preserve the file access or modification times of the original file for the
copied file.
# cp -p test1 test3
# ls -il
total 4
1954886 drwxr-xr-x 2 root root 4096 Sep 1 09:42 dir1/
1954793 -rw-r--r-- 1 root root 0 Dec 25 2008 test1
50
1954794 -rw-r--r-- 1 root root 0 Sep 1 09:39 test2
1954888 -rw-r--r-- 1 root root 0 Dec 25 2008 test3
#
Now, even though the test3 file is a completely new file, it has the same timestamps as the
original test1 file.
The -R parameter is extremely powerful. It allows you to recursively copy the contents of an
entire directory in one command:
# cp -R dir1 dir2
# ls -l
total 8
drwxr-xr-x 2 root root 4096 Sep 6 09:42 dir1/
drwxr-xr-x 2 root root 4096 Sep 6 09:45 dir2/
-rw-r--r-- 1 root root 0 Dec 25 09:30 test1
-rw-r--r-- 1 root root 0 Sep 6 09:39 test2
-rw-r--r-- 1 root root 0 Dec 25 11:30 test3
#
Now dir2 is a complete copy of dir1. You can also use wildcard characters in your cp commands:
# cp -f test* dir2
# ls -al dir2
total 12
drwxr-xr-x 2 root root 4096 Sep 6 10:55 ./
drwxr-xr-x 4 root root 4096 Sep 6 10:46 ../
-rw-r--r-- 1 root root 0 Dec 25 2008 test1
-rw-r--r-- 1 root root 0 Sep 6 10:55 test2
-rw-r--r-- 1 root root 0 Dec 25 2008 test3
#
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The cp Command Parameters
Linking files You may have noticed a couple of the parameters for the cp command referred to linking files.
This is a pretty cool option available in the Linux filesystems. If you need to maintain two (or
more) copies of the same file on the system, instead of having separate physical copies, you can
use one physical copy and multiple virtual copies, called links. A link is a placeholder in a
directory that points to the real location of the file. There are two different types of file links in
Linux:
■ A symbolic, or soft, link
■ A hard link
Figure 26 cp command parameters
52
The hard link creates a separate file that contains information about the original file and where to
locate it. When you reference the hard link file, it‘s just as if you‘re referencing the original file:
# cp -l test1 test4
# ls -il
total 16
1954886 drwxr-xr-x 2 root root 4096 Sep 1 09:42 dir1/
1954889 drwxr-xr-x 2 root root 4096 Sep 1 09:45 dir2/
1954793 -rw-r--r-- 2 root root 0 Sep 1 09:51 test1
1954794 -rw-r--r-- 1 root root 0 Sep 1 09:39 test2
1954888 -rw-r--r-- 1 root root 0 Dec 25 2008 test3
1954793 -rw-r--r-- 2 root root 0 Sep 1 09:51 test4
#
The -l parameter created a hard link for the test1 file called test4. When I performed the file
listing, you can see that the inode number of both the test1 and test4 files are the same, indicating
that, in reality, they are both the same file. Also notice that the link count (the third item in the
listing) now shows that both files have two links.
(NOTE:- You can only create a hard link between files on the same physical medium. You
can’t create a hard link between files under separate mount points. In that case, you’ll have to
use a soft link.)
On the other hand, the -s parameter creates a symbolic, or soft, link:
# cp -s test1 test5
# ls -il test*
total 16
1954793 -rw-r--r-- 2 root root 6 Sep 1 09:51 test1
1954794 -rw-r--r-- 1 root root 0 Sep 1 09:39 test2
1954888 -rw-r--r-- 1 root root 0 Dec 25 2008 test3
1954793 -rw-r--r-- 2 root root 6 Sep 1 09:51 test4
53
1954891 lrwxrwxrwx 1 root root 5 Sep 1 09:56 test5 -> test1
There are a couple of things to notice in the file listing, First, you‘ll notice that the new test5 file
has a different inode number than the test1 file, indicating that the Linux system treats it as a
separate file. Second, the file size is different. A linked file needs to store only information about
the source file, not the actual data in the file. The filename area of the listing shows the
relationship between the two files.
(TIP: Instead of using the cp command, if you want to link files you can also use the ln
command. By default the ln command creates hard links. If you want to create a soft link,
you’ll still need to use the -s parameter.)
Be careful when copying linked files. If you use the cp command to copy a file that‘s linked to
another source file, all you‘re doing is making another copy of the source file. This can quickly
get confusing. Instead of copying the linked file, you can create another link to the original file.
You can have many links to the same file with no problems. However, you also don‘t want to
create soft links to other soft-linked files. This creates a chain of links that can not only be
confusing but also be easily broken, causing all sorts of problems.
Renaming files
In the Linux world, renaming files is called moving. The mv command is available to move both
files and directories to another location:
# mv test2 test6
# ls -il test*
1954793 -rw-r--r-- 2 root root 6 Sep 1 09:51 test1
1954888 -rw-r--r-- 1 root root 0 Dec 25 2008 test3
1954793 -rw-r--r-- 2 root root 6 Sep 1 09:51 test4
1954891 lrwxrwxrwx 1 root root 5 Sep 1 09:56 test5 -> test1
1954794 -rw-r--r-- 1 root root 0 Sep 1 09:39 test6
54
#
Notice that moving the file changed the filename but kept the same inode number and the
timestamp value. Moving a file with soft links is a problem:
# mv test1 test8
# ls -il test*
total 16
1954888 -rw-r--r-- 1 root root 0 Dec 25 2008 test3
1954793 -rw-r--r-- 2 root root 6 Sep 1 09:51 test4
1954891 lrwxrwxrwx 1 root root 5 Sep 1 09:56 test5 -> test1
1954794 -rw-r--r-- 1 root root 0 Sep 1 09:39 test6
1954793 -rw-r--r-- 2 root root 6 Sep 1 09:51 test8
[root@test2 clsc]# mv test8 test1
The test4 file that uses a hard link still uses the same inode number, which is perfectly fine.
However, the test5 file now points to an invalid file, and it is no longer a valid link. You can also
use the mv command to move directories:
# mv dir2 dir4
The entire contents of the directory are unchanged. The only thing that changes is the name of
the directory.
Deleting files
Most likely at some point in your Linux career you‘ll want to be able to delete existing files.
Whether it‘s to clean up a filesystem or to remove a software package, there‘s always
opportunities to delete files.
In the Linux world, deleting is called removing. The command to remove files in the bash shell is
rm. The basic form of the rm command is pretty simple:
55
# rm -i test2
rm: remove `test2‘? y
# ls -l
total 16
drwxr-xr-x 2 root root 4096 Sep 1 09:42 dir1/
drwxr-xr-x 2 root root 4096 Sep 1 09:45 dir2/
-rw-r--r-- 2 root root 6 Sep 1 09:51 test1
-rw-r--r-- 1 root root 0 Dec 25 2008 test3
-rw-r--r-- 2 root root 6 Sep 1 09:51 test4
lrwxrwxrwx 1 root root 5 Sep 1 09:56 test5 -> test1
#
Notice that the command prompts you to make sure that you‘re serious about removing the file.
There‘s no trashcan in the bash shell. Once you remove a file it‘s gone forever. Now, here‘s an
interesting tidbit about deleting a file that has links to it:
# rm test1
# ls -l
total 12
drwxr-xr-x 2 root root 4096 Sep 1 09:42 dir1/
drwxr-xr-x 2 root root 4096 Sep 1 09:45 dir2/
-rw-r--r-- 1 root root 0 Dec 25 2008 test3
-rw-r--r-- 1 root root 6 Sep 1 09:51 test4
lrwxrwxrwx 1 root root 5 Sep 1 09:56 test5 -> test1
# cat test4
hello
# cat test5
cat: test5: No such file or directory
#
56
I removed the test1 file, which had both a hard link with the test4 file and a soft link with the
test5 file. Noticed what happened. Both of the linked files still appear, even though the test1 file
is now gone (although on my color terminal the test5 filename now appears in red). When I look
at the contents of the test4 file that was a hard link, it still shows the contents of the file. When I
look at the contents of the test5 file that was a soft link, bash indicates that it doesn‘t exist any
more.
Remember that the hard link file uses the same inode number as the original file. The hard link
file maintains that inode number until you remove the last linked file, preserving the data! All the
soft link file knows is that the underlying file is now gone, so it has nothing to point to. This is an
important feature to remember when working with linked files. One other feature of the rm
command, if you‘re removing lots of files and don‘t want to be bothered with the prompt, is to
use the -f parameter to force the removal. Just be careful!
Directory Handling
In Linux there are a few commands that work for both files and directories (such as the cp
command), and some that only work for directories. To create a new directory, you‘ll need to use
a specific command, which I‘ll discuss in this section. Removing directories can get interesting,
so we‘ll look at that as well in this section.
Creating directories
There‘s not much to creating a new directory in Linux, just use the mkdir command:
# mkdir dir3
# ls -il
total 16
1954886 drwxr-xr-x 2 root root 4096 Sep 1 09:42 dir1/
1954889 drwxr-xr-x 2 root root 4096 Sep 1 10:55 dir2/
1954893 drwxr-xr-x 2 root root 4096 Sep 1 11:01 dir3/
1954888 -rw-r--r-- 1 root root 0 Dec 25 2008 test3
1954793 -rw-r--r-- 1 root root 6 Sep 1 09:51 test4
57
#
The system creates a new directory and assigns it a new inode number.
Deleting directories
Removing directories can be tricky, but there‘s a reason for that. There are lots of opportunity for
bad things to happen when you start deleting directories. The bash shell tries to protect us from
accidental catastrophes as much as possible. The basic command for removing a directory is
rmdir:
# rmdir dir3
# rmdir dir1
rmdir: dir1: Directory not empty
#
By default, the rmdir command only works for removing empty directories. Since there is a file
in the dir1 directory, the rmdir command refuses to remove it. You can remove nonempty
directories using the --ignore-fail-on-non-empty parameter.
Our friend the rm command can also help us out some when handling directories. If you try
using it with not parameters, as with files, you‘ll be somewhat disappointed:
# rm dir1
rm: dir1: is a directory
#
However, if you really want to remove a directory, you can use the -r parameter to recursively
remove the files in the directory, then the directory itself:
# rm -r dir2
rm: descend into directory `dir2‘? y
rm: remove `dir2/test1‘? y
rm: remove `dir2/test3‘? y
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rm: remove `dir2/test4‘? y
rm: remove directory `dir2‘? y
#
While this works, it‘s somewhat awkward. Notice that you still must verify every file that gets
removed. For a directory with lots of files and subdirectories, this can become tedious. The
ultimate solution for throwing caution to the wind and removing an entire directory, contents and
all, is the rm command with both the -r and -f parameters:
# rm -rf dir2
#
That‘s it. No warnings, no fanfare, just another shell prompt. This, of course, is an extremely
dangerous tool to have, especially if you‘re logged in as the root user account. Use it sparingly,
and only after triple checking to make sure that you‘re doing exactly what you want to do.
Viewing File Contents
So far we‘ve covered everything there is to know about files, except for how to peek inside of
them. There are several commands available for taking a look inside files without having to pull
out an editor . This section demonstrates a few of the commands you have available to help you
examine files.
Viewing file statistics
You‘ve already seen that the ls command can be used to provide lots of useful information about
files. However, there‘s still more information that you can‘t see in the ls command (or at least
not all at once).
The stat command provides a complete rundown of the status of a file on the filesystem:
# stat test10
File: "test10"
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Size: 6 Blocks: 8 Regular File
Device: 306h/774d Inode: 1954891 Links: 2
Access: (0644/-rw-r--r--) Uid: ( 501/ root) Gid: ( 501/ root)
Access: Sat Sep 1 12:10:25 2007
Modify: Sat Sep 1 12:11:17 2007
Change: Sat Sep 1 12:16:42 2007
#
The results from the stat command show just about everything you‘d want to know about the file
being examined, even down the major and minor device numbers of the device where the file is
being stored.
Viewing the file type
Despite all of the information the stat command produces, there‘s still one piece of information
missing — the file type. Before you go charging off trying to list out a 1000-byte file, it‘s usually
a good idea to get a handle on what type of file it is. If you try listing a binary file, you‘ll get lots
of gibberish on your monitor, and possibly even lock up your terminal emulator.
The file command is a handy little utility to have around. It has the ability to peek inside of a file
and determine just what kind of file it is:
# file test1
test1: ASCII text
# file myscript
myscript: Bourne shell script text executable
# file myfile
myfile: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV),
dynamically linked (uses shared libs), not stripped
#
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The file command classifies files into three categories:
■ Text files: Files that contain printable characters
■ Executable files: Files that you can run on the system
■ Data files: Files that contain nonprintable binary characters, but that you can‘t run on the
system
The first demo shows a text file. The file command determined not only that the file contains text
but also the character code format of the text. The second demo shows a text script file. While
the file is text, since it‘s a script file you can execute (run) it on the system. The final demo is a
binary executable program. The file command determines the platform that the program was
compiled for and what types of libraries it requires. This is an especially handy feature if you
have a binary executable program from an unknown source.
Viewing the whole file
If you have a large text file on your hands, you may want to be able to see what‘s inside of it.
There are three different commands in Linux that can help you out here.
The cat command The cat command is a handy tool for displaying all of the data inside a text file:
# cat test1
hello
This is a test file.
That we‘ll use to test the cat command.
#
Nothing too exciting, just the contents of the text file. There are a few parameters you can use
with the cat command, though, that can help you out.
The -n parameter numbers all of the lines for us:
# cat -n test1
1 hello
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2 This is a test file.
3 That we‘ll use to test the cat command.
#
That feature will come in handy when you‘re examining scripts. If you just want to number the
lines that have text in them, the -b parameter is for you:
# cat -b test1
1 hello
2 This is a test file.
3 That we‘ll use to test the cat command.
Fancy! If you need to compress multiple blank lines into a single blank line, use the -s parameter:
# cat -s test1
hello
This is a test file.
That we‘ll use to test the cat command.
#
Finally, if you don‘t want tab characters to appear, use the -T parameter:
# cat -T test1
hello
This is a test file.
That we‘ll use to^Itest the cat command.
#
The -T parameter replaces any tabs in the text with the ^I character combination.
For large files, the cat command can be somewhat annoying. The text in the file will just quickly
scroll off of the monitor without stopping. Fortunately, there‘s a simple way to solve this
problem.
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The more command
The main drawback of the cat command is that you can‘t control what‘s happening once you
start it. To solve that problem, developers created the more command. The more command
displays a text file, but stops after it displays each page of data.
The more Command Options
The less command
Although from its name it sounds like it shouldn‘t be as advanced as the more command, the less
command is actually a play on words and is an advanced version of the more command (the less
command uses the phrase ‗‗less is more‘‘). It provides several very handy features for scrolling
both forward and backward through a text file, as well as some pretty advanced searching
capabilities.
The less command also has the feature of being able to display the contents of a file before it
finishes reading the entire file. This is a serious drawback for both the cat and more commands
Figure 27 more commands parameter
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when using extremely large files. The less command operates much the same as the more
command, displaying one screen of text from a file at a time.
Notice that the less command provides additional information in its prompt, showing the total
number of lines in the file, and the range of lines currently displayed. The less command
supports the same command set as the more command, plus lots more options. To see all of the
options available, look at the man pages for the less command. One set of features is that the less
command recognizes the up and down arrow keys, as well as the page up and page down keys
(assuming that you‘re using a properly defined terminal). This gives you full control when
viewing a file.
Viewing parts of a file
Often the data you want to view is located either right at the top or buried at the bottom of a text
file. If the information is at the top of a large file, you still need to wait for the cat or more
commands to load the entire file before you can view it. If the information is located at the
bottom of a file (such as a log file), you need to wade through thousands of lines of text just to
get to the last few entries. Fortunately, Linux has specialized commands to solve both of these
problems.
The tail command
The tail command displays the last group of lines in a file. By default, it‘ll show the last 10 lines
in the file, but you can change that with command line parameters. The -f parameter is a pretty
cool feature of the tail command. It allows you to peek inside a file as it‘s being used by other
processes. The tail command stays active and continues to display new lines as they appear in the
text file. This is a great way to monitor the system log file in real-time mode.
The head command
While not as exotic as the tail command, the head command does what you‘d expect, it displays
the first group of lines at the start of a file. By default, it‘ll display the first 10 lines of text.
64
Similar to the tail command, it supports the -c, and -n parameters so that you can alter what‘s
displayed.
The tail Command Line Parameters
VI EDITOR The VI editor is a screen-based editor used by many Unix users. The VI editor has powerful
features to aid programmers. The VI editor lets a user create new files or edit existing files. The
command to start the VI editor is vi, followed by the filename. For demo, to edit a file called
XYZ, you would type vi XYZ and then return. You can start VI without a filename, but when you
want to save your work, you will have to tell VI which filename to save it into later. At the
bottom of your screen, the filename should be shown, if you specified an existing file, and the
size of the file will be shown as well, like this:
"filename" 21 lines, 385 characters
If the file you specified does not exist, then it will tell you that it is a new file, like this:
"newfile" [New file]
If you started VI without a filename, the bottom line of the screen will just be blank when VI
starts. If the screen does not show you these expected results, your terminal type may be set
wrong. Just type :q and return to get out of VI.
Figure 28 tail command parameter
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Getting Out of VI
Now that you know how to get into VI, it would be a good idea to know how to get out of it. The
VI editor has two modes and in order to get out of VI, you have to be in command mode. Hit the
key labelled "Escape" or "Esc" to get into command mode.
The command to quit out of VI is :q. Once in command mode, type colon, and 'q', followed by
return. If your file has been modified in any way, the editor will warn you of this, and not let you
quit. To ignore this message, the command to quit out of VI without saving is :q!. This lets you
exit VI without saving any of the changes.
you would want to save the changes you have made. The command to save the contents of the
editor is :w. You can combine the above command with the quit command, or :wq. You can
specify a different file name to save to by specifying the name after the :w. For demo, if you
wanted to save the file you were working as another filename called filename2, you would type:
w filename2 and return.
Another way to save your changes and exit out of VI is the ZZ command. When in command
mode, type ZZ and it will do the equivalent of :wq. If any changes were made to the file, it will
be saved. This is the easiest way to leave the editor, with only two keystrokes.
The Two Modes of VI
The first thing most users learn about the VI editor is that it has two modes: command and insert.
The command mode allows the entry of commands to manipulate text. These commands are
usually one or two characters long, and can be entered with few keystrokes. The insert mode puts
anything typed on the keyboard into the current file.
VI starts out in command mode. There are several commands that put the VI editor into insert
mode. The most commonly used commands to get into insert mode are a and i.
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Some Simple VI Commands
Here is a simple set of commands to get a beginning VI user started. There are many other
convenient commands, which will be discussed in later sections.
a
enter insert mode, the characters typed in will be inserted after the current cursor position. If you
specify a count, all the text that had been inserted will be repeated that many times.
h
move the cursor to the left one character position.
i
enter insert mode, the characters typed in will be inserted before the current cursor position. If
you specify a count, all the text that had been inserted will be repeated that many times.
j
move the cursor down one line.
k
move the cursor up one line.
l
move the cursor to the right one character position.
r
replace one character under the cursor. Specify count to replace a number of characters
u
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undo the last change to the file. Typing u again will re-do the change.
x
delete character under the cursor. Count specifies how many characters to delete. The characters
will be deleted after the cursor.
Cutting and Yanking
The command commonly used command for cutting is d. This command deletes text from the
file. The command is preceded by an optional count and followed by a movement specification.
If you double the command by typing dd, it deletes the current line. Here are some combinations
of these:
d^
deletes from current cursor position to the beginning of the line.
d#
deletes from current cursor position to the end of the line.
dw
deletes from current cursor position to the end of the word.
3dd
deletes three lines from current cursor position downwards.
There is also the y command which operates similarly to the d command which take text from
the file without deleting the text.
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Pasting
The commands to paste are p and P. The only differ in the position relative to the cursor where
they paste. p pastes the specified or general buffer after the cursor position, while P pastes the
specified or general buffer before the cursor position. Specifying count before the paste
command pastes text the specified number of times.
Word and Character Searching
The VI editor has two kinds of searches: string and character. For a string search, the / and ?
commands are used. When you start these commands, the command just typed will be shown on
the bottom line, where you type the particular string to look for. These two commands differ only
in the direction where the search takes place. The / command searches forwards (downwards) in
the file, while the ? command
searches backwards (upwards) in the file. The n and N commands repeat the previous search
command in the same or opposite direction, respectively. Some characters have special meanings
to VI, so they must be preceded by a backslash (\) to be included as part of the search expression.
Special characters:
^
Beginning of the line. (At the beginning of a search expression.)
.
Matches a single character.
*
Matches zero or more of the previous character.
#
End of the line (At the end of the search expression.)
[
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Starts a set of matching, or non-matching expressions... For demo: /f[iae]t matches either of
these: fit fat fet In this form, it matches anything except these: /a[^bcd] will not match any of
these, but anything with an a and another letter: ab ac ad
<
Put in an expression escaped with the backslash to find the ending or beginning of a word. For
demo: /\<the\> should find only word the, but not words like these: there and other.
>
See the '<' character description above.
Abbreviations and Mapping Keys to Other Keys
One EX editor command that is useful in the VI editor is the abbreviate command. This lets you
set up abbreviations for specific strings. The command looks like this: :ab string thing to
substitute for. For demo, if you had to type the name, "Humuhumunukunukuapua`a" but you
didn't want to type the whole name, you could use an abbreviation for it. For this demo, the
command is entered like this:
:ab 9u Humuhumunukunukuapua`a
Now, whenever you type 9u as a separate word, VI will type in the entire word(s) specified. If
you typed in 9university, it will not substitute the word. To remove a previously defined
abbreviation, the command is unabbreviate. To remove the previous demo, the command would
be ":una 9u" To get your listing of abbreviations, simply just type :ab without any definitions.
Another EX editor command that is useful for customization is the mapping command. There are
two kinds of mapping commands. One for command mode, and the other for insert mode. These
two commands are :map and :map! respectively. The mapping works similarly to the
abbreviation, and you give it a key sequence and give it another key sequence to substitute it
with. (The substituted key sequences are usually VI commands.)
Inserting New Text
70
A
Append at the end of the current line.
I
Insert from the beginning of a line.
O
(letter oh) Enter insert mode in a new line above the current cursor position.
a
Enter insert mode, the characters typed in will be inserted after the current cursor
position. A count inserts all the text that had been inserted that many times.
i
Enter insert mode, the characters typed in will be inserted before the current cursor
position. A count inserts all the text that had been inserted that many times.
o
Enter insert mode in a new line below the current cursor position.
Linux Files and File Permission
There are three types of access:
1. read
2. write
3. execute
Each file belongs to a specific user and group. Access to the files is controlled by user, group,
and what is called other. The term, other, is used to refer to someone who is not the user (owner)
of the file, nor is the person a member of the group the file belongs to. When talking about
setting permissions for "other" users to use, it is commonly referred to as setting the world
71
execute, read, or write bit since anyone in the world will be able to perform the operation if the
permission is set in the other category.
File names and permission characters
File names can be up to 256 characters long with "-", "_", and "." characters along with letters
andnumbers.
When a long file listing is done, there are 10 characters that are shown on the left that indicate
type and permissions of the file. File permissions are shown according to the following syntax
demo:drwxrwxrwx
There are a total of 10 characters in this demo, as in all Linux files. The first character indicates
the type of file, and the next three indicate read, write, and execute permission for each of the
three user types, user, group and other. Since there are three types of permission for three users,
there are a total of nine permission bits. The table below shows the syntax:
1 2 3 4 5 6 7 8 9 10
File User Permissions Group Permissions Other Permissions
Type Read Write Execute Read Write Execute Read Write Execute
D R w x R w x r w x
Character 1 is the type of file: - is ordinary, d is directory, l is link.
Characters 2-4 show owner permissions. Character 2 indicates read permission, character
3 indicates write permission, and character 4 indicates execute permission.
Characters 5-7 show group permissions. Character 5=read, 6=write, 7=execute
Characters 8-10 show permissions for all other users. Character 8=read, 9=write,
10=execute
72
There are 5 possible characters in the permission fields. They are:
r = read - This is only found in the read field.
w = write - This is only found in the write field.
x = execute - This is only found in the execute field.
s = setuid - This is only found in the execute field.
If there is a "-" in a particular location, there is no permission. This may be found in any
field whether read, write, or execute field.
Demos
Type "ls -l" and a listing like the following is displayed:
total 10
drwxrwxrwx 4 FIRST FIRST 122 Dec 15 11:02 Projects
-rw-rw-rw- 1 FIRST FIRST 1873 Aug 25 03:34 Test
-rw-rw-rw- 1 FIRST FIRST 1234 Sep 12 11:13 datafile
Which means the following:
Type and # of Files's File's Size in Date of last Filename
Permission field Links Owner Group Bytes modification
| | | | | | |
drwxrwxrwx 4 FIRST FIRST 127 Dec 19 12:02 Project2
The fields are as follows:
1. Type field: The first character in the field indicates a file type of one of the following:
o d = directory
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o l = symbolic link
o s = socket
o p = named pipe
o - = regular file
o c= character (unbuffered) device file special
o b=block (buffered) device file special
2. Permissions are explained above.
3. Links: The number of directory entries that refer to the file. In our demo, there are four.
4. The file's owner in our demo is FIRST.
5. The group the file belongs to. In our demo, the group is FIRST.
6. The size of the file in bytes
7. The last modification date. If the file is recent, the date and time is shown. If the file is
not in the current year, the year is shown rather than time.
8. The name of the file.
Set User Identification Attribute
The file permissions bits include an execute permission bit for file owner, group and other. When
the execute bit for the owner is set to "s" the set user ID bit is set. This causes any persons or
processes that run the file to have access to system resources as though they are the owner of the
file. When the execute bit for the group is set to "s", the set group ID bit is set and the user
running the program is given access based on access permission for the group the file belongs to.
The following command:
chmod +s myfile
sets the user ID bit on the file "myfile". The command:
chmod g+s myfile
sets the group ID bit on the file "myfile".
74
The listing below shows a listing of two files that have the group or user ID bit set.
-rws--x--x 1 root root 14024 Sep 9 1999 chfn
-rwxr-sr-x 1 root mail 12072 Aug 16 1999 lockfile
The files chfn and lockfile are located in the directory "/usr/bin". The "s" takes the place of the
normal location of the execute bit in the file listings above. This special permission mode has no
meaning unless the file has execute permission set for either the group or other as well. This
means that in the case of the lockfile, if the other users (world execute) bit is not set with
permission to execute, then the user ID bit set would be meaningless since only that same group
could run the program anyhow. In both files, everyone can execute the binary. The first program,
when run is executed as though the program is the root user. The second program is run as
though the group "mail" is the user's group.
For system security reasons it is not a good idea to set many program's set user or group ID bits
any more than necessary, since this can allow an unauthorized user privileges in sensitive system
areas. If the program has a flaw that allows the user to break out of the intended use of the
program, then the system can be compromised.
Directory Permissions
There are two special bits in the permissions field of directories. They are:
s - Set group ID
t - Save text attribute (sticky bit) - The user may delete or modify only those files in the
directory that they own or have write permission for.
Save text attribute
The /tmp directory is typically world-writable and looks like this in a listing:
drwxrwxrwt 13 root root 4096 Apr 15 08:05 tmp
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Everyone can read, write, and access the directory. The "t'' indicates that only the user (and root,
of course) that created a file in this directory can delete that file.
To set the sticky bit in a directory, do the following:
chmod +t data
This option should be used carefully. A possible alternative to this is
1. Create a directory in the user's home directory to which he or she can write temporary
files.
2. Set the TMPDIR environment variable using each user's login script.
3. Programs using the tempnam(3) function will look for the TMPDIR variable and use it,
instead of writing to the /tmp directory.
Directory Set Group ID
If the setgid bit on a directory entry is set, files in that directory will have the group ownership as
the directory, instead of than the group of the user that created the file.
This attribute is helpful when several users need access to certain files. If the users work in a
directory with the setgid attribute set then any files created in the directory by any of the users
will have the permission of the group. For demo, the administrator can create a group called
ABC and add the users Ram and Jain to the group ABC. The directory ABCdir can be created
with the set GID bit set and Ram and Jain although in different primary groups can work in the
directory and have full access to all files in that directory, but still not be able to access files in
each other's primary group.
The following command will set the GID bit on a directory:
chmod g+s ABCdir
The directory listing of the directory "ABCdir":
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drwxrwsr-x 2 kathy ABC 1674 Sep 17 1999 ABCdir
The "s'' in place of the execute bit in the group permissions causes all files written to the
directory "ABCdir" to belong to the group "ABC" .
Demos
Below are demos of making changes to permissions:
chmod u+x myfile Gives the user execute permission on myfile.
chmod +x myfile Gives everyone execute permission on myfile.
chmod ugo+x myfile Same as the above command, but specifically specifies user, group and
other.
chmod 400 myfile Gives the user read permission, and removes all other permission. These
permissions are specified in octal, the first char is for the user, second for
the group and the third is for other. The high bit (4) is for read access, the
middle bit (2) os for write access, and the low bit (1) is for execute access.
chmod 764 myfile Gives user full access, group read and write access, and other read access.
chmod 751 myfile Gives user full access, group read and execute permission, and other,
execute permission.
chmod +s myfile Set the setuid bit.
chmod go=rx myfile Remove read and execute permissions for the group and other.
Below are demos of making changes to owner and group:
chown Garry test1 Changes the owner of the file test1 to the user Garry.
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chgrp Garry test1 Changes the file test1 to belong to the group "Garry".
Note: Linux files were displayed with a default tab value of 8 in older Linux versions. That
means that file names longer than 8 may not be displayed fully if you are using an old Linux
distribution. There is an option associated with the ls command that solves this problem. It is "-
T". Ex: "ls al -T 30" to make the tab length 30.
Umask Settings
The umask command is used to set and determine the default file creation permissions on the
system. It is the octal complement of the desired file mode for the specific file type. Default
permissions are:
777 - Executable files
666 - Text files
These defaults are set allowing all users to execute an executable file and not to execute a text
file. The defaults allow all users can read and write the file.
The permission for the creation of new executable files is calculated by subtracting the umask
value from the default permission value for the file type being created. An demo for a text file is
shown below with a umask value of 022:
666 Default Permission for text file
-022 Minus the umask value
-----
644 Allowed Permissions
Therefore the umask value is an expression of the permissions the user, group and world will not
have as a default with regard to reading, writing, or executing the file. The umask value here
means the group the file belongs to and users other than the owner will not be able to write to the
file. In this case, when a new text file is created it will have a file permission value of 644, which
means the owner can read and write the file, but members of the group the file belongs to, and all
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others can only read the file. A long directory listing of a file with these permissions set is shown
below.
-rw-r--r-- 1 root FIRST 14233 Apr 24 10:32 textfile.txt
A demo command to set the umask is:
umask 022
The most common umask setting is 022. The /etc/profile script is where the umask command is
usually set for all users.
Red Hat Linux has a user and group ID creation scheme where there is a group for each user and
only that user belongs to that group. If you use this scheme consistently you only need to use 002
for your umask value with normal users.
Hard Link and Soft Link
Hard Link is a mirror copy of the original file. Hard links share the same inode. Any changes
made to the original or Hard linked file will reflect the other. Even if you delete any one of the
files, nothing will happen to the other. Hard links can't cross file systems.
Soft Link is a symbolic link to the original file. Soft Links will have a different Inode value. A
soft link points to the original file. If you delete the original file, the soft link fails. If you delete
the soft link, nothing will happen. Hard links can cross file systems.
Lets learn the difference with an demo
Demo: Create a file " original-file.txt "
:~/test# echo "Learning about Hard and Soft Link" > original-file.txt
:~/test# cat original-file.txt
Learning about Hard and Soft Link
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Now lets create a Hard Link
Demo: HardLink-file.txt
:~/test# ln original-file.txt HardLink-file.txt
:~/test# ls –il
total 8
840388 -rw-r--r-- 2 FIRST FIRST 33 2009-05-18 09:16 HardLink-file.txt
840388 -rw-r--r-- 2 FIRST FIRST 33 2009-05-18 09:16 original-file.txt
Now lets create a Soft Link
Demo: SoftLink-file.txt
:~/test# ln -s original-file.txt SoftLink-file.txt
:~/test# ls –il
total 8
840388 -rw-r--r-- 2 FIRST FIRST 33 2009-05-18 09:16 HardLink-file.txt
840388 -rw-r--r-- 2 FIRST FIRST 33 2009-05-18 09:16 original-file.txt
840186 lrwxrwxrwx 1 FIRST FIRST 17 2009-05-18 09:23 SoftLink-file.txt -> original-file.txt
From the above ls -il result, you find the same inode for " HardLink-file.txt " and " original-
file.txt ".
Inode value is different for the soft link " SoftLink-file.txt ".
Now lets try editing the original file:
Demo:
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:~/test# cat >> original-file.txt
Editing this file to learn more!!
:~/test# cat original-file.txt
Learning about Hard and Soft Link
Editing this file to learn more!!
:~/test# cat HardLink-file.txt
Learning about Hard and Soft Link
Editing this file to learn more!!
:~/test# cat SoftLink-file.txt
Learning about Hard and Soft Link
Editing this file to learn more!!
Now lets try changing the permissions:
Demo:
:~/test# chmod 700 original-file.txt
:~/test# ls -il
total 8
840388 -rwx------ 2 FIRST FIRST 67 2009-05-18 09:34 HardLink-file.txt
840388 -rwx------ 2 FIRST FIRST 67 2009-05-18 09:34 original-file.txt
840186 lrwxrwxrwx 1 FIRST FIRST 17 2009-05-18 09:23 SoftLink-file.txt -> original-file.txt
From the above demo its clear that changing the permission of " original-file.txt " will update the
permission set of " HardLink-file.txt ".
The soft link remains unchanged.
Now lets try deleting the original file.
Demo:
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:~/test# rm original-file.txt
:~/test# ls -il
total 4
840388 -rwx------ 1 FIRST FIRST 67 2009-05-18 09:34 HardLink-file.txt
840186 lrwxrwxrwx 1 FIRST FIRST 17 2009-05-18 09:23 SoftLink-file.txt -> original-file.txt
:~/test# cat HardLink-file.txt
Learning about Hard and Soft Link
Editing this file to learn more!!
:~/test# cat SoftLink-file.txt
cat: SoftLink-file.txt: No such file or directory
So removing the original file will affect the Soft link. The Soft link fails. Hard link is unaffected.
Users and Groups
Who Is The Super User?
The super user with unrestricted access to all system resources and files is the user named "root".
You will need to log in as user root to add new users to your Linux box.
How To Add Users
When invoked without the -D option, the useradd command creates a new user account using
the values specified on the command line and the default values from the system. The new user
account will be entered into the system files as needed, the home directory will be created, and
initial files copied, depending on the command line options. The version provided with Red Hat
Linux will create a group for each user added to the system, unless the -n option is given. The
options which apply to the useradd command are:
useradd [-c comment] [-d home_dir] [-e expire_date] [-f inactive_time] [-g initial_group] [-G
group[,...]] [-m [-k skeleton_dir]] [-p passwd] [-s shell] [-u uid [ -o]] login
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useradd -D [-g default_group] [-b default_home] [-f default_inactive] [-e default_expire_date] [-s
default_shell]
-c comment The new user's password file comment field.
-d home_dir The new user will be created using home_dir as the value for the user's login
directory. The default is to append the login name to default_home and use
that as the login directory name.
-e expire_date The date on which the user account will be disabled. The date is specified in
the format YYYY-MM-DD.
-f inactive_time The number of days after a password expires until the account is permanently
disabled. A value of 0 disables the account as soon as the password has
expired, and a value of -1 disables the feature. The default value is -1.
-g initial_group The group name or number of the user's initial login group. The group name
must exist. A group number must refer to an already existing group. The
default group number is 1.
-G group,[,...] A list of supplementary groups which the user is also a member of. Each group
is separated from the next by a comma, with no intervening whitespace. The
groups are subject to the same restrictions as the group given with the -g
option. The default is for the user to belong only to the initial group.
-m The user's home directory will be created if it does not exist. The files
contained in skeleton_dir will be copied to the home directory if the -k option
is used, otherwise the files contained in /etc/skel will be used instead. Any
directories contained in skeleton_dir or /etc/skel will be created in the user's
home directory as well. The -k option is only valid in conjunction with the -m
option. The default is to not create the directory and to not copy any files.
-p passwd The encrypted password, as returned by crypt. The default is to disable the
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account.
-s shell The name of the user's login shell. The default is to leave this field blank,
which causes the system to select the default login shell.
-u uid The numerical value of the user's ID. This value must be unique, unless the -o
option is used. The value must be non-negative. The default is to use the
smallest ID value greater than 99 and greater than every other user. Values
between 0 and 99 are typically reserved for system accounts.
-b default_home The initial path prefix for a new user's home directory. The user's name will be
affixed to the end of default_home to create the new directory name if the -d
option is not used when creating a new account.
-e
default_expire_date
The date on which the user account is disabled.
-f default_inactive The number of days after a password has expired before the account will be
disabled.
-g default_group The group name or ID for a new user's initial group. The named group must
exist, and a numerical group ID must have an existing entry.
-s default_shell The name of the new user's login shell. The named program will be used for
all future new user accounts.
Changing/Setting password for USER
passwd - entering just passwd would allow you to change the password. After entering passwd
you will receive the following three prompts:
Current Password:
New Password:
Confirm New Password:
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Each of these prompts must be entered and entered correctly for the password to be successfully
changed.
passwd newperson
If you have just setup an account using the useradd command you would then type a command
similar to the above command to set the password for the "newperson" account. Only root users
have the ability to set passwords for other accounts.
Adding user to a group
You can use the useradd or usermod commands to add a user to a group. The useradd command
creates a new user or update default new user information. The usermod command modifies a
user account i.e. it is useful to add user to existing group. There are two types of group. First is
primary user group and other is secondary group. All user account related information is stored
in /etc/passwd, /etc/shadow and /etc/group files to store user information.
useradd Demo - Add A New User To Secondary Group
You need to the useradd command to add new users to existing group (or create a new group and
then add user). If group does not exist, create it. The syntax is as follows:
useradd -G {group-name} username
In this demo, create a new user called FIRST and add it to group called DEMO. First login as a
root user (make sure group developers exists), enter:
If you do not see any group then you need to add group DEMO using groupadd command:
# groupadd DEMO
Next, add a user called FIRST to group DEMO:
# useradd -G DEMO FIRST
Setup password for user FIRST:
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# passwd FIRST
Ensure that user added properly to group DEMO:
# id FIRST
Output:
uid=1122(FIRST) gid=1125(FIRST) groups=1125(FIRST),1124(DEMO)
Please note that capital G (-G) option add user to a list of supplementary groups. Each group is
separated from the next by a comma, with no intervening whitespace. For demo, add user A to
groups admins, ftp, www, and DEMO, enter:
# useradd -G admins,ftp,www,DEMO A
useradd demo - Add a new user to primary group
To add a user AA to group DEMO use following command:
# useradd -g DEMO AA
# id AA
Sample outputs:
uid=1123(AA) gid=1124(DEMO) groups=1124(DEMO)
Please note that small -g option add user to initial login group (primary group). The group name
must exist. A group number must refer to an already existing group.
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usermod demo - Add a existing user to existing group
Add existing user tony to ftp supplementary/secondary group with usermod command using -a
option ~ i.e. add the user to the supplemental group(s). Use only with -G option :
# usermod -a -G ftp AA
Change existing user AA primary group to www:
# usermod -g www AA
/etc/passwd file
Passwd file is a text file, that contains a list of the system's accounts, giving for each account
some useful information like user ID, group ID, home directory, shell, etc. Often, it also contains
the encrypted passwords for each account. It should have general read permission (many utilities,
like ls use it to map user IDs to user names), but write access only for the superuser.
Password file format
account:password:UID:GID:GECOS:directory:shell
The /etc/passwd file consists of user records, one to a line. Each record contains multiple fields,
separated by colons (:). The fields are:
username
encrypted password (or x if shadow passwords are in use)
UID
default GID
real name (also known as the GECOS field)
home directory
default shell
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/etc/shadow file
shadow file contains the encrypted password information for user's accounts and optional the
password aging information.
Shadow file format
smithj:Ep6mckrOLChF.:10063:0:99999:7:::
if shadow passwords are being used, the /etc/shadow file contains users' encrypted passwords
and other information about the passwords. Its fields are colon-separated as for /etc/passwd, and
are as follows:
username
encrypted password
Days since Jan 1, 1970 that password was last changed
Days before password may be changed
Days after which password must be changed
Days before password is to expire that user is warned
Days after password expires that account is disabled
Days since Jan 1, 1970 that account is disabled
A reserved field
The password expiry related fields are modified by the change program.
/etc/group file
There is one entry per line, and each line has the format:
Group file format
group_name:passwd:GID:user_list
The /etc/group file consists of group records, one to a line. Each record contains multiple fields,
separated by colons (:). The fields are:
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group name
encrypted group password (or x if shadow passwords are in use)
GID
group members' usernames, comma-separated
USING THE BASH SHELL
SHELL VARIABLES
demo, to print out the The shell has built in variables that control the behavior of the shell. To
see a list of the names and values of all currently defined shell variables, use the set command
without any paramaters.
To reference a shell variable in a command, simply place the # in front of the variable name.
This will cause the shell to replace the variable reference with the current value of that shell
variable. This can save a lot of typing. For current value for the PATH variable:
# echo #PATH
# /bin:/usr/bin:/home/fred/bin
The two variables you will be most concerned with are PATH and PS1.
PATH shell variable
The value of the PATH shell variable is a colon-separated list of directories in which the shell
should automatically look for executable files when it is trying to run a command. For demo,
when you run the command
# netscape
the shell will look through all of the directories in the PATH looking for an executable file
named "netscape" that it can run.
If you want the shell to always look in the cwd for executable files, you need to put the cwd on
the PATH. If you don't have the cwd in your PATH variable, then in order to execute a file in the
cwd you have to type ./filename. If the cwd is in the PATH variable you can just type the
filename.
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# filename
# command filename not found
# ./filename
//program runs
To set the value of a shell variable, simply type the name of the variable, an equal sign, followed
by its new value. So, to add the cwd to the PATH variable, use the following command:
# PATH=#PATH:.
This simply takes the current value of PATH, appends :. to it, and reassigns the new value to
PATH. After executing this command, programs in the cwd can be executed by simply typing
their names:
# filename
// program runs
PS1 shell variable
PS1 is the variable controlling the prompt. You can change the prompt to be anything you want.
For demo, the following command would change the prompt to be the string "DAN IS THE
MAN #".
# PS1="DAN IS THE MAN \#"
The prompt format string stored in PS1 may contain special variables that are automatically
replaced with useful information. Some of these are:
\w current working directory
\W last element of current working directory
\u user name
\h host name
\d current date
\t current time
The most common format for PS1 is
PS1="[\u@\h \w]\#"
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FILE NAME EXPANSION
Many Linux commands accept multiple file or directory names as arguments.
There are many file name expansion operators, some of which are listed below:
Operator Meaning Demo
* match any string of zero or more characters # cp *.txt backup
? match any single character # cp ?.txt backup
[abc…] match any of the enclosed characters # cp [abc].txt backup
[!abc…] match anything but the enclosed characters # cp [!abc].txt backup
[a-z] match any character in the range # cp [a-c].txt backup
[!a-z] match any character not in the range # cp [!a-c].txt backup
{str1,str2,…} match any of the enclosed strings # cp {dog,cat,duck}.txt backup
~ substitute user‘s home directory # cp ~/foodle/*.txt backup
~name substitute some other user‘s home directory # cp ~ram/foodle/*.txt backup
QUOTING
File name expansion is very convenient, but what if we need to pass arguments that contain file
expansion operators to a program? For demo, what if you wanted to pass an asterisk as an
argument to a command?
# print the message "* is an asterisk"
# echo * is an asterisk
Normally, the shell will perform file name expansion on all arguments that look like patterns,
even if we don't want it to. In the demo above, before calling echo the shell would replace the
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asterisk with the names of all files/directories in the current directory, which is not the intended
effect.
The solution to this problem is quoting. Quoting is a mechanism for turning off file name
expansion, causing the shell to pass arguments through unmodified, even if they contain file
name expansion operators. File name expansion can be turned off for a single character by
preceding it with \, like this:
# print the message "* is an asterisk"
# echo \* is an asterisk
Any character that is preceded by \ is taken literally and passed through untouched (i.e.,
unexpanded).
You can also quote entire strings by enclosing them in single-quotes, like this:
# print the message "* is an asterisk"
# echo '* is an asterisk'
All characters between the single-quotes are taken literally.
You can use double-quotes instead of single-quotes, but the meaning is slightly different. The
meaning of single-quotes is: "Shell, don't change the string between single- quotes, not matter
what!". The meaning of double-quotes is: "Shell, don't change the string between double-quotes,
unless it contains a shell variable reference. In that case, it's OK to replace the variable with its
value.". The following demo demonstrates the difference between using single and double
quotes.
# echo ‘Is your name #USERNAME ?‘
Is your name #USERNAME ?
# echo ―Is your name #USERNAME ?‖
Is your name user ?
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In addition to turning off file name expansion, quoting is also useful for handling file/directory
names that contain spaces. For demo, suppose that you have a directory named "my foodle files".
Using the cd command to change to this directory would look like this:
# cd my foodle files
The problem with this is that "my foodle files" looks like three arguments instead of one, and the
cd command will not work. The solution to this problem is to quote the directory name:
# cd 'my foodle files'
FILTER COMMANDS/TEXT PROCESSING TOOLS: FINDING AND PROCESSING FILES:
wc
The wc command can be used to count the number of lines, words, and bytes in text files.
# wc homework.txt
4 7 280 homework.txt
The output above means that the file homework.txt contains 4 lines, 7 words, and 280 bytes.
By default, wc prints out all three pieces of information The –l, -w, and –c options can be used to
tell wc to display only one or two of these pieces of information
# wc –l homework.txt
4 homework.txt
wc can also compute totals for multiple files
# wc –l *.txt
grep
The grep command can be used to display all lines in a text file that match a particular pattern
(i.e, regular expression). The user provides one or more file names, along with the pattern they
are searching for. Grep will look through all of the specified files and print out all lines that
match the specified pattern. Grep is useful for searching source code or other text files for
particular strings. For demo, the following command searches all of the C++ source files in a
directory for the string "main":
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# where is the main function?
# grep main *.cpp
By default, grep performs case-sensitive searches (i.e., case matters). The –i option can be used
to perform a case-insensitive search:
# where is the main function?
# grep –i mAiN *.cpp
So far we have used only literal strings as our search patterns. The search pattern can also be a
regular expression. For demo, the following command searches a file for lines containing the text
BYU, with any number of spaces between the letters:
# grep ‘B *Y *U‘ file.txt
Grep supports a variety of pattern operators, some of which are listed below:
Operator Meaning
. match any character
* match zero or more of the preceding
(Note that the meaning of * here is different than with file name expansion)
^ match beginning of line
# match end of line
\ escape the following character
[abc] match one in the list
[a-z] match one in the range
[^abc] match one not in the list
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[^a-z] match one not in the range
\{n,m\} match between n and m of the preceding
\{n\} match exactly n of the preceding
\{n,\} match n or more of the preceding
Some of the grep pattern operators are also shell file expansion operators. For this reason, when
using grep it is frequently necessary to quote the pattern argument so that the shell does not
perform file name expansion on it.
find
The find command is used to recursively search a directory structure for files/directories that
match specified criteria. Possible search criteria include name, modification date, file type, and
many other possibilities. Once a matching file/directory has been found, it can be operated upon
by copying it, deleting it, running wc or grep on it, etc. Typical uses of find include the
following:
Recursively finding files with particular names
Doing recursive search & replace on files
Recursively deleting selected files
Find is a complex command with many options. Here we cover only a few of the most useful
options. The find command-line has the following form:
# find pathnames conditions operations
pathnames is a list of directories to search
conditions are the search criteria
operations are the operations to be performed on each matching file/directory
Search Criteria
The following demos show how to provide search criteria for several common types of searches.
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Search by Name
# find pathnames -name pattern operations
# print the paths of all files/dirs named core in the CWD
# find . -name core -print
# print the paths of all .cpp files in foodle directory
# find ~/foodle -name '*.cpp' -print
Search by Type
# find pathnames -type c operations
# print the paths of all regular files in the CWD
# find . -type f -print
# print the paths of all directories in foodle directory
# find ~/foodle -type d -print
Search by Modification Time
# find pathnames -mtime (+n|-n|n) operations
# print the paths of all files/dirs in the CWD that
# were last modified more than 7 days ago
# find . -mtime +7 -print
# print the paths of all files/dirs in the foodle
# directory that were last modified less than 7 days ago
# find ~/foodle -mtime -7 -print
Operations on Matching Files
The next demos show how to perform common operations on matching files/directories.
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Print the full path of each matching file/directory
# find pathnames conditions -print
# print the paths of all .cpp files in foodle directory
# find ~/foodle -name '*.cpp' -print
Execute a command on each matching file/directory
# find pathnames conditions -exec command {} \;
# search all .cpp files in the CWD for "main"
# find . -name '*.cpp' -print -exec grep main {} \;
# delete all .o files in the foodle directory
# find ~/foodle -name '*.o' -print -exec rm {} \;
In this case, {} is replaced by the name of the matching file/directory, and \; is used to
mark the end of the command.
CUT
The cut command takes a vertical slice of a file, printing only the specified columns or fields.
Like the sort command, thecut command defines a field as a word set off by blanks, unless you
specify your own delimiter. It's easiest to think of a column as just the nth character on each line.
In other words, "column 5" consists of the fifth character of each line.
Consider a slight variation on the company.data file we've been playing with in this section:
406378:Sales:Itorre:Jan
031762:Marketing:Nasium:Jim
636496:Research:Ancholie:Mel
396082:Sales:Jucacion:Ed
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If you want to print just columns 1 to 6 of each line (the employee serial numbers), use the -c1-
6 flag, as in this command:
cut -c1-6 company.data
406378
031762
636496
396082
If you want to print just columns 4 and 8 of each line (the first letter of the department and the
fourth digit of the serial number), use the -c4,8 flag, as in this command:
cut -c4,8 company.data
3S
7M
4R
0S
And since this file obviously has fields delimited by colons, we can pick out just the last names
by specifying the -d: and -f3 flags, like this:
cut -d: -f3 company.data
Itorre
Nasium
Ancholie
Jucacion
It's often the case that you want to use a space as the delimiter. To do so, you must put the
delimiter in single quotes, like this: -d' '
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Also, when you want to cut from a starting point to the end of the line, just leave off the final
field number, as shown in the demo below.
Let's say this is your test.txt file:
abc def ghi jkl
mno pqr stu vwx
yz1 234 567 890
To cut only columns 2-END, do this: cut -d' ' -f2- test.txt
And the results are:
def ghi jkl
pqr stu vwx
234 567 890
Here is a summary of the most common flags for the cut command:
-c [n | n,m | n-m] Specify a single column, multiple columns (separated by a comma), or range of
columns (separated by a dash).
-f [n | n,m | n-m] Specify a single field, multiple fields (separated by a comma), or range of fields
(separated by a dash).
-dc Specify the field delimiter.
-s Suppress (don't print) lines not containing the delimiter.
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Paste
paste command is used to paste the content from one file to another file. It is also used to set
column format for each line.
The Syntax is
paste [options]
OPTIONS:
-s Paste one file at a time instead of in parallel.
-d Reuse characters from LIST instead of TABs .
SED (Stream Editor)
You can use the sed command to change all occurrences of one string to another within a file,
just like the search-and-replace feature of your word processor. The sed command can also
delete a range of lines from a file. Since sed is a stream editor, it takes the file given as input, and
sends the output to the screen, unless you redirect output to a file. In other words, sed does not
change the input file.
The general forms of the sed command are as follows:
Substitution sed 's/<oldstring>/<newstri ng>/g' <file>
Deletion sed '<start>,<end>d' < file>
Let's start with a substitution demo. If you want to change all occurrences of raga to hama in
the poem.txt file in the grepdemo, enter this:
sed 's/raga/hama/g' poem.txt
Mary had a little hama
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Mary fried a lot of spam
Jack ate a Spam sandwich
Jill had a hama spamwich
In the quoted string, the "s" means substitute, and the "g" means make a global change. You can
also leave off the "g" (to change only the first occurrence on each line) or specify a number
instead (to change the first n occurrences on each line).
Now let's try an demo involving deletion of lines. The values for start and end can be either a
line number or a pattern to match. All lines from the start line to the end line are removed from
the output. This demo will delete starting at line 2, up to and including line 3:
sed '2,3d' poem.txt
Mary had a little raga
Jill had a raga spamwich
This demo will delete starting at line 1, up to and including the next line containing Jack:
sed '1,/Jack/d' poem.txt
Jill had a raga spamwich
The most common use of sed is to change one string of text to another string of text. But I should
mention that the strings that sed uses for search and delete are actually regular expressions. This
means you can use pattern matching, just as with grep. Although you'll probably never need to
do anything like this, here's an demo anyway. To change any occurrences of raga at the end of a
line to hama, and save the results in a new file, enter this:
sed 's/raga#/hama/g' poem.txt > new.file
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Since we directed output to a file, sed didn't print anything on the screen. If you look at the
contents of new.file it will show these lines:
Mary had a little hama
Mary fried a lot of spam
Jack ate a Spam sandwich
Jill had a raga spamwich
Sort
The sort command sorts a file according to fields--the individual pieces of data on each line. By
default, sort assumes that the fields are just words separated by blanks, but you can specify an
alternative field delimiter if you want (such as commas or colons). Output from sort is printed to
the screen, unless you redirect it to a file.
If you had a file like the one shown here containing information on people who contributed to
your presidential reelection campaign, for demo, you might want to sort it by last name, donation
amount, or location. (Using a text editor, enter those three lines into a file and save it
with donor.data as the file name.)
Bay Ching 500000 China
Jack Arta 250000 Indonesia
Cruella Lumper 725000 Malaysia
Let's take this sample donors file and sort it according to the donation amount. The following
shows the command to sort the file on the second field (last name) and the output from the
command:
sort +1 -2 donors.data
Jack Arta 250000 Indonesia
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Bay Ching 500000 China
Cruella Lumper 725000 Malaysia
The syntax of the sort command is pretty strange, but if you study the following demos, you
should be able to adapt one of them for your own use. The general form of the sort command is
sort <flags> <sort fields> <file name>
The most common flags are as follows:
-f Make all lines uppercase before sorting (so "Bill" and "bill" are treated the same).
-r Sort in reverse order (so "Z" starts the list instead of "A").
-n Sort a column in numerical order
-tx Use x as the field delimiter (replace x with a comma or other character).
-u Suppress all but one line in each set of lines with equal sort fields (so if you sort on a field
containing last names, only one "Smith" will appear even if there are several).
Specify the sort keys like this:
+m Start at the first character of the m+1th field.
-n End at the last character of the nth field (if -N omitted, assume the end of the line).
Looks weird, huh? Let's look at a few more demos with the sample company.data file shown
here, and you'll get the hang of it. (Each line of the file contains four fields: first name, last name,
serial number, and department name.)
Jan Itorre 406378 Sales
Jim Nasium 031762 Marketing
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Mel Ancholie 636496 Research
Ed Jucacion 396082 Sales
To sort the file on the third field (serial number) in reverse order and save the results
in sorted.data, use this command:
sort -r +2 -3 company.data > sorted.data
Mel Ancholie 636496 Research
Jan Itorre 406378 Sales
Ed Jucacion 396082 Sales
Jim Nasium 031762 Marketing
Now let's look at a situation where the fields are separated by colons instead of spaces. In this
case, we will use the -t:flag to tell the sort command how to find the fields on each line. Let's
start with this file:
Itorre, Jan:406378:Sales
Nasium, Jim:031762:Marketing
Ancholie, Mel:636496:Research
Jucacion, Ed:396082:Sales
To sort the file on the second field (serial number), use this command:
sort -t: +1 -2 company.data
Nasium, Jim:031762:Marketing
Jucacion, Ed:396082:Sales
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Itorre, Jan:406378:Sales
Ancholie, Mel:636496:Research
To sort the file on the third field (department name) and suppress the duplicates, use this
command:
sort -t: -u +2 company.data
Nasium, Jim:031762:Marketing
Ancholie, Mel:636496:Research
Itorre, Jan:406378:Sales
Note that the line for Ed Jucacion did not print, because he's in Sales, and we asked the
command (with the -u flag) to suppress lines that were the same in the sort field.
Investigating and Managing Processes (crontab zip tar)
tar
Sometimes it is necessary to store a set of files and directories in a single file called an "archive".
This is useful for backing up files and for transmitting them over a network (email, web, ftp,
etc.). In Linux, the tar command is used to create archive files, and to extract files/directories
from archive files. Archive files created by tar are called "tar files", and frequently have a name
with the .tar extension. Tar can be used to perform one of three primary tasks:
Create a new archive file
Extract files from an existing archive file
List the contents of an existing archive file
The tar command-line has the following form:
# tar options tar-file other-files
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options is a string of characters that tells tar which options to use. Possible options are the
following:
You will always specify either c, x, or t (c = create, x = extract, t = list).
The v option requests verbose output, and causes tar to print file names as they are added
or extracted
The z option tells tar to use gzip compression
The f option is used to specify the name of the tar file
tar-file is the name of the tar file
other-files is a list of the files and directories that you want to include in the archive Here are
some demos that show how to use tar to perform some common tasks.
### create a tar file containing the source files for myfile
# ls
myfile.cpp myfile.h myfile.html
# tar cf myfile.tar *
# ls
myfile.cpp myfile.h myfile.html myfile.tar
### list the contents of myfile.tar
# tar tf myfile.tar
myfile.cpp
myfile.h
myfile.html
### extract the myfile source files
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# rm *.cpp *.h *.html
# ls
myfile.tar
# tar xf myfile.tar
# ls
myfile.cpp myfile.h myfile.html myfile.tar
### create a compressed a tar file containing the entire myfile directory
# cd ..
# ls
myfile
# tar czf myfile.tgz myfile
# ls
myfile myfile.tgz
### list the contents of myfile.tgz
# tar tzf myfile.tgz
myfile/myfile.cpp
myfile/myfile.h
myfile/myfile.html
### extract the myfile directory
# rm -R myfile
# ls
myfile.tgz
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# tar xzf myfile.tgz
# ls
myfile myfile.tgz
COMPRESSION AND DECOMPRESSION
gzip
gzip does file compression/decompression, but not archiving. However, it works well when used
together with an archiving tool - in particular, gzip and tar play nicely together.
gzip, like zip, uses a variation on the DEFLATE algorithm. However, when used on a tar
archive, it'll compress better than zip, because it treats the whole archive together, rather than
compressing file-by-file, and thus can look for (and eliminate!) duplication between files as well
as within them.
Gzip commands:
gzip file.tar
compresses file.tar to create file.tar.gz.
tar czf file.tar.gz mydir/
archives and compresses mydir/ to create file.tar.gz
gunzip file.tar.gz or gzip -d file.tar.gz
uncompress file.tar.gz.
tar zxf file.tar.gz
uncompress file.tar.gz and also unpack the archive.
Note that you can also use gzcat (use exactly as gunzip) to decompress directly to standard
output.
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bzip2
bzip2, like gzip, does compression but not archiving. Again, it works well with tar if you want
both archiving and compression. It compresses better than gzip, but the cost of that is that it takes
longer to run.
It uses a series of stacked transforms to compress data, the most important of which is the
Burrows-Wheeler transform. This reorders blocks to maximise the number of repeated symbols.
Runs of repeated symbols can then be replaced by the symbol and a code which represents the
length of the run, reducing the size taken up by each run. The Huffman coding used in
DEFLATE (where common symbols are swapped in for shorter versions of them) is also used
after that, to reduce the space still further, along with a couple of other more minor changes.
Given all of that, the time it takes is hardly surprising!
Bzip2 commands:
bzip2 file.tar
compresses file.tar to create file.tar.bz2.
bunzip2 file.tbz
uncompresses file.tbz to create file.tar.
tar jxf file.tbz
uncompresses file.tbz and unpacks the archive.
Note that you can also use bzcat to decompress directly to standard output.
If you're interested in compression algorithms, you could also check out 7-zip (the Linux
command-line version is p7zip), which implements the LZM algorithm. In most cases it
compresses better than bzip2, and it also decompresses faster (although it compresses slower).
However, it's much less popular (I've never seen it in the wild).
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CRONTAB
The crontab (cron derives from chronos, Greek for time; tab stands for table) command, found
in Unix and Unix-like operating systems, is used to schedule commands to be executed
periodically. To see what crontabs are currently running on your system, you can open a
terminal and run:
crontab -e
This wil open a the default editor (could be vi or pico, if you want you can change the default
editor) to let us manipulate the crontab. If you save and exit the editor, all your cronjobs are
saved into crontab. Cronjobs are written in the following format:
* * * * * /bin/execute/this/script.sh
Scheduling explained
As you can see there are 5 stars. The stars represent different date parts in the following order:
1. minute (from 0 to 59)
2. hour (from 0 to 23)
3. day of month (from 1 to 31)
4. month (from 1 to 12)
5. day of week (from 0 to 6) (0=Sunday)
Execute every minute
If you leave the star, or asterisk, it means every. Maybe that's a bit unclear. Let's use the the
previous demo again:
* * * * * /bin/execute/this/script.sh
They are all still asterisks! So this means execute /bin/execute/this/script.sh:
1. every minute
2. of every hour
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3. of every day of the month
4. of every month
5. and every day in the week.
In short: This script is being executed every minute. Without exception.
Execute every Friday 1AM
So if we want to schedule the script to run at 1AM every Friday, we would need the following
cronjob:
0 1 * * 5 /bin/execute/this/script.sh
Get it? The script is now being executed when the system clock hits:
1. minute: 0
2. of hour: 1
3. of day of month: * (every day of month)
4. of month: * (every month)
5. and weekday: 5 (=Friday)
Execute on workdays 1AM
So if we want to schedule the script to Monday till Friday at 1 AM, we would need the
following cronjob:
0 1 * * 1-5 /bin/execute/this/script.sh
Get it? The script is now being executed when the system clock hits:
1. minute: 0
2. of hour: 1
3. of day of month: * (every day of month)
4. of month: * (every month)
5. and weekday: 1-5 (=Monday til Friday)
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Figure 29 crontab
STANDARD INPUT/OUTPUT
C++ programs can read input from the keyboard. This is called ―standard input‖. C++ programs
can also write output to the screen (i.e., the shell). This is called ―standard output‖.
Many Linux programs accept file names as command-line arguments (e.g., ls, cp, mv, rm, wc,
etc.). If you don't provide any file names on the command-line, these programs will read their
input from standard input (i.e., the keyboard). This allows the user to interactively type the input
to the program rather than having it read from a file. For demo, the following command will
count the number of lines typed in by the user (CTRL-D is used to terminate interactive input).
### count the number of lines typed, hit CTRL-D to end
# wc -l
Because no file name was provided on the command-line, wc will take its input from the
keyboard. Once CTRL-D is hit, wc knows that the user is done typing.
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STANDARD I/O REDIRECTION
The shell lets you redirect a program‘s standard input so that it comes from a file instead of from
the keyboard. The command below instructs the shell to run the program "myfile" with its
standard input coming from a file named "file.input". This means that whenever "myfile" reads
input from the keyboard, it will actually be reading from "file.input".
# myfile < file.input
The shell also lets you redirect a program‘s standard output so that it goes to a file instead of the
screen. The command below instructs the shell to run the program "myfile" with its standard
output going to a file named "file.output". This means that whenever "myfile" writes output to
the screen, the output will actually go to "file.output" instead of the screen.
# overwrite the output file
# myfile > file.output
# append to the output file
# myfile >> file.output
You can also redirect both standard input and standard output at the same time, as shown below.
# myfile < file.input > file.output
PIPELINES
The shell allows you to run multiple commands simultaneously, with the output of one command
becoming the input to the next command. This allows commands to be assembled into a
"pipeline" where each program reads input, transforms the data, and then sends the transformed
data to the next command in the pipeline for further processing.
For demo, suppose that you have a text file and you want to count the number of lines in the file
that contain the text "BYU". The grep command can be used to find the lines that contain
"BYU", but it doesn't know how to count them. Similarly, the wc command can be used to count
lines, but it doesn't know how to find only the lines that contain "BYU". By running grep and wc
together in a pipeline, we can achieve the desired effect.
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# grep ‘BYU‘ file.txt | wc –l
First, run grep on the file, searching for the pattern "BYU". Take the output of grep and connect
it to the input of wc. This will cause grep to find only those lines containing "BYU" and send
them to wc. When wc receives the "BYU" lines from grep, it will count them and print out the
answer.
As shown above, multiple commands may be assembled into a pipeline by placing | between the
commands. This tells the shell to run the programs so that the standard output of one program
becomes the standard input of the next program, thus connecting the programs together. The first
command in the pipeline will read its input from the keyboard or a file. The last command in the
pipeline will write its output to the screen or a file. All commands in the middle of the pipeline
will read their input from the previous command, and write their output to the next command.
Of course, command pipelines may contain more than two commands. For demo, instead of
counting "BYU" lines, we might want to sort them and send them to a printer.
# grep 'BYU' file.txt | sort | print
Pipelines work because programs like grep, wc, sort, and print read their input from standard
input if no file name is specified on the command line.
Basic Networking
Under Red Hat Enterprise Linux, all network communications occur between configured
software interfaces and physical networking devices connected to the system.
The configuration files for network interfaces are located in the /etc/sysconfig/network-
scripts/ directory. The scripts used to activate and deactivate these network interfaces are also
located here. Although the number and type of interface files can differ from system to system,
there are three categories of files that exist in this directory:
1. Interface configuration files
2. Interface control scripts
3. Network function files
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The files in each of these categories work together to enable various network devices.
This part explores the relationship between these files and how they are used.
Network Configuration Files
Before delving into the interface configuration files, let us first itemize the primary configuration
files used in network configuration. Understanding the role these files play in setting up the
network stack can be helpful when customizing a Red Hat Enterprise Linux system.
The primary network configuration files are as follows:
/etc/hosts
The main purpose of this file is to resolve hostnames that cannot be resolved any other
way. It can also be used to resolve hostnames on small networks with no DNS server.
Regardless of the type of network the computer is on, this file should contain a line
specifying the IP address of the loopback device (127.0.0.1) as localhost.localdomain.
For more information, refer to the hosts man page.
/etc/resolv.conf
This file specifies the IP addresses of DNS servers and the search domain. Unless
configured to do otherwise, the network initialization scripts populate this file. For more
information about this file, refer to the resolv.conf man page.
/etc/sysconfig/network
This file specifies routing and host information for all network interfaces.
/etc/sysconfig/network-scripts/ifcfg-<interface-name>
For each network interface, there is a corresponding interface configuration script. Each
of these files provide information specific to a particular network interface.
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Ethernet Interfaces
One of the most common interface files is ifcfg-eth0, which controls the first Ethernet network
interface card or NIC in the system. In a system with multiple NICs, there are multiple ifcfg-
eth<X> files (where <X> is a unique number corresponding to a specific interface). Because
each device has its own configuration file, an administrator can control how each interface
functions individually.
The following is a sample ifcfg-eth0 file for a system using a fixed IP address:
DEVICE=eth0
BOOTPROTO=none
ONBOOT=yes
NETWORK=10.0.1.0
NETMASK=255.255.255.0
IPADDR=10.0.1.27
USERCTL=no
The values required in an interface configuration file can change based on other values. For
demo, the ifcfg-eth0 file for an interface using DHCP looks different because IP information is
provided by the DHCP server:
DEVICE=eth0
BOOTPROTO=dhcp
ONBOOT=yes
The Network Administration Tool (system-config-network) is an easy way to make changes to
the various network interface configuration files. However, it is also possible to manually edit
the configuration files for a given network interface.
Below is a listing of the configurable parameters in an Ethernet interface configuration file:
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BOOTPROTO=<protocol>
where <protocol> is one of the following:
none — No boot-time protocol should be used.
bootp — The BOOTP protocol should be used.
dhcp — The DHCP protocol should be used.
BROADCAST=<address>
where <address> is the broadcast address. This directive is deprecated, as the value is
calculated automatically with ifcalc.
DEVICE=<name>
where <name> is the name of the physical device (except for dynamically-allocated PPP
devices where it is the logical name).
DHCP_HOSTNAME
Use this option only if the DHCP server requires the client to specify a hostname before
receiving an IP address.
DNS{1,2}=<address>
where <address> is a name server address to be placed in /etc/resolv.conf if
the PEERDNS directive is set to yes.
ETHTOOL_OPTS=<options>
where <options> are any device-specific options supported by ethtool. For demo, if you
wanted to force 100Mb, full duplex:
ETHTOOL_OPTS="autoneg off speed 100 duplex full"
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GATEWAY=<address>
where <address> is the IP address of the network router or gateway device (if any).
HWADDR=<MAC-address>
where <MAC-address> is the hardware address of the Ethernet device in the
form AA:BB:CC:DD:EE:FF. This directive is useful for machines with multiple NICs to
ensure that the interfaces are assigned the correct device names regardless of the
configured load order for each NIC's module. This directive should not be used in
conjunction with MACADDR.
IPADDR=<address>
where <address> is the IP address.
MACADDR=<MAC-address>
where <MAC-address> is the hardware address of the Ethernet device in the
form AA:BB:CC:DD:EE:FF. This directive is used to assign a MAC address to an
interface, overriding the one assigned to the physical NIC. This directive should not be
used in conjunction with HWADDR.
MASTER=<bond-interface>
where <bond-interface> is the channel bonding interface to which the Ethernet interface
is linked.
This directive is used in conjunction with the SLAVE directive.
NETMASK=<mask>
where <mask> is the netmask value.
NETWORK=<address>
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where <address> is the network address. This directive is deprecated, as the value is
calculated automatically with ifcalc.
ONBOOT=<answer>
where <answer> is one of the following:
yes — This device should be activated at boot-time.
no — This device should not be activated at boot-time.
PEERDNS=<answer>
where <answer> is one of the following:
yes — Modify /etc/resolv.conf if the DNS directive is set. If using DHCP,
then yes is the default.
no — Do not modify /etc/resolv.conf.
SLAVE=<bond-interface>
where <bond-interface> is one of the following:
yes — This device is controlled by the channel bonding interface specified in
the MASTER directive.
no — This device is not controlled by the channel bonding interface specified in
the MASTER directive.
This directive is used in conjunction with the MASTER directive.
SRCADDR=<address>
where <address> is the specified source IP address for outgoing packets.
USERCTL=<answer>
where <answer> is one of the following:
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yes — Non-root users are allowed to control this device.
no — Non-root users are not allowed to control this device.
The interface control scripts
The interface control scripts activate and deactivated system interfaces. There are two primary
interface control scripts that call on control scripts located in the /etc/sysconfig/network-
scripts/ directory: /sbin/ifdown and /sbin/ifup.
The ifup and ifdown interface scripts are symbolic links to scripts in the /sbin/ directory. When
either of these scripts are called, they require the value of the interface to be specified, such as:
ifup eth0
Command we use to open the Network-setting dialog box in text mode setup.
Advance Topics in User’s , group’s and Permissions
Advanced File Permissions in Linux
Here we will discuss about the 3 special attributes other than the common read/write/execute.
Demo:
drwxrwxrwt - Sticky Bits - chmod 1777
drwsrwxrwx - SUID set - chmod 4777
drwxrwsrwx - SGID set - chmod 2777
Sticky bit
Sticky bits are mainly set on directories. If the sticky bit is set for a directory, only the owner of
that directory or the owner of a file can delete or rename a file within that directory.
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Demo:
Consider you have a directory " test ".
chmod it to " 777 ". This gives permissions for all the users to read, write and execute.
chmod +t test
Demo: ls –al
drwxrwxrwt 2 a1 a1 4096 Jun 13 2008
-rw-rw-r-- 1 a1 a1 0 Jun 11 17:30 1.txt
-rw-rw-r-- 1 b2 b2 0 Jun 11 22:52 2.txt
From the above demo a1 is the owner of the test directory.
a1 can delete or rename the files 1.txt and 2.txt.
b2 can delete or rename the file 2.txt only.
SUID - [ Set User ID ]
SUID bit is set for files ( mainly for scripts ). The SUID permission makes a script to run as
the user who is the owner of the script, rather than the user who started it.
Demo:
If a1 is the owner of the script and b2 tries to run the same script, the script runs with the
ownership of a1.
If the root user wants to give permissions for some scripts to run by different users, he can
set the SUID bit for that particular script.
So if any user on the system starts that script, it will run under the root ownership.
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SGID - [ Set Group ID ]
If a file is SGID, it will run with the privileges of the files group owner, instead of the
privileges of the person running the program. This permission set also can make a similar
impact. Here the script runs under the groups ownership.
You can also set SGID for directories.
Consider you have given 2777 permission for a directory. Any files created by any users
under this directory will come as follows.
Demo:
-rw-rw-r-- 1 b2 a1 0 Jun 11 17:30 1.txt
In the above demo you can see that the owner of the file 1.txt is b2 and the group owner is
a1.
So both b2 and a1 will have access to the file 1.txt.
Now lets make this more intresting and complicated.
Create a directory "test". Chmod it to 2777. Add sticky bit to it.
Demo:
mkdir test
chmod 2777 test
chmod +t test
ls -al test
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drwxrwsrwt 2 a1 a1 4096 Jun 13 2008 test
From the above permission set you can understand that SGID and sticky bit is set for the
folder "test".
Now any user can create files under the test directory.
Demo:
drwxrwsrwt 2 a1 a1 4096 Jun 13 2008
-rw-rw-r-- 1 b2 a1 0 Jun 11 17:30 1.txt
-rw-rw-r-- 1 c3 a1 0 Jun 11 17:30 2.txt
-rw-rw-r-- 1 d4 a1 0 Jun 11 17:30 3.txt
So all the a1 user has access to all the files under the test directory. He can edit, rename or
remove the file.
b2 user has access to 1.txt only, c3 has access to 2.txt only...
If sticky bit was not set for the test directory, any user can delete any files from the test
directory, since the test directory has 777 permissions.
But now it not possible.
Demo:
If d4 tries to remove 1.txt
rm -f 1.txt
rm: cannot remove `1.txt': Operation not permitted
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INODE NUMBER’S
The inode (index node) is a fundamental concept in the Linux and UNIX filesystem. Each
object in the filesystem is represented by an inode. But what are the objects? Let us try to
understand it in simple words. Each and every file under Linux (and UNIX) has following
attributes:
1. File type (executable, block special etc)
2. Permissions (read, write etc)
3. Owner
4. Group
5. File Size
6. File access, change and modification time (remember UNIX or Linux never stores file creation
time,this is favorite question asked in UNIX/Linux sys admin job interview)
7. File deletion time
8. Number of links (soft/hard)
9. Extended attribute such as append only or no one can delete file including root user
(immutability)
10. Access Control List (ACLs)
All the above information stored in an inode. In short the inode identifies the file and its
attributes (as above) . Each inode is identified by a unique inode number within the file system.
Inode is also know as index number.
inode definition
An inode is a data structure on a traditional Unix-style file system such as UFS or ext3. An inode
stores basic information about a regular file, directory, or other file system object.
How do I see file inode number?
You can use ls -i command to see inode number of file
$ ls -i /etc/passwd
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Sample Output
32820 /etc/passwd
You can also use stat command to find out inode number and its attribute:
$ stat /etc/passwdOutput:
File: `/etc/passwd'
Size: 1988 Blocks: 8 IO Block: 4096 regular file
Device: 341h/833d Inode: 32820 Links: 1
Access: (0644/-rw-r--r--) Uid: ( 0/ root) Gid: ( 0/ root)
Access: 2005-11-10 01:26:01.000000000 +0530
Modify: 2005-10-27 13:26:56.000000000 +0530
Change: 2005-10-27 13:26:56.000000000 +0530
Package management
RPM (RedHat Package Management)
The RPM Package Manager (RPM) is a powerful command line driven package management
system capable of installing, uninstalling, verifying, querying, and updating computer software
packages. Each software package consists of an archive of files along with information about the
package like its version, a description, and the like. There is also a library API, permitting
advanced developers to manage such transactions from programming languages such as C or
Python.
RPM is free software, released under the GNU GPL.
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RPM is a core component of many Linux distributions, such as Red Hat Enterprise Linux,
the Fedora Project, SUSE Linux Enterprise, openSUSE, CentOS, Meego, Mageia and many
others.
It is also used on many other operating systems as well, and the RPM format is part of the Linux
Standard Base.
To install RPM Package:
1. Log in as root user at the workstation on which you want to install the software.
2. Download the package you wish to install. The package will be named something
like gcc.401.i386.rpm.
3. To install the package, enter the following command at the prompt:
rpm -i gcc.401.i386.rpm
If you are upgrading from an earlier version of the software package, run RPM in
upgrade mode, as in the following demo:
rpm -U gcc.401.i386.rpm
xmms-1.2.10-9.i386.rpm
Package Name Version Type of Extension
architecture
# rpm <options> <package name>
# rpm -ivh <package name>
--aid (install package along with dependencies)
--force (forcefully)
--nodebs (to remove package along with dependencies)
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# rpm -e <package name> (To uninstall package)
-U (upgrade the package)
# rpm -q <package name> (Show whether package is install or not)
-qa (queries all installed rpm packages)
-qc (lists only the configuration files stored in the queried package)
-qd (lists only the documentation files stored in the queried rpm)
-qi (displays whole information about the queried rpm)
-qs (displays the states of files in the queried rpm)
-ql (displays all files related to the queried rpm)
SSH (Secure SHell)
There are a couple of ways that you can access a shell (command line) remotely on most
Linux/Unix systems. One of the older ways is to use the telnet program, which is available on
most network capable operating systems. Accessing a shell account through the telnet method
though poses a danger in that everything that you send or receive over that telnet session is
visible in plain text on your local network, and the local network of the machine you are
connecting to. So anyone who can "sniff" the connection in between can see your username,
password, email that you read, and commands that you run. For these reasons you need a more
sophisticated program than telnet to connect to a remote host.
SSH, which is an acronym for Secure SHell, was designed and created to provide the best
security when accessing another computer remotely. Not only does it encrypt the session, it also
provides better authentication facilities, as well as features like secure file transfer, X session
forwarding, port forwarding and more so that you can increase the security of other protocols. It
can use different forms of encryption ranging anywhere from 512 bit on up to as high as 32768
bits and includes ciphers like AES (Advanced Encryption Scheme), Triple DES, Blowfish,
CAST128 or Arcfour. Of course, the higher the bits, the longer it will take to generate and use
keys as well as the longer it will take to pass data over the connection.
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Sometimes it may be necessary to identify the SSH client that you are currently running and it‘s
corresponding version number, which can be identified as shown below. Please note that Linux
comes with OpenSSH.
$ ssh -V
OpenSSH_3.9p1, OpenSSL 0.9.7a Feb 19 2003
$ ssh -V
ssh: SSH Secure Shell 3.2.9.1 (non-commercial version) on i686-pc-linux-gnu
Login to remote host:
The First time when you login to the remotehost from a localhost, it will display the host key
not found message and you can give ―yes‖ to continue. The host key of the remote host will
be added under .ssh2/hostkeys directory of your home directory, as shown below.
localhost$ ssh -l demo remotehost.demo.com
Host key not found from database.
Key fingerprint:
xabie-dezbc-manud-bartd-satsy-limit-nexiu-jambl-title-jarde-tuxum
You can get a public key‘s fingerprint by running
% ssh-keygen -F publickey.pub
on the keyfile.
Are you sure you want to continue connecting (yes/no)? yes
Host key saved to /home/demo/.ssh2/hostkeys/key_22_remotehost.demo.com.pub
host key for remotehost.demo.com, accepted by demo Mon Apr 26 2010 17:05:50 -0700
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[email protected] password:
remotehost.demo.com$
The Second time when you login to the remote host from the localhost, it will prompt only for
the password as the remote host key is already added to the known hosts list of the ssh client.
localhost$ ssh -l demo remotehost.demo.com
[email protected] password:
remotehost.demo.com$
For some reason, if the host key of the remote host is changed after you logged in for the first
time, you may get a warning message as shown below. This could be because of various reasons
such as 1) Sysadmin upgraded/reinstalled the SSH server on the remote host 2) someone is doing
malicious activity etc., The best possible action to take before saying ―yes‖ to the message
below, is to call your sysadmin and identify why you got the host key changed message and
verify whether it is the correct host key or not.
localhost$ ssh -l demo remotehost.demo.com
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@@@@@
@ WARNING: HOST IDENTIFICATION HAS CHANGED! @
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@@@@@
IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!
Someone could be eavesdropping on you right now (man-in-the-middle attack)!
It is also possible that the host key has just been changed.
Please contact your system administrator.
Add correct host key to "/home/demo/.ssh2/hostkeys/key_22_remotehost.demo.com.pub"
to get rid of this message.
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Received server key's fingerprint:
xabie-dezbc-manud-bartd-satsy-limit-nexiu-jambl-title-jarde-tuxum
You can get a public key's fingerprint by running
% ssh-keygen -F publickey.pub
on the keyfile.
Agent forwarding is disabled to avoid attacks by corrupted servers.
Are you sure you want to continue connecting (yes/no)? yes
Do you want to change the host key on disk (yes/no)? yes
Agent forwarding re-enabled.
Host key saved to /home/demo/.ssh2/hostkeys/key_22_remotehost.demo.com.pub
host key for remotehost.demo.com, accepted by demo Mon May 26 2008 16:17:31 -0700
demo @remotehost.demo.com's password:
remotehost$
File transfer to/from remote host:
Another common use of ssh client is to copy files from/to remote host using scp.
Copy file from the remotehost to the localhost:
localhost$ scp [email protected]:/home/demo/remotehostfile.txt
remotehostfile.txt
Copy file from the localhost to the remotehost:
localhost$ scp localhostfile.txt
[email protected]:/home/demo/localhostfile.txt
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Debug SSH Client:
Sometimes it is necessary to view debug messages to troubleshoot any SSH connection issues.
For this purpose, pass -v (lowercase v) option to the ssh as shown below.
Demo without debug message:
localhost$ ssh -l demo remotehost.demo.com
warning: Connecting to remotehost.demo.com failed: No address associated to the name
localhost$
Demo with debug message:
locaclhost$ ssh -v -l demo remotehost.demo.com
debug: SshConfig/sshconfig.c:2838/ssh2_parse_config_ext: Metaconfig parsing stopped
at line 3.
debug: SshConfig/sshconfig.c:637/ssh_config_set_param_verbose: Setting variable
'VerboseMode' to 'FALSE'.
debug: SshConfig/sshconfig.c:3130/ssh_config_read_file_ext: Read 17 params from config
file.
debug: Ssh2/ssh2.c:1707/main: User config file not found, using defaults. (Looked for
'/home/demo/.ssh2/ssh2_config')
debug: Connecting to remotehost.demo.com, port 22... (SOCKS not used)
warning: Connecting to remotehost.demo.com failed: No address associated to
SUDO
sudo allows a permitted user to execute a command as the superuser or another user, as specified
in the sudoers file. The real and effective uid and gid are set to match those of the target user as
specified in the passwd file (the group vector is also initialized when the target user is not root).
By default, sudo requires that users authenticate themselves with a password (NOTE: by default
this is the user's password, not the root password). Once a user has been authenticated, a
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timestamp is updated and the user may then use sudo without a password for a short period of
time (5 minutes unless overridden in sudoers).
The file /etc/sudoers, has the rules that users have to follow when using sudo command.
Two of the best advantages about using sudo are:
Restringed privileges
Logs of the actions done by users
Well but in order to use sudo we first need to configure the sudoers file.
“Do not edit directly the file”
To edit it, use the command
visudo
You will see a file more or less like this.
# /etc/sudoers
#
# This file MUST be edited with the 'visudo' command as root.
#
# See the man page for details on how to write a sudoers file.
#
Defaults env_reset
# Host alias specification
# User alias specification
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# Cmnd alias specification
# User privilege specification
root ALL=(ALL) ALL
As you can see there is basically one line
root ALL=(ALL) ALL
This lines means that the user root can execute from ALL terminals, acting as ALL (any) users,
and run ALL (any) command.
So the first part is the user, the second is the terminal from where the user can use sudo, the third
is as which user he may act, and the last one, is which commands he may run.
Let's see some other examples.
User ALL= /sbin/poweroff
This makes that users user can from any terminal, run the command poweroff.
You can also create aliases for: users
These are some examples:
User_Alias DEMO = demo
Runas_Alias OP = root, operator
Host_Alias OFNET = 10.1.2.0/255.255.255.0
Cmnd_Alias PRINTING = /usr/sbin/lpc, /usr/bin/lprm
As you can see the alias DEMO include the users demo, the alias OP includes the users root and
operator, alias OFNET includes the network 10.1.2.0 (all the C class), and the command alias
PRINTING includes the commands lpc and lprm.
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So a typical sudoers file may look like this.
User_Alias DEMO = demo
Runas_Alias OP = root, operator
Host_Alias OFNET = 10.1.2.0/255.255.255.0
Cmnd_Alias PRINTING = /usr/sbin/lpc, /usr/bin/lprm
DEMO ALL=ALL
#The users in the DEMO group can run any command from
any terminal.
linus ALL=(OP) ALL
# The user linus can run any command from any terminal as any
user in the OP group (root or operator).
user2 OFNET=(ALL) ALL
# user user2 may run any command from any machine in the
OFNET network, as any user.
user3 ALL= PRINTING
# user user3 may run lpc and lprm from any machine.
go2linux ALL=(ALL) ALL
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YUM (Yellowdog Updater Modified)
The Yellowdog Updater, Modified (YUM) is an open-source command-line package-
management utility for RPM-compatible Linux operating systems and has been released under
the GNU General Public License. It was developed by Seth Vidal and a group of volunteer
programmers. Though yum has a command-line interface, several other tools provide graphical
user interfaces to yum functionality.
The Yellowdog Updater, Modified. Yum is an automatic updater and package installer/remover
for rpm-based systems. It automatically computes dependencies and figures out what things
should occur in order to safely install, remove, and update rpm packages. Yum also efficiently
and easily retrieves information on any package installed or available in a repository to the
installer. Yum makes it easier to maintain groups of machines without having to manually update
each one using rpm or other tools. Yum can manage package groups, multiple repositories,
fallback repositories and more to permit centralized package management performed by just one
or two individuals to scale over an entire organization.
Now to create a YUM in your Linux Server
1. Create a directory named repo
[root@localhost ~]# mkdir /repo
2. Insert DVD of rhel6 then Mount CD ROM
[root@localhost ~]# mount /dev/cdrom /mnt
3. Copy directory named Packages - from DVD of RHEL-6
[root@localhost ~]# cd /mnt
[root@localhost mnt# cp -Rvf Server Packages /repo
4. Now create a file named anyname.repo
# vi /etc/yum.repos.d/base.repo
[base]
name= base
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baseurl=file:///repo/Packages
enabled=1
gpgcheck=0
(save the file)
5. Import GPG Key from DVD
# rpm --import /mnt/RPM-GPG*
6. Now we have to group files to create repodata
7. Install standalone service
# rpm -ivh /mnt/Packages/createrepo*
It will ask for dependency named deltaparm & python-deltaparm, install both of the
packages.
8. Now we have to create repository
[root@localhost ~]# createrepo -v /repo/Packages
9. # yum clean all
10. # yum list all
11. Now install package from command
# yum install <package name>
12. To uninstall package
# yum remove <package name>
13. Now on graphics you will find add/remove program and all packages are there to install.
Printing in Linux
From the command line, use the lp or lpr command.
lp file(s)
lpr file(s)
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These commands can read from a pipe, so you can print the output of commands using
command | lp
There are many options available to tune the page layout, the number of copies, the printer that
you want to print to if you have more than one available, paper size, one-side or double-sided
printing if your printer supports this feature, margins and so on.
Status of your print jobs
Once the file is accepted in the print queue, an identification number for the print job is assigned:
[[email protected] ~]# lp /etc/profile
request id is Printer -253 (1 file(s))
To view (query) the print queue, use the lpq or lpstat command. When entered without
arguments, it displays the contents of the default print queue.
[[email protected] ~]# lpq
Printer is ready and printing
Rank Owner Job File(s) Total Size
active root 253 profile 1024 bytes
[[email protected] ~]# lpstat
Printer -253 root 1024 Tue 25 Jul 2006 10:20_01 AM IST
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Status of your printer
[[email protected] ~]#lpstat -d
system default destination: Printer
What is the status of my printer(s)?
[[email protected] ~]# lpstat -p
printer Printer now printing Printer -253. enabled since Jan 01 18:01
Removing jobs from the print queue
If you don't like what you see from the status commands, use lprm or cancel to delete jobs.
[[email protected] ~]# lprm 253
In the graphical environment, you may see a popup window telling you that the job has been
canceled.
In larger environments, lpc may be used to control multiple printers. See the Info or man pages
on each command.
System Monitoring
Being able to monitor the performance of your system is essential. If system resources become to
low it can cause a lot of problems. System resources can be taken up by individual users, or by
services your system may host such as email or web pages. The ability to know what is
happening can help determine whether system upgrades are needed, or if some services need to
be moved to another machine.
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The top command.
The most common of these commands is top. The top will display a continually updating report
of system resource usage.
# top
12:10:49 up 1 day, 3:47, 7 users, load average: 0.23, 0.19, 0.10
125 processes: 105 sleeping, 2 running, 18 zombie, 0 stopped
CPU states: 5.1% user 1.1% system 0.0% nice 0.0% iowait 93.6% idle
Mem: 512716k av, 506176k used, 6540k free, 0k shrd, 21888k buff
Swap: 1044216k av, 161672k used, 882544k free 199388k cached
PID USER PRI NI SIZE RSS SHARE STAT %CPU %MEM TIME CPU COMMAND
2330 root 15 0 161M 70M 2132 S 4.9 14.0 1000m 0 X
2605 FIRST 15 0 8240 6340 3804 S 0.3 1.2 1:12 0 kdeinit
3413 FIRST 15 0 6668 5324 3216 R 0.3 1.0 0:20 0 kdeinit
18734 root 15 0 1192 1192 868 R 0.3 0.2 0:00 0 top
1619 root 15 0 776 608 504 S 0.1 0.1 0:53 0 dhclient
1 root 15 0 480 448 424 S 0.0 0.0 0:03 0 init
2 root 15 0 0 0 0 SW 0.0 0.0 0:00 0 keventd
3 root 15 0 0 0 0 SW 0.0 0.0 0:00 0 kapmd
4 root 35 19 0 0 0 SWN 0.0 0.0 0:00 0 ksoftirqd_CPU0
9 root 25 0 0 0 0 SW 0.0 0.0 0:00 0 bdflush
5 root 15 0 0 0 0 SW 0.0 0.0 0:00 0 kswapd
10 root 15 0 0 0 0 SW 0.0 0.0 0:00 0 kupdated
11 root 25 0 0 0 0 SW 0.0 0.0 0:00 0 mdrecoveryd
15 root 15 0 0 0 0 SW 0.0 0.0 0:01 0 kjournald
81 root 25 0 0 0 0 SW 0.0 0.0 0:00 0 khubd
1188 root 15 0 0 0 0 SW 0.0 0.0 0:00 0 kjournald
1675 root 15 0 604 572 520 S 0.0 0.1 0:00 0 syslogd
1679 root 15 0 428 376 372 S 0.0 0.0 0:00 0 klogd
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1707 rpc 15 0 516 440 436 S 0.0 0.0 0:00 0 portmap
1776 root 25 0 476 428 424 S 0.0 0.0 0:00 0 apmd
1813 root 25 0 752 528 524 S 0.0 0.1 0:00 0 sshd
1828 root 25 0 704 548 544 S 0.0 0.1 0:00 0 xinetd
1847 ntp 15 0 2396 2396 2160 S 0.0 0.4 0:00 0 ntpd
1930 root 24 0 76 4 0 S 0.0 0.0 0:00 0 rpc.rquotad
The top portion of the report lists information such as the system time, uptime, CPU usage,
physical ans swap memory usage, and number of processes. Below that is a list of the processes
sorted by CPU utilization.
You can modify the output of top while is is running. If you hit an i, top will no longer display
idle processes. Hit i again to see them again. Hitting M will sort by memory usage, S will sort by
how long they processes have been running, and P will sort by CPU usage again.
In addition to viewing options, you can also modify processes from within the top command.
You can use u to view processes owned by a specific user, k to kill processes, and r to renice
them.
For more in-depth information about processes you can look in the /proc filesystem. In
the /proc filesystem you will find a series of sub-directories with numeric names. These
directories are associated with the processes ids of currently running processes. In each directory
you will find a series of files containing information about the process.
The iostat command.
The iostat will display the current CPU load average and disk I/O information. This is a great
command to monitor your disk I/O usage.
# iostat
Linux 2.6.32-24.9 (myhost) 12/23/2003
avg-cpu: %user %nice %sys %idle
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62.09 0.32 2.97 34.62
Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn
dev3-0 2.22 15.20 47.16 1546846 4799520
For 2.4 kernels the devices is names using the device's major and minor number. In this case the
device listed is /dev/hda. To have iostat print this out for you, use the -x.
# iostat –x
Linux 2.6.32-24.9 (myhost) 12/23/2003
avg-cpu: %user %nice %sys %idle
62.01 0.32 2.97 34.71
Device: rrqm/s wrqm/s r/s w/s rsec/s wsec/s rkB/s wkB/s avgrq-sz avgqu-sz await svctm %util
/dev/hdc 0.00 0.00 .00 0.00 0.00 0.00 0.00 0.00 0.00 2.35 0.00 0.00 14.71
/dev/hda 1.13 4.50 .81 1.39 15.18 47.14 7.59 23.57 28.24 1.99 63.76 70.48 15.56
/dev/hda1 1.08 3.98 .73 1.27 14.49 42.05 7.25 21.02 28.22 0.44 21.82 4.97 1.00
/dev/hda2 0.00 0.51 .07 0.12 0.55 5.07 0.27 2.54 30.35 0.97 52.67 61.73 2.99
/dev/hda3 0.05 0.01 .02 0.00 0.14 0.02 0.07 0.01 8.51 0.00 12.55 2.95 0.01
The ps command
The ps will provide you a list of processes currently running. There is a wide variety of options
that this command gives you.
A common use would be to list all processes currently running. To do this you would use the ps -
ef command. (Screen output from this command is too large to include, the following is only a
partial output.)
UID PID PPID C STIME TTY TIME CMD
root 1 0 0 Dec22 ? 00:00:03 init
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root 2 1 0 Dec22 ? 00:00:00 [keventd]
root 3 1 0 Dec22 ? 00:00:00 [kapmd]
root 4 1 0 Dec22 ? 00:00:00 [ksoftirqd_CPU0]
root 9 1 0 Dec22 ? 00:00:00 [bdflush]
root 5 1 0 Dec22 ? 00:00:00 [kswapd]
root 6 1 0 Dec22 ? 00:00:00 [kscand/DMA]
root 7 1 0 Dec22 ? 00:01:28 [kscand/Normal]
root 8 1 0 Dec22 ? 00:00:00 [kscand/HighMem]
root 10 1 0 Dec22 ? 00:00:00 [kupdated]
root 11 1 0 Dec22 ? 00:00:00 [mdrecoveryd]
root 15 1 0 Dec22 ? 00:00:01 [kjournald]
root 81 1 0 Dec22 ? 00:00:00 [khubd]
root 1188 1 0 Dec22 ? 00:00:00 [kjournald]
root 1675 1 0 Dec22 ? 00:00:00 syslogd -m 0
root 1679 1 0 Dec22 ? 00:00:00 klogd -x
rpc 1707 1 0 Dec22 ? 00:00:00 portmap
root 1813 1 0 Dec22 ? 00:00:00 /usr/sbin/sshd
ntp 1847 1 0 Dec22 ? 00:00:00 ntpd -U ntp
root 1930 1 0 Dec22 ? 00:00:00 rpc.rquotad
root 1934 1 0 Dec22 ? 00:00:00 [nfsd]
root 1942 1 0 Dec22 ? 00:00:00 [lockd]
root 1943 1 0 Dec22 ? 00:00:00 [rpciod]
root 1949 1 0 Dec22 ? 00:00:00 rpc.mountd
root 1961 1 0 Dec22 ? 00:00:00 /usr/sbin/vsftpd /etc/vsftpd/vsftpd.conf
root 2057 1 0 Dec22 ? 00:00:00 /usr/bin/spamd -d -c -a
root 2066 1 0 Dec22 ? 00:00:00 gpm -t ps/2 -m /dev/psaux
bin 2076 1 0 Dec22 ? 00:00:00 /usr/sbin/cannaserver -syslog -u bin
root 2087 1 0 Dec22 ? 00:00:00 crond
daemon 2195 1 0 Dec22 ? 00:00:00 /usr/sbin/atd
root 2215 1 0 Dec22 ? 00:00:11 /usr/sbin/rcd
FIRST 3414 3413 0 Dec22 pts/1 00:00:00 /bin/bash
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FIRST 4342 3413 0 Dec22 pts/2 00:00:00 /bin/bash
FIRST 19121 18668 0 12:58 pts/2 00:00:00 ps –ef
The first column shows who owns the process. The second column is the process ID. The Third
column is the parent process ID. This is the process that generated, or started, the process. The
forth column is the CPU usage (in percent). The fifth column is the start time, of date if the
process has been running long enough. The sixth column is the tty associated with the process, if
applicable. The seventh column is the cumulitive CPU usage (total amount of CPU time is has
used while running). The eighth column is the command itself.
With this information you can see exacly what is running on your system and kill run-away
processes, or those that are causing problems.
The vmstat command
The vmstat command will provide a report showing statistics for system processes, memory,
swap, I/O, and the CPU. These statistics are generated using data from the last time the
command was run to the present. In the case of the command never being run, the data will be
from the last reboot until the present.
# vmstat
procs memory swap io system cpu
r b w swpd free buff cache si so bi bo in cs us sy id
0 0 0 181604 17000 26296 201120 0 2 8 24 149 9 61 3 36
The following was taken from the vmstat man page.
FIELD DESCRIPTIONS
Procs
r: The number of processes waiting for run time.
b: The number of processes in uninterruptable sleep.
w: The number of processes swapped out but otherwise runnable. This
field is calculated, but Linux never desperation swaps.
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Memory
swpd: the amount of virtual memory used (kB).
free: the amount of idle memory (kB).
buff: the amount of memory used as buffers (kB).
Swap
si: Amount of memory swapped in from disk (kB/s).
so: Amount of memory swapped to disk (kB/s).
IO
bi: Blocks sent to a block device (blocks/s).
bo: Blocks received from a block device (blocks/s).
System
in: The number of interrupts per second, including the clock.
cs: The number of context switches per second.
CPU
These are percentages of total CPU time.
us: user time
sy: system time
id: idle time
The lsof command
The lsof command will print out a list of every file that is in use. Since Linux considers
everything a file, this list can be very long. However, this command can be useful in diagnosing
problems. An example of this is if you wish to unmount a filesystem, but you are being told that
it is in use. You could use this command and grep for the name of the filesystem to see who is
using it.
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Or suppose you want to see all files in use by a particular process. To do this you would use lsof
-p -processid-.
Monitoring Devices
The df command
The df is the simplest tool available to view disk usage. Simply type in df and you'll be shown
disk usage for all your mounted filesystems in 1K blocks
[[email protected] ~]# df
Filesystem 1K-blocks Used Available Use% Mounted on
/dev/hda3 5242904 759692 4483212 15% /
tmpfs 127876 8 127868 1% /dev/shm
/dev/hda1 127351 33047 87729 28% /boot
/dev/hda9 10485816 33508 10452308 1% /home
/dev/hda8 5242904 932468 4310436 18% /srv
/dev/hda7 3145816 32964 3112852 2% /tmp
/dev/hda5 5160416 474336 4423928 10% /usr
/dev/hda6 3145816 412132 2733684 14% /var
You can also use the -h to see the output in "human-readable" format. This will be in K, Megs,
or Gigs depending on the size of the filesystem. Alternately, you can also use the -B to specify
block size.
In addition to space usage, you could use the -i option to view the number of used and available
inodes.
[[email protected] ~]# df –I
Filesystem Inodes IUsed IFree IUse% Mounted on
/dev/hda3 0 0 0 - /
tmpfs 31969 5 31964 1% /dev/shm
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/dev/hda1 32912 47 32865 1% /boot
/dev/hda9 0 0 0 - /home
/dev/hda8 0 0 0 - /srv
/dev/hda7 0 0 0 - /tmp
/dev/hda5 656640 26651 629989 5% /usr
/dev/hda6 0 0 0 - /var
The du command
Now that you know how much space has been used on a filesystem how can you find out where
that data is? To view usage by a directory or file you can use du. Unless you specify a
filename du will act recursively. For example:
[[email protected] ~]# du file.txt
1300 file.txt
Or like the df I can use the -h and get the same output in "human-readable" form.
[[email protected] ~]# du -h file.txt
1.3M file.txt
Unless you specify a filename du will act recursively.
[[email protected] ~]# du -h /usr/local
4.0K /usr/local/games
16K /usr/local/include/nessus/net
180K /usr/local/include/nessus
208K /usr/local/include
62M /usr/local/lib/nessus/plugins/.desc
97M /usr/local/lib/nessus/plugins
164K /usr/local/lib/nessus/plugins_factory
97M /usr/local/lib/nessus
12K /usr/local/lib/pkgconfig
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2.7M /usr/local/lib/ladspa
104M /usr/local/lib
112K /usr/local/man/man1
4.0K /usr/local/man/man2
4.0K /usr/local/man/man3
4.0K /usr/local/man/man4
16K /usr/local/man/man5
4.0K /usr/local/man/man
If you just want a summary of that directory you can use the -s option.
[[email protected] ~]# du -hs /usr/local
210M /usr/local
Monitoring Users
From time to time there are going to be occasions where you will want to know exactly what
people are doing on your system. Maybe you notice that a lot of RAM is being used, or a lot of
CPU activity. You are going to want to see who is on the system, what they are running, and
what kind of resources they are using.
The who command
The easiest way to see who is on the system is to do a who or w. The --> who is a simple tool
that lists out who is logged --> on the system and what port or terminal they are logged on at.
[[email protected] ~ ]# who
demo pts/0 May 23 09:33
user1 pts/3 May 20 11:35
user2 pts/1 May 22 11:03
user2 pts/2 May 23 15:04
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The ps command
In the previous section we can see that user user2 is logged onto both pts/1 and pts/2, but what if
we want to see what they are doing? We could to a ps -u user2 and get the following output
[[email protected] ~]# ps -u user2
20876 pts/1 00:00:00 bash
20904 pts/2 00:00:00 bash
20951 pts/2 00:00:00 ssh
21012 pts/1 00:00:00 ps
From this we can see that the user is doing a ps ssh.
This is a much more consolidated use of the ps than discussed previously.
The w command
Even easier than using the who and ps -u commands is to use the w. w will print out not only
who is on the system, but also the commands they are running.
[[email protected] ~]# w
user2 :0 09:32 ?xdm? 30:09 0.02s -:0
user2 pts/0 09:33 5:49m 0.00s 0.82s kdeinit: kded
user2 pts/2 09:35 8.00s 0.55s 0.36s vi sag-0.9.sgml
user2 pts/1 15:03 59.00s 0.03s 0.03s /bin/bash
Kernel Monitoring
uname is one of the most useful commands when it comes to gathering basic information about
your system. You can use it to find out the hostname of the system you're on, the hardware
architectures supported by the currently used kernel and the exact release of your system.
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Basic uname usage
uname -n
This command shows you the node (host) name of your system:
[[email protected] ~]# uname -n
demo.com
uname -i
If you're interested in confirming the hardware platform of your system, this is the command to
use.
For Linux, it will return i386 for 32-bit processors or x86_64 for 64-bit ones. For Solaris, it will
confirm the actual server type used:
[[email protected] ~]# uname -i
i386
Release and version of the Unix kernel
To find out the release and version of your Unix kernel, you need to use uname -r and uname -
v.
uname -r
This allows you to confirm the release of Unix kernel used in your OS.
On Linux, it looks like this:
[[email protected] ~]# uname -r
2.6.32-8.el6
For the version of Unix kernel, use uname -v:
Typical Linux output:
[[email protected] ~]# uname -v
#1 SMP Fri Jan 26 14:15:14 EST 2007
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Common uname usage
uname -a
Most usually, you simply use uname to output everything it knows about your system.
Mounting External Devices
Unix systems have a single directory tree. All accessible storage must have an associated
location in this single directory tree. This is unlike Windows where (in the most common syntax
for file paths) there is one directory tree per storage component (drive).
Mounting is the act of associating a storage device to a particular location in the directory tree.
For example, when the system boots, a particular storage device (commonly called the root
partition) is associated with the root of the directory tree, i.e., that storage device is mounted
on / (the root directory).
Let's say you now want to access files on a CD-ROM. You must mount the CD-ROM on a
location in the directory tree (this may be done automatically when you insert the CD). Let's say
the CD-ROM device is /dev/cdrom and the chosen mount point is /media/cdrom. The
corresponding command is
[[email protected] ~]# mount /dev/cdrom /media
After that command is run, a file whose location on the CD-ROM is /dir/file is now accessible on
your system as /media/cdrom/dir/file. When you've finished using the CD, you run the
commandumount /dev/cdrom or umount /media/cdrom (both will work; typical desktop
environments will do this when you click on the ―eject‖ or ‖safely remove‖ button).
Mounting applies to anything that is made accessible as files, not just actual storage devices. For
example, all Linux systems have a special filesystem mounted under /proc. That filesystem
(called proc) does not have underlying storage: the files in it give information about running
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processes and various other system information; the information is provided directly by the
kernel from its in-memory data structures.
To mount your hard drive you need to use the mount command. (To unmount the drive when you
are ready to unplug it and take it home, use the umount command.) Here‘s an example:
To check the id of harddrive or flash drive which we want to mount command is:
[[email protected] ~]# blkid
[[email protected] ~]# mkdir /mnt/usbdrive
[[email protected] ~]# mount /dev/sda1 /mnt/usbdrive
The above command first creates a directory in your /mnt folder, typically the folder will be
descriptive of the device, so we use /mnt/usbdrive. This only has to be done once, as you won‘t
be deleting this folder. This folder will be your mount point for the external hard drive.
The second line mounts the device at the /dev/sda1 location to the /mnt/usbdrive directory.
Now you can reference your hard drive from the command line using the folder /mnt/usbdrive.
Creating New Partition
Mounted File Systems vs. Logical Volumes
There are two ways to configure a new disk drive into a RHEL system. One very simple method
is to create one or more Linux partitions on the new drive, create Linux file systems on those
partitions and then mount them at specific mount points so that they can be accessed. This is the
approach that will be covered in this chapter.
Another approach is to add the new space to an existing volume group or create a new volume
group. When RHEL is installed using the default disk configuration layout, a volume group is
created and called VolGroup00. Within this volume group are two logical volumes named
LogVol00 and LogVol01 that are used to store the / file system and swap partition respectively.
By configuring the new disk as part of a volume group we are able to increase the disk space
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available to the existing logical volumes. Using this approach we are able, therefore, to increase
the size of the / file system by allocating some or all of the space on the new disk to LogVol00.
Let‘s assumes that the new physical hard drive has been installed on the system and is visible to
the RHEL operating system. The best way to do this is to enter the system BIOS during the boot
process and ensuring that the BIOS sees the disk drive. Sometimes the BIOS will provide a menu
option to scan for new drives. If the BIOS does not see the disk drive double check the
connectors and jumper settings (if any) on the drive.
Finding the New Hard Drive in RHEL 6
Assuming the drive is visible to the BIOS it should automatically be detected by the operating
system. Typically, the disk drives in a system are assigned device names beginning hd or sd
followed by a letter to indicate the device number. Typically hd devices are attached to an IDE
disk controller and sd devices are attached to a SATA controller. For example, the first device on
a SATA controller might be /dev/sda, the second /dev/sdb and so on.
The following is output from a system with only one physical disk drive:
[[email protected] ~]# ls /dev/sd*
/dev/sda /dev/sda1 /dev/sda2 /dev/sda3 /dev/sda4 /dev/sda5
This shows that the disk drive represented by /dev/sda is itself divided into 5 partitions,
represented by /dev/sda1 through /dev/sda5. In all likelihood, when a second disk drive is
detected by the system it will be assigned to /dev/sdb.
The following output is from the same system after a second hard disk drive has been installed:
[[email protected] ~]# ls /dev/sd*
/dev/sda /dev/sda1 /dev/sda2 /dev/sda3 /dev/sda4 /dev/sda5 /dev/sdb
As shown above, the new hard drive has been assigned to the device file /dev/sdb. At this point
the drive has no partitions shown (because we have yet to create any).
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At this point we have a choice of creating partitions and file systems on the new drive and
mounting them for access or adding the disk as a physical volume as part of a volume group.
Creating Linux Partitions
The next step is to create one or more Linux partitions on the new disk drive. This is achieved
using the fdisk utility which takes as a command-line argument the device to be partitioned:
[[email protected] ~]# fdisk /dev/sdb
Device contains neither a valid DOS partition table, nor Sun, SGI or OSF disklabel
Building a new DOS disklabel. Changes will remain in memory only,
until you decide to write them. After that, of course, the previous
content won't be recoverable.
The number of cylinders for this disk is set to 4177.
There is nothing wrong with that, but this is larger than 1024,
and could in certain setups cause problems with:
1) software that runs at boot time (e.g., old versions of LILO)
2) booting and partitioning software from other OSs
(e.g., DOS FDISK, OS/2 FDISK)
Warning: invalid flag 0x0000 of partition table 4 will be corrected by w(rite)
Command (m for help):
In order to view the current partitions on the disk enter the p command:
Command (m for help): p
Disk /dev/sdb: 34.3 GB, 34359738368 bytes
255 heads, 63 sectors/track, 4177 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Device Boot Start End Blocks Id System
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As we can see from the above fdisk output the disk currently has no partitions because it is a
previously unused disk. The next step is to create a new partition on the disk, a task which is
performed by entering n (for new partition) and p (for primary partition):
Command (m for help): n
Command action
e extended
p primary partition (1-4)
p
Partition number (1-4):
In this example we only plan to create one partition which will be partition 1. Next we need to
specify where the partition will begin and end. Since this is the first partition we need it to start at
cylinder 1 and since we want to use the entire disk we specify the last cylinder as the end. Note
that if you wish to create multiple partitions you can specify the size of each partition by
cylinders, bytes, kilobytes or megabytes.
Command (m for help): n
Command action
e extended
p primary partition (1-4)
p
Partition number (1-4): 1
First cylinder (1-4177, default 1):
Using default value 1
Last cylinder or +size or +sizeM or +sizeK (1-4177, default 4177):
Using default value 4177
Now that we have specified the partition we need to write it to the disk using the w command:
Command (m for help): w
The partition table has been altered!
Calling ioctl() to re-read partition table.
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Syncing disks.
If we now look at the devices again we will see that the new partition is visible as /dev/hdb1:
[[email protected] ~]# partprobe
[[email protected] ~]# ls /dev/sd*
/dev/sda /dev/sda2 /dev/sda4 /dev/sdb
/dev/sda1 /dev/sda3 /dev/sda5 /dev/sdb1
The next step is to create a file system on our new partition.
Creating a File System on an RHEL Disk Partition
We now have a new disk installed, it is visible to RHEL and we have configured a Linux
partition on the disk. The next step is to create a Linux file system on the partition so that the
operating system can use it to store files and data. The easiest way to create a file system on a
partition is to use the mkfs.ext4 utility which takes as arguments the label and the partition
device:
[[email protected] ~]# mkfs.ext4 –L /userdata /dev/sdb1
Filesystem label=/userdata
OS type: Linux
Block size=4096 (log=2)
Fragment size=4096 (log=2)
4194304 inodes, 8387930 blocks
419396 blocks (5.00%) reserved for the super user
First data block=0
Maximum filesystem blocks=4294967296
256 block groups
32768 blocks per group, 32768 fragments per group
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16384 inodes per group
Superblock backups stored on blocks:
32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632, 2654208,
4096000, 7962624
Writing inode tables: done
Creating journal (32768 blocks): done
Writing superblocks and filesystem accounting information: done
This filesystem will be automatically checked every 34 mounts or
180 days, whichever comes first. Use tune2fs -c or -i to override.
Mounting a File System
Now that we have created a new file system on the Linux partition of our new disk drive we need
to mount it so that it is accessible to the RHEL system and its users. In order to do this we need
to create a mount point. A mount point is simply a directory or folder into which the file system
will be mounted. For the purposes of this example we will create a /userdata directory to match
our file system label (although it is not necessary that these values match):
[[email protected] ~]# mkdir /userdata
The file system may then be manually mounted using the mount command:
[[email protected] ~]# mount /dev/sdb1 /userdata
Running the mount command with no arguments shows us all currently mounted file systems
(including our new file system):
[[email protected] ~]# mount
/dev/mapper/VolGroup00-LogVol00 on / type ext4 (rw)
proc on /proc type proc (rw)
sysfs on /sys type sysfs (rw)
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devpts on /dev/pts type devpts (rw,gid=5,mode=620)
/dev/sda3 on /boot type ext4 (rw)
tmpfs on /dev/shm type tmpfs (rw)
none on /proc/sys/fs/binfmt_misc type binfmt_misc (rw)
sunrpc on /var/lib/nfs/rpc_pipefs type rpc_pipefs (rw)
/dev/sdb1 on /userdata type ext4 (rw)
Configuring RHEL 6 to Automatically Mount a File System
In order to set up the system so that the new file system is automatically mounted at boot time an
entry needs to be added to the
/etc/fstab file.
The following example shows an fstab file configured to automount our /userdata partition:
/dev/VolGroup00/LogVol00 / ext4 defaults 1 1
LABEL=/boot /boot ext4 defaults 1 2
tmpfs /dev/shm tmpfs defaults 0 0
ysfs /sys sysfs defaults 0 0
proc /proc proc defaults 0 0
/dev/VolGroup00/LogVol01 swap swap defaults 0 0
LABEL=/userdata /userdata ext4 defaults 1 2
With the appropriate configuration line added to the fstab file, the file system will automatically
mount on the next system restart.
Creating Swap Partition
1. Firstly if there is swap partition in your system, stop swap
[[email protected] ~]# free -m (to check swap is working)
[[email protected] ~]# swapon -s (to check swap is on which partition)
2. First remove the entry of swap from fstab
3. [[email protected] ~]# fdisk /dev/hda
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(delete swap and reboot)
4. Now create
:n
:l (for seeing the swap code)
:t (to give code)
Partition no: 5
Hex code :82 (for swap)
:w (save)
5. [[email protected] ~]# partprobe
6. [[email protected] ~]# mkswap /dev/hda5 (to make partition swap)
7. [[email protected] ~]# swapon -a /dev/hda5
8. [[email protected] ~]# e2label /dev/hda5 hda5-swap (to give label to partition)
9. Now in fstab
LABEL=hda5-swap swap swap defaults 0 0
:wq (save)
10. Now reboot your system and you will find a new swap partition.
Logical Volume Manager
Logical Volume Manager (LVM) enables you to be much more flexible with your disk usage
than you can be with conventional old-style file partitions. Normally if you create a partition,
you have to keep the partition at that size indefinitely. For example, if your system logs have
grown immensely, and you‘ve run out of room on your /var partition, increasing a partition size
without LVM is a big pain.
You would have to get another disk drive, create a /var mount point on there too, and copy all
your data from the old /var to the new /var disk location. With LVM in place, you could add
another disk, and then assign that disk to be part of the /var partition. Then you‘d use the LVM
file system resizing tool to increase the file system size to match the new partition size.
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Normally you might think of disk drives as independent entities, each containing some data
space. When you use LVMs, you need a new way of thinking about disk space. First you have to
understand that space on any disk can be used by any file system. A Volume Group is the term
used to describe various disk spaces (either whole disks or parts of disks) that have been grouped
together into one volume.
Volume groups are then bunched together to form Logical volumes. Logical volumes are akin to
the historic idea of partitions. You can then use a file system creation tool such as fdisk to create
a file system on the logical volume. The Linux kernel sees a logical volume in the same way it
sees a regular partition. Some Linux tools for modifying logical volumes are pvcreate for
creating physical volumes, vgcreate for creating volume groups, vgdisplay for showing volume
groups, and mke2fs for creating a file system on your logical volume.
First create Physical Volume (PV)
[[email protected] ~]# pvcreate /dev/hda5 /dev/hda6 or
[[email protected] ~]# pvcreate /dev/hda{5,6}
[[email protected] ~]# pvdisplay (to see whether pv is created successfully)
Now create Volume Group
[[email protected] ~]# vgcreate vg0 /dev/hda{5,6} (200 mb)
[[email protected] ~]# vgdisplay (to see whether vg is created successfully)
Now we will create logical volume
[[email protected] ~]# lvcreate -L +50M /dev/vg0 -n lv0 (it will create a logical volume of
50 Mb{52 Mb})
To extend the logical volume
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[[email protected] ~]# lvextend -L +25M /dev/vg0/lv0 (it will extend the logical volume to
approx. 80 Mb)
To reduce the logical volume
[[email protected] ~]# lvreduce -L -25M /dev/vg0/lv0 (it will reduce the logical volume to
25 Mb)
For removing Logical volumes first we have to remove logical volume then volume group and
then physical volume.
[[email protected] ~]# lvremove /dev/vg0/lv0
[[email protected] ~]# vgremove /dev/vg0
[[email protected] ~]# pvremove /dev/hda5 /dev/hda6
RAID (Redundant Array of Independent Disk)
Metadevices are virtual block devices that are made up of other block devices. An example of a
metadevice is a disk array that makes many disks look like one large disk. When a disk that‘s
mounted as a regular block device dies, then the data on it becomes unavailable. If a disk dies in
a metadevice, the metadevice is still up. As long as the criteria are met for the minimum number
of working devices in the metadevice, the metadevice still functions.
RAID are of two types:
1. Software RAID
2. Hardware RAID
RAID 1 IN SOFTWARE
RAID stands for Redundant Array of Independent Disks. RAID 1, known as disk mirroring, is a
redundant RAID disk mode. A mirror of the first disk is kept on the other disks. If all disks crash
but one, all data can still be recovered. To work properly, RAID1 needs two or more disks, and
zero or more spare disks.
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RAID 5 IN SOFTWARE
RAID 5 combines the ability to use a large number of disks while still maintaining some
redundancy. It uses three or more disks, and spare disks are optional. The final RAID 5 array
contains the combined file size of all disks except one. The equivalent of one disk‘s worth of
space is taken up by the parity information, which is written evenly across all the disks. A RAID
5 array can survive one disk loss, but if more than one disk fails, all data is lost.
RAID IN HARDWARE
The principles of the software RAID levels also apply to hardware RAID setups. The main
difference is that in hardware RAID the disks have their own RAID controller with built-in
software that handles the RAID disk setup, and I/O. To Linux, the hard RAID interface is
transparent, so the hardware RAID disk array looks like one giant disk.
First create 4 (ex- had 7,8,9,10) partition of equal size.
Now, for Raid level 5 (minimum three harddisk)
[[email protected] ~]# mdadm -C /dev/md0 -n3 /dev/had{8,9,10} -l5 (by this command we
have successfully created raid level 5)
To format the md0
[[email protected] ~]# mkfs.ext4 /dev/md0
Now mount md0 in a folder
[[email protected] ~]# mkdir /raid
[[email protected] ~]# mount /dev/md0 /raid
To check the status of raid devices
[[email protected] ~]# mdadm -D /dev/md0
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[[email protected] ~]# cat /proc /mdstat
To make any partition faulty
[[email protected] ~]# mdadm -f /dev/md0 /dev/hda10
To remove the partition from raid
[[email protected] ~]# mdadm -r /dev/md0 /dev/hda10
To add new partition in raid array
[[email protected] ~]# mdadm -a /dev/md0 /dev/hda7
To stop the raid
[[email protected] ~]# mdadm -S /dev/md0
Before stopping, we have to unmount raid
To activate raid
[[email protected] ~]# mdadm -A /dev/md0 /dev/hda {7,8,9}
(mount before activating)
Now for RAID level 1 (Mirroring) (minimum two harddisk required)
[[email protected] ~]# mdadm -C /dev/md0 --chunk=64 --level=1 --raid-devices=2
/dev/hda{6,7}
Now format & mount same as above.
User Quota
Quota allows you to specify limits on two aspects of disk storage: the number of inodes a user or
a group of users may possess; and the number of disk blocks that may be allocated to a user or a
group of users.
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The idea behind quota is that users are forced to stay under their disk consumption limit, taking
away their ability to consume unlimited disk space on a system. Quota is handled on a per user,
per file system basis. If there is more than one file system which a user is expected to create files,
then quota must be set for each file system separately.
Firstly edit file /etc/fstab for adding quota for users
[[email protected] ~]# vi /etc/fstab
LABEL=home /home ext4 defaults,usrquota 0 0
Save the file
[[email protected] ~]# mount -o remount /home (remount home partitions)
[[email protected] ~]# quotacheck -cu /home ( to check whether quota is applied
or not)
[[email protected] ~]# quotaon /home (to start the quota)
Now, add two users and edit file for user
[[email protected] ~]# edquota user (edit this file) or
[[email protected] ~]# setquota user 512 512 40 50 /home
It will set quota for the user that he can only use 512kb of home partition space.
When you type command edquota user, it will open a file, now in that file there are two terms
hard limit and soft limit.
Soft Limit
Soft limit indicates the maximum amount of disk usage a quota user has on a partition. When
combined with grace period, it acts as the border line, which a quota user is issued warnings
about his impending quota violation when passed.
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Hard Limit
Hard limit works only when grace period is set. It specifies the absolute limit on the disk usage,
which a quota user can't go beyond his hard limit.
[[email protected] ~]# quotaoff /home (to stop quota)
Using Access Control list
On an ext4 filesystem, read, write, and execute permissions can be set for the owner of the file,
the group associated with the file, and for everyone else who has access to the filesystem. These
files are visible with the ls -l command.
In most cases, these standard file permissions along with restricted access to mounting
filesystems are all that an administrator needs to grant file privileges to users and to prevent
unauthorized users from accessing important files. However, when these basic file permissions
are not enough, access control lists, or ACLs, can be used on an ext4 filesystem. ACLs expand
the basic read, write, and execute permissions to more categories of users and groups. In addition
to permissions for the owner and group for the file, ACLs allow for permissions to be set for any
user, any user group, and the group of all users not in the group for the user. An effective rights
mask, which is explained later, can also be set
to restrict permissions.
There are four categories of ACLs per file: for an individual user, for a user group, via the
effective rights mask, and for users not in the user group associated with the file. To view the
existing ACLs for a file, execute the following:
[[email protected] ~]# getfacl <file>
If ACLs are enabled, the output should look similar to Listing
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# file: testfile
# owner: tfox
# group: tfox
user::rwx
group::r-x
mask::rwx
other::r-x
To set or modify existing ACLs, use the following syntax:
[[email protected] ~]# setfacl -m <rules> <file>
Replace <file> with one or more space-separated file or directory names. Rules can be set for
four different rule types. Replace <rules> with one or more of the following, and replace
<perms> in these rules with one or more of r, w, and x (which stand for read, write, and execute):
For an individual user:
u: <uid> : <perms>
For a specific user group:
g: <gid> : <perms>
For users not in the user group associated with the file:
o: <perms>
Via the effective rights mask:
m: <perms>
The first three rule types (individual user, user group, or users not in the user group for the file)
are pretty self-explanatory. They allow you to give read, write, or execute permissions to users in
these three categories. A user or group ID may be used, or the actual username or group name.
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EXAMPLES
Granting an additional user read access
setfacl -m u:user:r file
Revoking write access from all groups and all named users (using the effective rights mask)
setfacl -m m::rx file
Removing a named group entry from a file's ACL
setfacl -x g:staff file
Copying the ACL of one file to another
getfacl file1 | setfacl --set-file=- file2
Copying the access ACL into the Default ACL
getfacl -a dir | setfacl -d -M- dir
Runlevel Service Management in Linux
We can indicate desired runlevels for each services in Linux
Services are located in: /etc/init.d
ntsysv:
Usage:
1. ntsysv - manages services in the current run-level
2. ntsysv 35 - manages services for run-levels 3 & 5
Note: ntsysv or chkconfig starts|stops services
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Chkconfig
Usage:
1. chkconfig --list ntpd - returns run-level environment for 'ntpd'
Note: items listed as 'off' have K (kill) scripts Note: items listed as 'on' have S (start) scripts
2. chkconfig --level 3 ntpd off - creates a K(kill) script in run-level 3
3. chkconfig --level 35 ntpd off
4. chkconfig ntpd on - enables 'ntpd' in levels 2-5
5. chkconfig ntpd off - disables 'ntpd' in levels 0-6
Note: Use 'chkconfig' from the shell or a script
Note: Use 'ntsysv' from the shell in interactive mode
Note: When controlling services using 'chkconfig', reference the name of the service as it's
specified in: /etc/init.d
system-config-services - GUI tool to manage services
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NFS Server
The Network File System (NFS) was developed to allow machines to mount a disk partition on a
remote machine as if it were a local disk. It allows for fast, seamless sharing of files across a
network.
It also gives the potential for unwanted people to access your hard drive over the network (and
thereby possibly read your email and delete all your files as well as break into your system) if
you set it up incorrectly.
Packages:
nfs
Port Number:
2049 – nfsd
Configuration File:
/etc/exports
Service:
rpcbind
nfs
Daemons:
nfsd
1. Create a directory and make files or paste files you want to share from in network. (ex.-
/dump)
2. Edit file for sharing
[[email protected] ~]# vi /etc/exports
/dump *(rw)
/share 192.168.1.15(ro) (save the file)
In first line, we have share /dump for all users in a network with read & write permission & in
second line, we have shared /share only for the PC having IP address 192.168.1.15 with read
only permission
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3. [[email protected] ~]# service nfs restart
4. [[email protected] ~]# service rpcbind restart
5. [[email protected] ~]# showmount -e
To check NFS Server and its shared directory from Server itself
6. [[email protected] ~]# showmount -e <IP address of Server >
To check NFS Server and its shared doc from client
7. CLIENT END: For taking NFS server service we have to mount directory from server to
client
[[email protected] ~]# mount -t nfs <server IP>:/var/dump /mnt
[[email protected] ~]# cd /mnt
here you will find all the shared files or
[[email protected] ~]# cd /net
[[email protected] ~]# cd /IP of NFS Server
8. For installation from NFS, copy whole DVD/CD in a folder and share it.
9. Boot from CD & press Tab Key and write linux askmethod on client side, here it will
ask to select the installation media, select NFS, now it will ask to give IP for your client
computer, give IP, Now it will ask for Name of NFS Server: <Give IP of NFS Server>,
and directory : <Give the path>. Now your system will install very quickly with the help
of NFS server.
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SAMBA SERVER
Samba is a suite of utilities that allows your Linux PC to share files and other resources, such as
printers, with Windows PC. This part describe you how can you make your Linux PC into a
Windows Primary Domain Controller (PDC) or a server for a Windows Workgroup. Either
configuration will allow everyone at home to have:
Their own logins on all the home windows PC while having their files on the Linux
PC appear to be located on a new Windows drive
Shared access to printers on the Linux PC
Shared files accessible only to members of their Linux user group.
What's the difference between a PDC and Windows Workgroup member?
A PDC stores the login information in a central database on its hard drive. This
allows each user to have a universal username and password when logging in from all
PCs on the network.
In a Windows Workgroup, each PC stores the usernames and passwords locally so
that they are unique for each PC.
This part will only cover the much more popular PDC methodology used at home. By default,
Samba mimics a Windows PDC in almost every way needed for simple file sharing. Linux
functionality doesn't disappear when you do this. Samba Domains and Linux share the same
usernames so you can log into the Samba based Windows domain using your Linux password
and immediately gain access to files in your Linux user's home directory. For added security you
can make your Samba and Linux passwords different.
When it starts up, and with every client request, the Samba daemon reads the configuration file
/etc/samba/smb.conf to determine its various modes of operation. You can create your own
smb.conf using a text editor or the Web-based SWAT utility which is easier. Keep in mind,
however, that if you create /etc/samba/smb.conf with a text editor then subsequently use SWAT
to edit the file, you will lose all the comments you inserted with the text editor.
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First we will learn about the simple sharing between a Samba server and a windows PC
1. [[email protected] ~]# yum install samba* (Install samba package) or
[[email protected] ~]# rpm -ivh samba*
2. Now, edit file
[[email protected] ~]# vi /etc/samba/smb.conf
(at last of file, pressing ‗G‘ copy the following line and paste the lines and then remove
‗;‘ infont of the line.)
[My Share] (Share name)
comment= (any comment)
path= (here give the path which you want to share)
valid users= (Here give samba users)
public= no
writable=no (Here give the permission you want to give)
printable=no
save the file.
3. [[email protected] ~]# service smb restart
4. To create Samba users
[[email protected] ~]# useradd <username>
[[email protected] ~]# smbpasswd -a <username>
5. To check samba is configured of not
[[email protected] ~]# testparm
6. Samba users entry can be find in /etc/samba/smbpasswd
7. On client computers: on run: \\<IP address of samba server>
Now it will ask for samba user give username and password.
Now we make samba server as windows domain controller
[[email protected] ~]# vi /etc/samba/smb.conf (edit file)
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Global Setting
Workgroup=FIRST #remove semicolon and give workgroup name
Netbios name=FIRST # remove semicolon and give Netbios name
# Domain controller options
Domain logons =yes # remove semicolons
Domain master=yes # remove semicolons
#Browser Control options
# remove semicolons
Local master = no
OS level = 35 #increase from 33 i.e. default
Preffered master = yes
#Share Definations
Net logon # remove semicolons
Profiles # remove semicolons
[Profiles] # remove semicolons
Path=/home/%u # remove semicolons and give the profile path
Browseable =yes # remove semicolons
Writable=yes # remove semicolons
Guest ok =yes
and save file :wq
Create samba user
[[email protected] ~]# smbpasswd -a root
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[[email protected] ~]# smbpasswd
give samba passwd
[[email protected] ~]# service smb restart
Now at client side, windows PC, MY Computer Properties Computer Name Change
domain name FIRST. Give root samba password, restart the system and then login from the
samba user.
SQUID SERVER
The utility squid is an internet proxy server that can be used within a network to distribute an
internet connection to all the computers within the network. One central computer is connected
to the internet through any means such as dial-up, cable modem, ISDN, DSL, or T1, runs squid,
and thus acts as the firewall to the internet. Because it is a proxy, it has the capabilities to log all
user actions such as the URLs visited. There are many features that can be configured in squid.
Reasons to Create a Squid Proxy
Two important goals of many small businesses are to:
Reduce Internet bandwidth charges
Limit access to the Web to only authorized users.
The Squid web caching proxy server can achieve these fairly easily.
Users configure their web browsers to use the Squid proxy server instead of going to the web
directly. The Squid server then checks its web cache for the web information requested by the
user. It will return any matching information that finds in its cache, and if not, it will go to the
web to find it on behalf of the user. Once it finds the information, it will populate its cache with
it and also forward it to the user's web browser.
As you can see, this reduces the amount of data accessed from the web. Another advantage is
that you can configure your firewall to only accept HTTP web traffic from the Squid server and
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no one else. Squid can then be configured to request usernames and passwords for each user that
users its services. This provides simple access control to the Internet.
Packages required:
Squid
Port Number:
3128 (default)
Configuration File:
/etc/squid/squid.conf
Service/Daemon:
squid
For squid your system must have two lancard one for internet line & other for your
private network.
1. [[email protected] ~]# yum install squid* or
[[email protected] ~]# rpm -ivh Squid*
2. Edit file
[[email protected] ~]# vi /etc/squid/squid.conf
(in this file set line no. & remove ‗#‘ and do following changes)
Search for the line, if not found then write the same
http_port 3128 #deafult port number on which squid will
work
Cache_dir ufs /var/spool/squid 100 16 256 # directory where cache is stored
access_log /var/log/squid/access.log squid #Path where access logs are
maintained
Cache_log /var/log/Squid/cache.log # Path where cache logs are
maintained
cache_mgr [email protected] #change with your email-address
or name
Now for Access list
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Search for the line acl CONNECT method CONNECT, type under that line
acl <any name> url_regex <Websites you want to block or initials,
downloadings>
acl xyz url_regex orkut mp3 downloading www.gmail.com
http_access deny <above name> (xyz) or
acl xyz url_regex www.google.com (Only this website you want your client
to access)
http_access allow xyz
http_access deny all (save the file)
3. [[email protected] ~]# service squid restart/reload
4. [[email protected] ~]# netstat -tulpn |grep 3128
to check whether squid is working or not
5. To check the record of website opened by client
[[email protected] ~]# vi /var/log/squid/access.log
6. Now on client PC
Redhat- FireFox- edit menu- preferences- connection setting- Manual Proxy setting-
Proxy Server IP- __________ Port no. 3128- check (use this proxy for all protocols)
On Windows:- Internet explorer- Tools- Internet Options- connections- Lan Setting-
check (use a proxy for this lan)- Give IP & Port no.- OK.
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DNS SERVER
Introduction to DNS
DNS Domains
Everyone in the world has a first name and a last, or family, name. The same thing is true in the
DNS world: A family of Web sites can be loosely described a domain. For example, the domain
demo.com has a number of children, such as www.demo.com and mail.demo.com for the Web
and mail servers, respectively.
BIND
BIND is an acronym for the Berkeley Internet Name Domain project, which is a group that
maintains the DNS-related software suite that runs under Linux. The most well known program
in BIND is named, the daemon that responds to DNS queries from remote machines.
DNS Clients
A DNS client doesn't store DNS information; it must always refer to a DNS server to get it. The
only DNS configuration file for a DNS client is the /etc/resolv.conf file, which defines the IP
address of the DNS server it should use. You shouldn't need to configure any other files. You'll
become well acquainted with the /etc/resolv.conf file soon.
Authoritative DNS Servers
Authoritative servers provide the definitive information for your DNS domain, such as the names
of servers and Web sites in it. They are the last word in information related to your domain.
How DNS Servers Find Out Your Site Information
There are 13 root authoritative DNS servers (super duper authorities) that all DNS servers query
first. These root servers know all the authoritative DNS servers for all the main domains - .com,
.net, and the rest. This layer of servers keep track of all the DNS servers that Web site systems
administrators have assigned for their sub domains.
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For example, when you register your domain mynewsite.com, you are actually inserting a record
on the .com DNS servers that point to the authoritative DNS servers you assigned for your
domain.
When To Use A DNS Caching Name Server
Most servers don‘t ask authoritative servers for DNS directly, they usually ask a caching DNS
server to do it on their behalf. These servers, through a process called recursion, sequentially
query the authoritative servers at the root, main domain and sub domain levels to get eventually
get the specific information requested. The most frequently requested information is then stored
(or cached) to reduce the lookup overhead of subsequent queries.
If you want to advertise your Web site www.mynewsite.com to the rest of the world, then a
regular DNS server is what you require. Setting up a caching DNS server is fairly
straightforward and works whether or not your ISP provides you with a static or dynamic
Internet IP address.
After you set up your caching DNS server, you must configure each of your home network PCs
to use it as their DNS server. If your home PCs get their IP addresses using DHCP, then you
have to configure your DHCP server to make it aware of the IP address of your new DNS server,
so that the DHCP server can advertise the DNS server to its PC clients. Off-the-shelf
router/firewall appliances used in most home networks usually can act as both the caching DNS
and DHCP server, rendering a separate DNS server is unnecessary.
Basic DNS Testing of DNS Resolution
As you know, DNS resolution maps a fully qualified domain name (FQDN), such as
www.demo.com, to an IP address. This is also known as a forward lookup. The reverse is also
true: By performing a reverse lookup, DNS can determining the fully qualified domain name
associated with an IP address.
Many different Web sites can map to a single IP address, but the reverse isn't true; an IP address
can map to only one FQDN. This means that forward and reverse entries frequently don't match.
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The reverse DNS entries are usually the responsibility of the ISP hosting your site, so it is quite
common for the reverse lookup to resolve to the ISP's domain. This isn't an important factor for
most small sites, but some e-commerce applications require matching entries to operate
correctly. You may have to ask your ISP to make a custom DNS change to correct this.
There are a number of commands you can use do these lookups. Linux uses the host command,
for example, but Windows uses nslookup.
Provides resolution of names to IP address and resolution of IP address to names, defines a
hierarchical namespace where each level of a namespace is separated by a ‖.‖.
Zone- Zone is a storage database which contains all zones records.
Forward lookup zone:- Used to resolve hostname to IP address. It maintains host to IP
mapping information.
Types of Records:-
SOA Record:- The first record in any zone file.
NS Record :- Identifies the DNS Server for each zone.
A record:- Resolves a hostname to IP address.
CNAME Record:- Resolves an alias name to a host name.
PTR Record:- Resolves IP address for hostname.
MX Record:- Used by mail server.
DNS server with bind
In order to install bind nameserver service, the bind package must be installed :
# yum install bind
The bind nameserver root directory is on /var/named and the configuration files are stored on /etc
as usual :
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/etc/sysconfig/named
Configuration file that set up how is executed on the system the bind daemon.
/etc/named.conf
Main DNS configuration file that includes data from other files.
/etc/named.rfc1912.zones
It contains appropriate zones for localhost names and addresses.
/var/named/my.internal.zone.db
Zone file for the local network.
/var/named/slaves/my.slave.internal.zone.db
Zone file for a slave nameserver.
/var/named/localdomain.zone
Zone file for localhost domain.
/var/named/localhost.zone
Zone file for localhost computer.
/var/named/named.broadcast
Broadcast record for localhost.
/var/named/named.ca
List of the root DNS servers for the Internet consulted by nameserver when a nameserver
resolution can not be done by the nameserver.
/var/named/named.ip6.local
IPv6 version of named.local.
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/var/named/named.zero
Defaults to the broadcast record for the localhost.
/var/named/data/named.stats.txt
Nameserver statistics.
Templates of these files can be found on /usr/share/doc/bind*/sample. If the package bind-chroot
is installed the nameserver root directory will be in /var/named/chroot/var/named and the
configuration directory will be in /var/named/chroot/etc.
Caching-only nameserver
When a DNS query is performed against a cache-only name server the query will be forwarded
to another nameserver if the answer for this query is not located on the dns caching-only name
server cache. When the external nameserver answers to the DNS query the caching-only name
server puts the answer on his cache and forwards the answer to the client who has made the
query. If somebody repeats the same query against the caching-only name server it will be
answered directly (faster) from the caching-only nameserver because the answer for this query
will be on the cache.
In order to configure a caching-only name server for all your LAN the file /etc/named.conf
installed by default by bind rpm must be changed in just two lines:
listen-on port 53 {127.0.0.1; }; --> listen-on port 53 {127.0.0.1; 192.168.1.10; };
allow-query {localhost; }; --> allow-query{localhost; 192.168.1.0/24; };
The first line will put the bind service listening on the localhost (127.0.0.1) and LAN
(192.168.1.10) network interfaces. The second line will give access to all hosts on
192.168.1.0/24 LAN to use this server as nameserver, once the service will be started and the
firewall open.
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The configuration file for a caching-only nameserver is as follows :
# cat /etc/named.conf
options {
listen-on port 53 { 127.0.0.1; 192.168.1.10; };
listen-on-v6 port 53 { ::1; };
directory "/var/named";
dump-file "/var/named/data/cache_dump.db";
statistics-file "/var/named/data/named_stats.txt";
memstatistics-file "/var/named/data/named_mem_stats.txt";
allow-query { localhost; 192.168.1.0/24; };
recursion yes;
dnssec-enable yes;
dnssec-validation yes;
dnssec-lookaside auto;
/* Path to ISC DLV key */
bindkeys-file "/etc/named.iscdlv.key"; };
logging {
channel default_debug {
file "data/named.run";
severity dynamic;
};
};
zone "." IN {
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type hint;
file "named.ca";
};
include "/etc/named.rfc1912.zones";
Once this is done the next step is start bind daemon and make sure it will start on boot. Make
sure that security considerations described on 'DNS security' are also applied. :
# /etc/init.d/named start
# chkconfig named on
With this configuration a cache-only nameserver will be serving name resolution on
192.168.1.10 for all 192.168.1.0/24 LAN. Have a look on lab1...
Forward nameserver
This kind of nameserver only needs a simple configuration option on /etc/named.conf file that
configures the nameservers that the forwarding nameserver will forward all DNS queries. The
/etc/named.conf from caching-only nameserver that is installed by default with bind RPM can be
used to generate the forward nameserver configuration file just adding the following lines at
'option' section :
# Set nameserver type to forward
forward only;
# Configures the nameservers IPs where DNS queries will be forwarded
forwarders {192.168.1.20; 192.168.1.30; }
The configuration file for a forwarding-only nameserver is as follows :
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# cat /etc/named.conf
options {
listen-on port 53 { 127.0.0.1; 192.168.1.10; };
listen-on-v6 port 53 { ::1; };
directory "/var/named";
dump-file "/var/named/data/cache_dump.db";
statistics-file "/var/named/data/named_stats.txt";
memstatistics-file "/var/named/data/named_mem_stats.txt";
allow-query { localhost; 192.168.1.0/24; };
recursion yes;
forward only;
forwarders {192.168.1.20; 192.168.1.30; };
dnssec-enable yes;
dnssec-validation yes;
dnssec-lookaside auto;
/* Path to ISC DLV key */
bindkeys-file "/etc/named.iscdlv.key";
};
logging {
channel default_debug {
file "data/named.run";
severity dynamic;
};
};
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zone "." IN {
type hint;
file "named.ca";
};
include "/etc/named.rfc1912.zones";
With this configuration the nameserver will be configured to forward all DNS queries that are
not cached only to 192.168.1.20 , 192.168.1.30 DNS servers. The nameserver is caching +
forward.
Once this is done the next step is start bind daemon and make sure it will start on boot. Make
sure that security considerations described on 'DNS security' are also applied. :
# /etc/init.d/named start
# chkconfig named on
Master nameserver
When the nameserver is configured to serve name resolution for an specified domain (local
domain or Internet domain) that server has the authoritative DNS records for that domain. This
nameserver is consulted by other nameservers when a resolution for the domain where it is
authoritative is performed on others servers.
In order to configure a nameserver as master nameserver for a domain the bind RPM must be
installed. Next step is copy the file /usr/share/doc/bind*/sample/etc/named.conf to
/etc/named.conf file (or /var/named/chroot/etc/named.conf if the package bind-chroot also has
been installed ) and perform the following changes :
# Make named daemon to listen on the nameserver IPv4 network IP (192.168.1.10 in this case)
plus the localhost IP (127.0.0.1)
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listen-on port 53 { 127.0.0.1; 192.168.1.10 };
# Allow only query from clients on your LAN plus localhost
allow-query { localhost; 192.168.1.0/24; };
allow-query-cache { localhost; 192.168.1.0/24; };
# Introduce on the 'view localhost_resolver' and 'view internal' the direct and reverse zone file.
The direct file maps your domain hostnames with the numerical IP address, the reverse zone file
maps the numerical IP values with their correspondents hostnames
view localhost_resolver {
...
# Direct authoritative zone file for our domain 'demo.com'
zone "demo.com" {
type master;
file "demo.com.zone";
};
# Reverse authoritative zone file for our domain 'demo.com'
zone "1.168.192.in-addr.arpa" IN {
type master;
file "demo.com.rr.zone";
allow-update { none; };
};
};
# The same for 'view internal'. On this view substitute 'zone "my.internal.zone"' by 'zone
"demo.com"' and create the reverse zone 'view localhost_resolver'.
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# On 'view internal' comment all 'zone "my.ddns.internal.zone', this nameserver is not going to
be updated dynamically...
# In order to prevent unauthorized access to the named daemon, BIND uses a shared secret key
authentication method to grant privileges to hosts. This means an identical key must be present in
both /etc/named.conf and the rndc configuration file, /etc/rndc.conf.
key ddns_key {
algorithm hmac-md5;
secret "N7ypFzAWQrEo2nzwigFIRSTKA==";
};
# Remove 'view external' section, in this case this nameserver is not going to allow DNS queries
from clients outside the LAN.
After all this changes the file /etc/named.conf will be as follows :
# cat /etc/named.conf
options
{
// Put files that named is allowed to write in the data/ directory:
directory "/var/named"; // "Working" directory
dump-file "data/cache_dump.db";
statistics-file "data/named_stats.txt";
memstatistics-file "data/named_mem_stats.txt";
listen-on port 53 { 127.0.0.1; 192.168.1.10; };
//listen-on port 53 { 127.0.0.1; };
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listen-on-v6 port 53 { any; };
//listen-on-v6 port 53 { ::1; };
allow-query { localhost; 192.168.1.0/24; };
allow-query-cache { localhost; 192.168.1.0/24; };
// Enable/disable recursion - recursion yes/no;
recursion yes;
/* DNSSEC related options. See information about keys ("Trusted keys", bellow) */
/* Enable serving of DNSSEC related data - enable on both authoritative
and recursive servers DNSSEC aware servers */
dnssec-enable yes;
/* Enable DNSSEC validation on recursive servers */
dnssec-validation yes;
/* Enable DLV by default, use built-in ISC DLV key. */
dnssec-lookaside auto;
};
logging
{
channel default_debug {
file "data/named.run";
severity dynamic;
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};
};
view "localhost_resolver"
{
/* This view sets up named to be a localhost resolver ( caching only nameserver ).
* If all you want is a caching-only nameserver, then you need only define this view:
*/
match-clients { localhost; };
recursion yes;
# all views must contain the root hints zone:
zone "." IN {
type hint;
file "/var/named/named.ca";
};
/* these are zones that contain definitions for all the localhost
* names and addresses, as recommended in RFC1912 - these names should
* not leak to the other nameservers:
*/
include "/etc/named.rfc1912.zones";
zone "demo.com" {
type master;
file "demo.com.zone";
};
zone "1.168.192.in-addr.arpa" IN {
type master;
file "demo.com.rr.zone";
allow-update { none; };
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};
};
view "internal"
{
/* This view will contain zones you want to serve only to "internal" clients
that connect via your directly attached LAN interfaces - "localnets" .
*/
match-clients { localnets; };
recursion yes;
zone "." IN {
type hint;
file "/var/named/named.ca";
};
/* these are zones that contain definitions for all the localhost
* names and addresses, as recommended in RFC1912 - these names should
* not leak to the other nameservers:
*/
include "/etc/named.rfc1912.zones";
// These are your "authoritative" internal zones, and would probably
// also be included in the "localhost_resolver" view above :
//zone "mynternal.zone".internal.zone" {
// type master;
// file "my.internal.zone.db";
//};
zone "my.slave.internal.zone" {
type slave;
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file "slaves/my.slave.internal.zone.db";
masters { /* put master nameserver IPs here */ 127.0.0.1; } ;
// put slave zones in the slaves/ directory so named can update them
};
//zone "my.ddns.internal.zone" {
// type master;
// allow-update { key ddns_key; };
// file "dynamic/my.ddns.internal.zone.db";
// // put dynamically updateable zones in the slaves/ directory so named can update
them
//};
zone "demo.com" {
type master;
file "demo.com.zone";
};
zone "1.168.192.in-addr.arpa" IN {
type master;
file "demo.com.rr.zone";
allow-update { none; };
};
};
key ddns_key
{
algorithm hmac-md5;
secret "N7ypFzAWQrEo2nzwigFIRSTKA==";
};
...
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Now is time to create the direct and reverse zone files on /var/named directory for our domain
'demo.com' :
# cat /var/named/demo.com.zone
$TTL 86400
@ IN SOA rhel6.demo.com. root.rhel6.demo.com. (
42 ; serial (d. adams)
3H ; refresh
15M ; retry
1W ; expiry
1D ) ; minimum
IN NS rhel6
IN MX 10 rhel6
node01 IN A 192.168.1.101
node02 IN A 192.168.1.102
rhel6 IN A 192.168.1.10
server IN CNAME rhel6
www IN CNAME rhel6
ftp IN CNAME rhel6
# cat /var/named/demo.com.rr.zone
@ IN SOA rhel6.demo.com. root.rhel6.demo.com. (
1997022700 ; Serial
28800 ; Refresh
14400 ; Retry
3600000 ; Expire
86400 ) ; Minimum
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IN NS rhel6.demo.com.
101 IN PTR node01.demo.com.
102 IN PTR node02.demo.com.
10 IN PTR rhel6.demo.com.
Once the direct and reverse zone files has been created the ownership must be changed to named
user and restart the named service :
# chown named:named demo.com.zone demo.com.rr.zone
# /etc/init.d/named restart
Now from node01, configuring rhel6 server as nameserver for demo.com domain , we can test
direct and reverse name resolution against demo.com domain:
node01> cat /etc/resolv.conf
search demo.com
nameserver 192.168.1.10
# Direct www.demo.com name resolution
node01> dig www.demo.com
...
;; ANSWER SECTION:
www.demo.com. 86400 IN CNAME rhel6.demo.com.
rhel6.demo.com. 86400 IN A 192.168.1.10
...
;; SERVER: 192.168.1.10#53(192.168.1.10)
# Reverse 192.168.1.102 resolution
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node01> dig -x 192.168.1.102
...
;; ANSWER SECTION:
102.1.168.192.in-addr.arpa. 86400 IN PTR node02.demo.com.
...
;; SERVER: 192.168.1.10#53(192.168.1.10)
Slave nameserver
In order to configure a nameserver as slave nameserver for a domain, the following configuration
must be added on the internal view :
zone "example.org" IN {
type slave;
file "slaves/example.com.org";
masters {
192.168.1.50
};
};
The task for a slave server is easier; it periodically checks with the master DNS server (in this
case 192.168.1.50.) When it does, it automatically reads the master DNS server data and creates
the slave.example.com. zone file in the /var/named/slaves directory.
The dig command can be used to force a zone transfer from the master nameserver to the slave
nameserver. From slave nameserver execute the following command:
# dig @master_nameserver_ip example.org axfr
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APACHE SERVER
Apache Server is used for hosting website for Internet/Intranet. By this server we can also
authenticate only limited users to access website. For this server you must have DNS server
configured or add your IP and server name in hosts file. Apache is probably the most popular
Linux-based Web server application in use. Once you have DNS correctly setup and your server
has access to the Internet, you'll need to configure Apache to accept surfers wanting to access
your Web site.
Packages:
httpd*
1. Install package for http.
[[email protected] ~]# yum install http* or
[[email protected] ~]# rpm -ivh http*
2. Now make a webpage.
[[email protected] ~]# vi /var/www/html/demo.html
(for single webpage, for virtual hosting make another page in another location, )
3. Now when we enter the DNS address, apache server welcome screen comes, if we want
to add our own homepage we have to edit a file.
[[email protected] ~]# vi /etc/httpd/conf/httpd.conf (at last of the file)
</virtual Host *: 80> (change * with IP address)
Document root /var/www/html
Servername www.demo.com
</virtual Host> (save the file)
4. [[email protected] ~]# service httpd restart
5. Now when you enter the Domain name (www.demo.com) it will open your webpage
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6. AUTHENTICATION:
open the same file and at the end of file, type the following
<Directory /var/www/html>
AuthUserfile /etc/httpd/conf/htpasswd
AuthName ―Web Authentication‖
AuthType Basic
Require Valid-User
</Directory> (save the file)
7. Adduser for http
[[email protected] ~]# useradd demo
[[email protected] ~]# htpasswd -c /etc/httpd/conf/htpasswd demo
(it creates a new file htpasswd and add user demo in it)
For adding more user
[[email protected] ~]# htpasswd /etc/httpd/conf/htpasswd <username>
again start service
8. Virtual Hosting-
First copy the above four lines
<Virtualhost *:80> (change * with IP address at both place)
Documentroot /var/www
Servername www.abc.com
</Virtualhost>
Come to the lines above these lines
Name Virtual Host *:80
remove * and # change * with IP address of the system
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FTP SERVER
The File Transfer Protocol (FTP) is used as one of the most common means of copying files
between servers over the Internet. Most web based download sites use the built in FTP
capabilities of web browsers and therefore most server oriented operating systems usually
include an FTP server application as part of the software suite. Linux is no exception.
This server will show you how to convert your Linux PC into an FTP server using the default
Very Secure FTP Daemon (VSFTPD) package included in RHEL6.
FTP relies on a pair of TCP ports to get the job done. It operates in two connection channels :
FTP Control Channel, TCP Port 21: All commands you send and the ftp server's responses to
those commands will go over the control connection, but any data sent back (such as "ls"
directory lists or actual file data in either direction) will go over the data connection.
FTP Data Channel, TCP Port 20: This port is used for all subsequent data transfers between
the client and server.
Regular FTP
By default, the VSFTPD package allows regular Linux users to copy files to and from their home
directories with an FTP client using their Linux usernames and passwords as their login
credentials.
VSFTPD also has the option of allowing this type of access to only a group of Linux users,
enabling you to restrict the addition of new files to your system to authorized personnel.
The disadvantage of regular FTP is that it isn't suitable for general download distribution of
software as everyone either has to get a unique Linux user account or has to use a shared
username and password. Anonymous FTP allows you to avoid this difficulty.
Anonymous FTP
Anonymous FTP is the choice of Web sites that need to exchange files with numerous unknown
remote users. Common uses include downloading software updates and MP3s and uploading
diagnostic information for a technical support engineers' attention. Unlike regular FTP where
you login with a preconfigured Linux username and password, anonymous FTP requires only a
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username of anonymous and your email address for the password. Once logged in to a VSFTPD
server, you automatically have access to only the default anonymous FTP directory (/var/ftp in
the case of VSFTPD) and all its subdirectories.
Required Package
vsftpd*
1. [[email protected] ~]# rpm -ivh vsftpd*
[[email protected] ~]# yum install vsftpd*
2. [[email protected] ~]# vi /etc/vsftpd/vsftpd.conf
We have to remove # infront of the following lines.
anonymous = default user
#FTP uses two users anonymous & FTP anonymous has no password
local_enable=yes #Local user can login through FTP
write_enable=yes #write permission is enabled to FTP
anon_upload_enable=yes #anonymous user can upload files
anon_mkdir_write_enable=yes #anonymous user can create directory
dir message enable=yes
Ftpd_banner= welcome to FTP
3. [[email protected] ~]# service vsftpd restart
4. Client end:
ftp <Server IP>
>cd pub
>get <filename> (download)
>put <filename> (upload)
>bye (exit)
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DHCP SERVER
As the network grows bigger , administering each and every computer can becomes difficult. If
you need to change critical information of your domain, then it is impossible to give these new
information to hundreds of computers in the network manually. A new member to your network
means adding a new user account to multiple computers.
DHCP provides a method of dynamically configuring IP addresses, network numbers and server
locations for the computers on your local LAN.
Setting Dynamic Host Configuration Protocol (DHCP) server allows you to centrally manage all
the IP and network information for client computers on your LAN. When you have configured a
DHCP server on your network all the clients will be able to choose between whether they will
get there information dynamically or statically. DHCP server, when properly configured can
provide IP address, network mask, DNS server, NetBIOS server, router (gateway), and other
information needed to get up and running on the LAN.
After the configuration, you can simplify the initial network configuration that each client
computer on your LAN need to do. The DHCP is a very robust server and you can easily expand
the network with it.
Packages required:
dhcp*
Port numbers:
67 Bootp, 68 DHCP
Configuration file:
/etc/dhcp/dhcpd.conf
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Service/Daemon:
dhcpd
Step 1. You need to install DHCP package.
Example:
[root@localhost ~]# yum install dhcp*
Sample file copy and paste the specified location /etc/dhcp/dhcpd.conf and get the dhcp service.
Step 2. [root@localhost ~]# cd /usr/share/doc/dhcp-4.1.1
Step 3. [root@localhost ~]# cp -v dhcpd.conf.sample /etc/dhcp/dhcpd.conf
#### Open the DHCP configuration file and add the following ########
Step 4.[root@localhost ~]# vim /etc/dhcp/dhcpd.conf
##### change subnet, netmask, and range ############
subnet 192.168.0.0 netmask 255.255.255.0 {
range 192.168.0.2 192.168.0.10;
option domain-name-servers ns1.internal.example.org;
option domain-name "internal.example.org";
#option routers 10.5.5.1;
#option broadcast-address 10.5.5.31;
default-lease-time 600;
max-lease-time 7200;
}
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Step 5. [root@localhost ~]# service dhcpd restart
Step 6. [root@localhost ~]# chkconfig dhcpd on
TELNET
1. # yum install xinetd*
2. # yum install telnet*
3. Now we have to edit a file for enabling telnet.
# vi /etc/xinetd.d/telnet
disabled= no (by default it is yes, we have do it no.)
(save the file)
4. Start the service
# service xinetd restart
5. Now you must have a user in the server pc with whom you login and after that with su
command you can take the root access.
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NIS Server (Centralization)
The Network Information Service distributes information on the network that must be shared
between the Linux machines. Sun Microsystems developed a system known as Yellow Pages
(YP) that eventually became known as NIS. The original YP is added to most commands for
NIS ypserver, ypbind, and yppasswd. Typically NIS is used to distribute information about the
users and how they authenticate to Linux. Specifically, /etc/passwd and /etc/group. NIS
provides a means to central administration of the network users. When a user logs into a system
using NIS, the system first checks the local /etc/passwd file , if it cannot find the user it will then
check the NIS server.
The structure used for NIS is the normal client-server setup . The configuration for NIS uses a
domain that is used to indicate a group of systems on a LAN or subnet. Each of these systems on
the domain will use the same map, which refers to a common database. These databases are
stored in a DBM format which is based on ASCII text files.
Each NIS domain needs to have a NIS server that acts as a information base for the domain. The
NIS clients will query the NIS server for information that they will need. Maps are the
information that is stored in the database.
Figure 30 NIS Server
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You can divide the NIS servers into masters and slaves to provide a level of redundancy for the
maps. The slave will maintain a copy of the maps and will update fro the master whenever a
change is made. The program that notifies the slave of any changes is yppush
There are four basic topologies used for NIS:
1. single domain – one master no slaves
2. single domain with slaves – one master, one or more slaves
3. multiple domains – each domain has a master but no slaves
4. multiple domains with slaves – each domain with a mast and one or more slaves
It only makes sense that if you are really interested in having NIS work right you will need at
least one master and one slave as a backup.
Configuring a NIS server
Install the necessary applications.
[[email protected] ~]# yum install ypserv*
Set Up the Domain Name
Use this command:
[[email protected] ~]# nisdomainname mydomain
You can set to see if the domain is active by using the command without the argument.
[[email protected] ~]#domainname mydomain
Start the Server
Before starting the server make sure portmap is running by issuing the command:
service portmap start
[[email protected] ~]# service portmap restart , service ypserv restart
You can use either start/stop/restart/status. Yo can also use rpcinfo to see information.
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rpcinfo -p localhost
Create Databases
The database is created with this command:
[[email protected] ~]# ./usr/lib/yp/ypinit -m
When you initialize the maps you will need to know what slave servers you may want to enter as
those names can be added at the time of the database creation. Specify the hostname and any
slave and then use Ctrl+D, select ―y: and it is complete.
The -m of the command tells NIS that the database is created for the master server.
Once the slave has been set up as a client you can add the database from the masterserver with
this command:
[[email protected] ~]# ./usr/lib/yp/ypinit -s masterhost
This tells NIS to get the database from the masterhost server.
[[email protected] ~]# service portmap restart , service ypserv restart
CLIENT SIDE:
Starting NIS Servers at boot
Install ypbind package. You need to make changes permanent and make them so they are
available at start time. Open /etc/sysconfig/network with a text editor and add a line:
NISDOMAIN=mydomainname
mydomainname must be the name that you have chosen.
Now open a terminal and use chkconfig.
chkconfig –level 35 yppasswd on
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Adding Users to the Database
Add a user with the normal commands and password set up.
[[email protected] ~]# useradd ab
[[email protected] ~]# passwd ab
Move into the /var/yp directory and run make.
cd /var/yp
make
NTP SERVER
The Network Time Protocol (NTP) is a protocol used to help synchronize your Linux system's
clock with an accurate time source. There are that allow the general public to synchronize with
them. They are divided into two types:
Stratum 1: NTP sites using an atomic clock for timing.
Stratum 2: NTP sites with slightly less accurate time sources.
It is good practice to have at least one server on your network be the local time server for all your
other devices. This makes the correlation of system events on different systems much easier. It
also reduces Internet bandwidth usage due to NTP traffic and reduces the need to manage
firewall rules for each NTP client on your network. Sometimes, not all your servers will have
Internet access; in such cases you'll need a central server that all can access
The NTP program is configured using either the /etc/ntp.conf file depending on what distribution
of Linux you have. I won't go into too much detail on how to configure NTP. Instead I'll just
cover the basics.
An example of a basic ntp.conf file would look like:
# --- GENERAL CONFIGURATION ---
server aaa.bbb.ccc.ddd
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server 127.127.1.0
fudge 127.127.1.0 stratum 10
# Drift file.
driftfile /etc/ntp/drift
The most basic ntp.conf file will simply list 2 servers, one that it wishes to synchronize with, and
a pseudo IP address for itself (in this case 127.127.1.0). The pseudo IP is used in case of network
problems or if the remote NTP server goes down. NTP will synchronize against itself until the it
can start synchronizing with the remote server again. It is recommended that you list at least 2
remote servers that you can synchronize against. One will act as a primary server and the other as
a backup.
You should also list a location for a drift file. Over time NTP will "learn" the system clock's
error rate and automatically adjust for it.
The restrict option can be used to provide better control and security over what NTP can do, and
who can effect it. For example:
# Prohibit general access to this service.
restrict default ignore
# Permit systems on this network to synchronize with this
# time service. But not modify our time.
restrict aaa.bbb.ccc.ddd nomodify
# Allow the following unrestricted access to ntpd
restrict aaa.bbb.ccc.ddd
restrict 127.0.0.1
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NTP slowly corrects your systems time. Be patient! A simple test is to change your system clock
by 10 minutes before you go to bed and then check it when you get up. The time should be
correct.
Many people get the idea that instead of running the NTP daemon, they should just setup
a cron job job to periodically run the ntpdate command.
Mantaining centralized logs via SYSLOG Server
There are hundreds of Linux applications on the market, each with their own configuration files
and help pages. This variety makes Linux vibrant, but it also makes Linux system administration
daunting. Fortunately, in most cases, Linux applications use the syslog utility to export all their
errors and status messages to files located in the /var/log directory.
This can be invaluable in correlating the timing and causes of related events on your system. It is
also important to know that applications frequently don't display errors on the screen, but will
usually log them somewhere. Knowing the precise message that accompanies an error can be
vital in researching malfunctions in product manuals, online documentation, and Web searches.
syslog, and the logrotate utility that cleans up log files, are both relatively easy to configure but
they frequently don't get their fair share of coverage in most texts. I've included syslog here as a
dedicated chapter to both emphasize its importance to your Linux knowledge and prepare you
with a valuable skill that will help you troubleshoot all the Linux various applications that will be
presented throughout the book
syslog
syslog is a utility for tracking and logging all manner of system messages from the merely
informational to the extremely critical. Each system message sent to the syslog server has two
descriptive labels associated with it that makes the message easier to handle.
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The first describes the function (facility) of the application that generated it. For example,
applications such as mail and cron generate messages with easily identifiable facilities named
mail and cron.
The second describes the degree of severity of the message. There are eight in all and they are
listed in :
You can configure syslog's /etc/rsyslog.conf configuration file to place messages of differing
severities and facilities in different files. This procedure will be covered next.
Syslog Facilities
0 Emergencies System unusable
1 Alerts Immediate action required
2 Critical Critical condition
3 Errors Error conditions
4 Warnings Warning conditions
5 Notifications Normal but significant conditions
6 Informational Informational messages
7 Debugging Debugging messages
Configuring the Linux Syslog Server
By default syslog doesn't expect to receive messages from remote clients. Here's how to
configure your Linux server to start listening for these messages.
As we saw previously, syslog checks its /etc/rsyslog.conf file to determine the expected names
and locations of the log files it should create. It also checks the file /etc/sysconfig/syslog to
determine the various modes in which it should operate. Syslog will not listen for remote
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messages unless the SYSLOGD_OPTIONS variable in this file has a -r included in it as shown
below.
# Options to syslogd
# -m 0 disables 'MARK' messages.
# -r enables logging from remote machines
# -x disables DNS lookups on messages received with -r
# See syslogd(8) for more details
SYSLOGD_OPTIONS="-m 0 -r"
# Options to klogd
# -2 prints all kernel oops messages twice; once for klogd to decode, and
# once for processing with 'ksymoops'
# -x disables all klogd processing of oops messages entirely
# See klogd(8) for more details
KLOGD_OPTIONS="-2"
Note: In Debian / Ubuntu systems you have to edit the syslog startup script /etc/init.d/sysklogd
directly and make the SYSLOGD variable definition become "-r".
# Options for start/restart the daemons
# For remote UDP logging use SYSLOGD="-r"
#
#SYSLOGD="-u syslog"
SYSLOGD="-r"
You will have to restart syslog on the server for the changes to take effect. The server will now
start to listen on UDP port 514, which you can verify using either one of the following netstat
command variations.
[root@demo tmp]# netstat -a | grep syslog
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udp 0 0 *:syslog *:*
[root@demo tmp]# netstat -an | grep 514
udp 0 0 0.0.0.0:514 0.0.0.0:*
[root@demo tmp]#
Configuring the Linux Client
The syslog server is now expecting to receive syslog messages. You have to configure your
remote Linux client to send messages to it. This is done by editing the /etc/hosts file on the Linux
client named smallfry. Here are the steps:
1) Determine the IP address and fully qualified hostname of your remote logging host.
2) Add an entry in the /etc/hosts file in the format:
IP-address fully-qualified-domain-name hostname "loghost"
Example:
192.168.1.100 demo.my-site.com demo loghost
Now your /etc/hosts file has a nickname of "loghost" for server demo.
3) The next thing you need to do is edit your /etc/rsyslog.conf file to make the syslog messages
get sent to your new loghost nickname.
*.debug @loghost
*.debug /var/log/messages
You have now configured all debug messages and higher to be logged to both server demo
("loghost") and the local file /var/log/messages. Remember to restart syslog to get the remote
logging started.
You can now test to make sure that the syslog server is receiving the messages with a simple test
such as restarting the lpd printer daemon and making sure the remote server sees the messages.
Linux Client
[root@smallfry tmp]# service lpd restart
Stopping lpd: [ OK ]
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Starting lpd: [ OK ]
[root@smallfry tmp]#
Linux Server
[root@demo tmp]# tail /var/log/messages
...
...
Apr 11 22:09:35 smallfry lpd: lpd shutdown succeeded
Apr 11 22:09:39 smallfry lpd: lpd startup succeeded
...
...
[root@demo tmp]#
Installing KVM Virtualization on RHEL 6
Virtualization is generically defined as the ability to run multiple operating systems
simultaneously on a single computer system. Whilst not necessarily a new concept,
Virtualization has come to prominence in recent years because it provides a way to fully utilize
the CPU and resource capacity of a server system whilst providing stability (in that if one
virtualized guest system crashes, the host and any other guest systems continue to run).
Virtualization is also useful in terms of trying out different operating systems without having to
configure dual boot environments. For example, you can run Windows in a virtual machine
without having to re-partition the disk, shut down Red Hat Enterprise Linux 6 and then boot
from Windows. You simply start up a virtualized version of Windows as a guest operating
system. Similarly, virtualization allows you to run other Linux distributions from within an
RHEL 6 system, providing concurrent access to both operating systems.
Prior to Red Hat Enterprise Linux 6 two virtualization platforms were provided with RHEL in
the form of KVM and Xen. With the release of RHEL 6, support for Xen has been removed
leaving KVM as the only bundled virtualization option supplied with RHEL 6. In addition to
KVM, third party solutions are available in the form of products such as VMware and Oracle
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VirtualBox. Since KVM is supplied with RHEL 6. Before plunging into installing and running
KVM it is worth taking a little time to talk about two different types of virtualization and the
corresponding hardware requirements.
Full Virtualization, Para-Virtualization and Emulation
There are two common approaches to virtualization - full virtualization and para-virtualization.
Full virtualization provides complete abstraction between the hardware and the guest operating
system. In this scenario, the guest operating system is provided a complete virtual physical
environment in which to run and, as such, is unaware that it is running inside a virtual machine.
One advantage of full virtualization is that the operating system does not need to be modified in
order to run in a virtualized environment. This means that proprietary operating systems such as
Windows can be run on Linux systems. Disadvantages of full virtualization are that performance
is slightly reduced as compared to para-virtualization, and some virtualization platforms, such as
Xen and KVM, require CPUs with special virtualization support built in (such as Intel-VT and
AMD-V).
Para-virtualization requires that a guest operating system be modified to support virtualization.
This typically means that guest operating systems are limited to open source systems such as
Linux. It is also not possible to migrate a running guest OS from one server to another. The
advantage to this approach, however, is that a para-virtualized guest system comes closer to
native performance than a fully virtualized guest, and the latest virtualization CPU support is not
needed. Whilst Xen supports both full and para-virtualization, KVM supports only full
virtualization. As such, KVM can only be used on CPUs that support either Intel-VT or AMD-V
extensions. If your processor does not have Intel-VT or AMD-V support you will still be able to
install guest operating systems though this will be implemented using the QEMU environment
instead of KVM virtualization. QEMU uses emulation to run guest operating systems and as
such results in slower performance than the hardware assisted full-virtualization offered by
KVM.
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KVM Hardware Requirements
Before proceeding with this chapter we need to take a moment to discuss the hardware
requirements for running virtual machines within a KVM environment. Firstly, KVM
virtualization is only available on certain processor types. These processors include either Intel-
VT or AMD-V technology.
To check for Intel-VT support, run the following command in a terminal window to check for
the presence of vmx:
# grep vmx /proc/cpuinfo
flags: fpu tsc msr pae mce cx8 apic mtrr mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2
ss ht tm syscall nx lm constant_tsc pni monitor ds_cpl vmx est tm2 cx16 xtpr lahf_lm
To check for AMD-V support, run the following command which checks for the presence of
svm:
# grep svm /proc/cpuinfo
flags: fpu tsc msr pae mce cx8 apic mtrr mca cmov pat pse36 clflush mmx fxsr sse sse2 ht
syscall nx mmxext fxsr_opt rdtscp lm 3dnowext 3dnow pni cx16 lahf_lm cmp_legacy svm
cr8_legacy
Note that while the above commands only report whether the processor supports the respective
feature, it does not indicate whether the feature is currently enabled in the BIOS. In practice
virtualization support is typically disabled in the BIOS of most systems. It is recommended,
therefore, that you check your BIOS settings to ensure the appropriate virtualization technology
is enabled before proceeding with this tutorial.
Unlike a dual booting environment a virtualized environment involves the running of two or
more complete operating systems concurrently on a single computer system. This means that the
system must have enough physical memory, disk space and CPU processing power to
comfortably accommodate all these systems in parallel. Before beginning the configuration and
installation process check on the minimum system requirements for both RHEL 6 and your
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chosen guest operating systems and verify that your system has sufficient resources to handle the
requirements of both systems.
32-bit Support for KVM on RHEL 6
Whilst previous versions of RHEL supported KVM on both 32-bit and 64-bit systems, as of
RHEL 5.4 support for KVM virtualization is only available for 64-bit (x86_64 architecture)
versions of the operating system.
Preparing RHEL 6 for KVM Virtualization
Unlike Xen, it is not necessary to run a special version of the kernel in order to support KVM. As
a result KVM support is already available for use with the standard kernel via the installation of
a KVM kernel module, thereby negating the need to install and boot from a special kernel.
To avoid conflicts, however, if a Xen enabled kernel is currently running on the system, reboot
the system and select a non-Xen kernel from the boot menu before proceeding with the
remainder of this chapter.
The tools required to setup and maintain a KVM based virtualized system are not installed by
default unless specifically selected during the RHEL 6 operating system installation process. To
install KVM from the command prompt, execute the following command in a terminal window
with root privileges:
yum install kvm virt-manager libvirt
If the installation fails with a message similar to the following then it is likely that you are
attempting to install KVM on a 32-bit system, or a 64-bit system running a 32-bit version of
RHEL 6:
Error: libvirt conflicts with kvm
Once the installation of KVM is complete, close any applications that are running and reboot the
system.
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Verifying the KVM Installation
Once the system has restarted after the reboot it is worthwhile checking that the installation
worked correctly before moving forward. When KVM is installed and running two modules will
have been loaded into the kernel. The presence or otherwise of these modules can be verified in a
terminal window by running the following command:
su -
lsmod | grep kvm
Assuming that the installation was successful the above command should generate output similar
to the following:
lsmod | grep kvm
kvm_intel 45578 0
kvm 291875 1 kvm_intel
Note that if the system contains an AMD processor the kvm module will likely read kvm_amd
rather than kvm_intel. The installation process should also have configured the libvirtd daemon
to run in the background. Once again using a terminal window with super user privileges, run the
following command to ensure libvirtd is running:
/sbin/service libvirtd status
libvirtd (pid 2958) is running...
If the process is not running, it may be started as follows:
/sbin/service libvirtd start
Finally, run the virt-manager tool by selecting the Applications -> System Tools -> Virtual
Machine
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Figure 31 Virtual Machine Manager
If the QEMU entry is not listed, select the File -> Add Connection menu option and in the
resulting dialog select the QEMU/KVM hypervisor before clicking on the Connect button:
Figure 32 connecting Virtual Machine
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If the manager is not currently connected to the virtualization processes, right click on the entry
listed and select Connect from the popup menu.
In theory the system is now ready to create virtual machines into which guest operating systems
may be installed and run. Before taking that step, it is worth spending some time in the next
chapter talking about virtual networks and network bridges.
Using TCP wrappers to secure Linux
TCP Wrappers can be used to GRANT or DENY access to various services on your machine to
the outside network or other machines on the same network. It does this by using simple Access
List Rules which are included in the two files /etc/hosts.allow and /etc/hosts.deny .
A remote machine remote_mc trying to connect to your local machinelocal_mc using ssh.
When the request from the remote_mc is received by the tcp wrapped service (SSH in this case),
it takes the following basic steps:
1. It checks the /etc/hosts.allow file and applies the first rule specified for that service. If it
finds a matching rule , it allows the connection. If no rule is found, it moves on to step 2
2. It checks the /etc/hosts.deny file and if a matching rule is found, it deny's the
connection.
Points to remember
Rules in hosts.allow takes precedence over rules in hosts.deny . Which means if a
matching rule is found in hosts.allow file, the remote_mc is allowed access to
the service even if there is a matching deny rule in hosts.deny file.
You can have only one rule per service in hosts.allow and hosts.deny file.
If there are no matching rules in either of the files or if the files don't exist, then
the remote_mc is allowed access to the service.
Any changes to hosts.allow and hosts.deny file takes immediate effect.
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Rule Syntax
The syntax for both hosts.allow and hosts.deny file takes the following form:
daemon : client [:option1:option2:...]
Where daemon can be a combination of ssh daemon, ftp daemon, portmap daemon and so on.
Basically any service which has support for libwrap.a library compiled into it is a good
candidate for utilizing the services of TCP Wrappers.
client is a comma separated list of hostnames, host IP addresses, special patterns or special
wildcards which identify the hosts effected by that rule.
options is an optional action like say sending mail to the administrator when this rule is matched,
log to a particular file and so on. It can be a colon separated list of actions too.
Examples of using TCP Wrappers
I want to allow SSH access to hosts in a particular domain say xyz.com and deny access to all the
others. I enter the following rule in the hosts.allow file.
sshd : .xyz.com...
and in the hosts.deny file I include the rule:
sshd : ALL
The next rule denys FTP access to all the hosts in the abc.co.in domain as well as hosts in
the192.168.1.0 network.
#FILE: /etc/hosts.deny
vsftpd : 192.168.1. , .abc.co.in : spawn /bin/echo `/bin/date` access denied >>
var/log/vsftpd.log : deny
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The backslash (\) in the above rule is used to break the line and prevents the failure of the rule
due to length.
spawn and deny are options. Spawn launches a shell command as a child process. In the above
rule, spawn logs a message to the vsftpd log file each time the rule matches. deny is optional if
you are including this rule in the hosts.deny file.
Note: The last line in the files hosts.allow and hosts.deny must be a new line character. Or else
the rule will fail.
For example, you can use spawn option to send mail to the admin when ever a deny rule is
matched.
Wildcards
You can use wildcards in the client section of the rule to broadly classify a set of hosts. These are
the valid wildcards that can be used.
ALL - Matches everything
LOCAL - Matches any host that does not contain a dot (.) like localhost.
KNOWN - Matches any host where the hostname and host addresses are known or
where the user is known.
UNKNOWN - Matches any host where the hostname or host address are unknown or
where the user is unknown.
PARANOID - Matches any host where the hostname does not match the host address.
Patterns
You can also use patterns in the client section of the rule . Some examples are as follows:
ALL : .xyz.com
Matches all hosts in the xyz.com domain . Note the dot (.) at the beginning.
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ALL : 123.12.
Matches all the hosts in the 123.12.0.0 network. Note the dot (.) in the end of the rule.
ALL : 192.168.0.1/255.255.255.0
IP address/Netmask can be used in the rule.
ALL : *.xyz.com
Asterisk * matches entire groups of hostnames or IP addresses.
sshd : /etc/sshd.deny
If the client list begins with a slash (/), it is treated as a filename. In the above rule, TCP
wrappers looks up the file sshd.deny for all SSH connections.
sshd : ALL EXCEPT 192.168.0.15
If the above rule is included in the /etc/hosts.deny file, then it will allow ssh connection for only
the machine with the IP address 192.168.0.15 and block all other connections. Here EXCEPT is
an operator.
Note: If you want to restrict use of NFS and NIS then you may include a rule for portmap .
Because NFS and NIS depend on portmap for their successful working. In addition, changes to
portmap rules may not take effect immediately.
Suppose I want to log all connections made to SSH with a priority of emergency. See my
previous post to know more on logging. I could do the following:
sshd : .xyz.com : severity emerg
Note: You can use the options allow or deny to allow or restrict on a per client basis in either of
the files hosts.allow and hosts.deny
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in.telnetd : 192.168.5.5 : deny
in.telnetd : 192.168.5.6 : allow
Shell Commands
As mentioned above, you can couple the rules to certain shell commands by using the following
two options.
spawn - This option launches a shell command as a child process. For example, look at the
following rule:
sshd : 192.168.5.5 : spawn /bin/echo `/bin/date` from %h >> /var/log/ssh.log : deny
Each time the rule is satisfied, the current date and the clients hostname %h is appended to
thessh.log file.
twist - This is an option which replaces the request with the specified command. For example, if
you want to send to the client trying to connect using ssh to your machine, that they are
prohibited from accessing SSH, you can use this option.
sshd : client1.xyz.com : twist /bin/echo "You are prohibited from accessing this service!!" :
deny
When using spawn and twist, you can use a set of expressions. They are as follows :
%a — The client's IP address.
%A — The server's IP address.
%c — Supplies a variety of client information, such as the username and hostname, or
the username and IP address.
%d — The daemon process name.
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%h — The client's hostname (or IP address, if the hostname is unavailable).
%H — The server's hostname (or IP address, if the hostname is unavailable).
%n — The client's hostname. If unavailable, unknown is printed. If the client's hostname and
host address do not match, paranoid is printed.
%N — The server's hostname. If unavailable, unknown is printed. If the server's hostname and
host address do not match, paranoid is printed.
%p — The daemon process ID.
%s — Various types of server information, such as the daemon process and the host or IP
address of the server.
%u — The client's username. If unavailable, unknown is printed.
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SHELL SCRIPTING
What is Linux Shell ?
Shell is a user program or it's environment provided for user interaction. Shell is an command
language interpreter that executes commands read from the standard input device (keyboard) or
from a file.
Shell is not part of system kernel, but uses the system kernel to execute programs, create files
etc.
Several shell available with Linux including:
Shell Name Developed by Where Remark
BASH ( Bourne-Again
SHell )
Brian Fox and Chet
Ramey
Free Software
Foundation
Most common shell in
Linux. It's Freeware
shell.
CSH (C SHell) Bill Joy University of
California (For BSD)
The C shell's syntax
and usage are very
similar to
the C programming
language.
KSH (Korn SHell) David Korn AT & T Bell Labs --
TCSH See the man page.
Type $ man tcsh
-- TCSH is an enhanced
but completely
compatible version of
the Berkeley UNIX C
shell (CSH).
To find all available shells in your system type following command:
$ cat /etc/shells
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To find your current shell type following command
$ echo $SHELL
What is Shell Script ?
Normally shells are interactive. It means shell accept command from you (via keyboard) and
execute them. But if you use command one by one (sequence of 'n' number of commands) , the
you can store this sequence of command to text file and tell the shell to execute this text file
instead of entering the commands. This is know as shell script.
Shell script defined as:
"Shell Script is series of command written in plain text file. Shell script is just like batch file is
MS-DOS but have more power than the MS-DOS batch file."
Why use Shell Script:
Shell script can take input from user, file and output them on screen.
Useful to create our own commands.
Save lots of time.
To automate some task of day today life.
System Administration part can be also automated.
Execute your script as
syntax:
bash your-script-name
sh your-script-name
./your-script-name
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Examples:
$ sh first.sh
$ ./first.sh
Variables in Shell
To process our data/information, data must be kept in computers RAM memory. RAM memory
is divided into small locations, and each location had unique number called memory
location/address, which is used to hold our data. Programmer can give a unique name to this
memory location/address called memory variable or variable (Its a named storage location that
may take different values, but only one at a time).
In Linux (Shell), there are two types of variable:
(1) System variables - Created and maintained by Linux itself. This type of variable defined in
CAPITAL LETTERS.
(2) User defined variables (UDV) - Created and maintained by user. This type of variable
defined in lower letters.
You can see system variables by giving command like $ set, some of the important System
variables are:
System Variable Meaning
BASH=/bin/bash Our shell name
BASH_VERSION=1.14.7(1) Our shell version name
COLUMNS=80 No. of columns for our screen
HOME=/home/User Our home directory
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LINES=25 No. of columns for our screen
LOGNAME=demo students Our logging name
OSTYPE=Linux Our Os type
PATH=/usr/bin:/sbin:/bin:/usr/sbin Our path settings
PS1=[\u@\h \W]\$ Our prompt settings
PWD=/home/demo/Common Our current working directory
SHELL=/bin/bash Our shell name
USERNAME=demo User name who is currently login to this PC
To define UDV use following syntax
Syntax:
variable name=value
'value' is assigned to given 'variable name' and Value must be on right side = sign.
Example:
$ no=22# this is ok
$ 22=no# Error, NOT Ok, Value must be on right side of = sign.
To define variable called 'abc' having value name
$ abc=name
To define variable called no having value 1
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$ no=1
ECHO COMMAND
The echo command displays a message on the screen and is primarily useful for programmers
writing shell scripts. But anyone can use echo to show the value of environment variables. Here
are some examples:
echo [options] [string, variables...]
Options
-n Do not output the trailing new line.
-e Enable interpretation of the following backslash escaped characters in the strings:
\a alert (bell)
\b backspace
\c suppress trailing new line
\n new line
\r carriage return
\t horizontal tab
\\ backslash
$ echo -e "Hello \a\t\tFreinds\n"
echo $PATH
/usr/bin:/usr/local/bin
echo My name is $USER - Home directory=$HOME.
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My name is demo - Home directory=/home/demo.
Example:
$ vi first.sh
#
#
# Script to print user information who currently login , current date & time
#
clear
echo "Hello $USER"
echo "Today is \c ";date
echo "Number of user login : \c" ; who | wc –l
echo "Calendar"
cal
exit 0
Arithmatic operations
EXPR Function
expr evaluates each numeric_expr command line argument as a separate numeric expression.
Thus a single expression cannot contain unescaped whitespaces or needs to be placed in a quoted
string (i.e. between "{" and "}"). Arithmetic is performed on signed integers (currently numbers
in the range from -2,147,483,648 to 2,147,483,647). Successful calculations cause no output (to
either standard out/error or environment variables). So each useful numeric_expr needs to
include an assignment (or op-assignment). Each numeric_expr argument supplied is evaluated in
the order given (i.e. left to right) until they all evaluate successfully (returning a true status). If
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evaluating a numeric_expr fails (usually due to a syntax error) then the expr command fails with
"error" as the exit status and the error message is written to the environment variable "error".
Syntax:
expr op1 math-operator op2
Examples:
$ expr 1 + 3
$ expr 2 – 1
$ expr 10 / 2
$ expr 20 % 3
$ expr 10 \* 3
$ echo `expr 6 + 3`
Note:
expr 20 %3 - Remainder read as 20 mod 3 and remainder is 2.
expr 10 \* 3 - Multiplication use \* and not * since its wild card.
For the last statement not the following points
(1) First, before expr keyword we used ` (back quote) sign not the (single quote i.e. ') sign. Back
quote is generally found on the key under tilde (~) on PC keyboard OR to the above of TAB key.
(2) Second, expr is also end with ` i.e. back quote.
(3) Here expr 6 + 3 is evaluated to 9, then echo command prints 9 as sum
(4) Here if you use double quote or single quote, it will NOT work
For e.g.
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$ echo "expr 6 + 3" # It will print expr 6 + 3
$ echo 'expr 6 + 3' # It will print expr 6 + 3
There are three types of quotes
Quotes Name Meaning
" Double
Quotes
"Double Quotes" - Anything enclose in double quotes removed
meaning of that characters (except \ and $).
' Single quotes 'Single quotes' - Enclosed in single quotes remains unchanged.
` Back quote `Back quote` - To execute command
Example:
$ echo "Today is date"
Can't print message with today's date.
$ echo "Today is `date`".
It will print today's date as, Today is Tue Jan ....,Can you see that the `date` statement uses back
quote?
Exit Status
By default in Linux if particular command/shell script is executed, it return two type of values
which is used to see whether command or shell script executed is successful or not.
(1) If return value is zero (0), command is successful.
(2) If return value is nonzero, command is not successful or some sort of error executing
command/shell script.
This value is know as Exit Status.
But how to find out exit status of command or shell script?
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Simple, to determine this exit Status you can use $? special variable of shell.
For e.g. (This example assumes that abc file doest not exist on your hard drive)
$ rm abc
It will show error as follows
rm: cannot remove `abc`: No such file or directory and after that if you give command
$ echo $?
it will print nonzero value to indicate error. Now give command
$ ls
$ echo $?
It will print 0 to indicate command is successful.
Command line arguments
$ vi arg.sh
#!/bin/sh
#
# Script that demos, command line args
#
echo "Total number of command line argument are $#"
echo "$0 is script name"
echo "$1 is first argument"
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echo "$2 is second argument"
echo "All of them are :- $* or $@"
Run the script:
$ sh arg.sh we are Indians .
Output:
Total number of command line argument are 4
arg.sh is script name
we is first argument
are is second argument
All of them are :- we are Indians .
In above example $# gives the total no. of arguments given on command line, $0 gives the name
of the script, $1 $2 ….. $n specifies the 1st 2
nd and so on arguments, $@ and $* prints all the
agruments.
The READ Statement
Use to get input (data from user) from keyboard and store (data) to variable.
Syntax:
read variable1, variable2,...variableN
Following script first ask user, name and then waits to enter name from the user via keyboard.
Then user enters name from keyboard (after giving name you have to press ENTER key) and
entered name through keyboard is stored (assigned) to variable myname.
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$ vi second.sh
#
#Script to read your name from key-board
#
echo –n "Your first name please:"
read myname
echo "Hello $myname, Let‘s be friend!"
Run it as follows:
$ ./second.sh
Your first name please: demo
Hello demo, Let‘s be friend!
BASH STRUCTURED LANGUAGE CONSTRUCTS
Making decision is important part in ONCE life as well as in computers logical driven program.
In fact logic is not LOGIC until you use decision making. This topic introduces to the bashs
structured language constricts such as:
Decision making
Loops
if condition
if condition which is used for decision making in shell script, If given condition is true then
command1 is executed.
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Syntax:
if condition
then
command1 if condition is true or if exit status
of condition is 0 (zero)
...
...
fi
Condition is defined as:
"Condition is nothing but comparison between two values."
For compression you can use test or [ expr ] statements or even exist status can be also used.
Expreession is defined as:
"An expression is nothing but combination of values, relational operator (such as >,<, <> etc)
and mathematical operators (such as +, -, / etc )."
Following are all examples of expression:
5 > 2
3 + 6
3 * 65
a < b
c > 5
c > 5 + 30 -1
Type following commands (assumes you have file called foo)
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$ cat abc
$ echo $?
The cat command return zero(0) i.e. exit status, on successful, this can be used, in if condition as
follows,
Write shell script as
$ cat > third.sh
!/bin/sh
#
#Script to print file
#
if cat $1
then
echo -e "\n\nFile $1, found and successfully echoed"
fi
Run above script as:
$./third.sh abc
Shell script name is third.sh ($0) and abc is argument (which is $1).Then shell compare it as
follows:
if cat $1 which is expanded to if cat abc.
test command or [ expr ]
test command or [ expr ] is used to see if an expression is true, and if it is true it return zero(0),
otherwise returns nonzero for false.
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Syntax:
test expression OR [ expression ]
Example:
Following script determine whether given argument number is positive.
$ cat > positive.sh
#!/bin/sh
#
# Script to see whether argument is positive
#
if test $1 -gt 0
then
echo "$1 number is positive"
fi
Run it as follows
$ ./positive.sh 5
5 number is positive
$./positive.sh -45
Nothing is printed
$./positive.sh
./ispostive: test: -gt: unary operator expected
Detailed explanation
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The line, if test $1 -gt 0 , test to see if first command line argument($1) is greater than 0. If it is
true(0) then test will return 0 and output will printed as 5 number is positive but for -45 argument
there is no output because our condition is not true(0) (no -45 is not greater than 0) hence echo
statement is skipped. And for last statement we have not supplied any argument hence error
./positive.sh: test: -gt: unary operator expected, is generated by shell , to avoid such error we can
test whether command line argument is supplied or not.
For Mathematics, use following operator in Shell Script
Mathematical
Operator in Shell
Script
Meaning Normal
Arithmetical/
Mathematical
Statements
But in Shell
For test statement
with if command
For [ expr ]
statement with if
command
-eq is equal to 3 == 7 if test 3 -eq 7 if [ 3 -eq 7 ]
-ne is not equal
to
3 != 7 if test 3 -ne 7 if [ 3 -ne 7 ]
-lt is less than 3 < 7 if test 3 -lt 7 if [ 3 -lt 7 ]
-le is less than or
equal to
3 <= 7 if test 3 -le 7 if [ 3 -le 7 ]
-gt is greater
than
3 > 7 if test 3 -gt 7 if [ 3 -gt 7 ]
-ge is greater
than or equal
to
3 >= 7 if test 3 -ge 7 if [ 3 -ge 7 ]
NOTE: == is equal, != is not equal.
For string Comparisons use
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Operator Meaning
string1 =
string2
string1 is equal to string2
string1 !=
string2
string1 is NOT equal to string2
string1 string1 is NOT NULL or not defined
-n string1 string1 is NOT NULL and does exist
-z string1 string1 is NULL and does exist
Shell also test for file and directory types
Test Meaning
-s file Non empty file
-f file Is File exist or normal file and not a directory
-d dir Is Directory exist and not a file
-w file Is writeable file
-r file Is read-only file
-x file Is file is executable
Logical Operators
Logical operators are used to combine two or more condition at a time
Operator Meaning
! expression Logical NOT
expression1 -a expression2 Logical AND
expression1 -o expression2 Logical OR
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if...else...fi
If given condition is true then command1 is executed otherwise command2 is executed.
Syntax:
if condition
then
condition is zero (true - 0)
execute all commands up to else statement
else
if condition is not true then
execute all commands up to fi
fi
Write Script as follows:
$ vi numbers.sh
#!/bin/sh
#
# Script to see whether argument is positive or negative
#
if [ $# -eq 0 ]
then
echo "$0 : You must give/supply one integers"
exit 1
fi
if test $1 -gt 0
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then
echo "$1 number is positive"
else
echo "$1 number is negative"
fi
Try it as follows:
$ numbers.sh 5
5 number is positive
$ numbers.sh -45
-45 number is negative
$ numbers.sh
./ numbers.sh : You must give/supply one integers
$ numbers.sh 0
0 number is negative
Multilevel if-then-else
Syntax:
if condition
then
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condition is zero (true - 0)
execute all commands up to elif statement
elif condition1
then
condition1 is zero (true - 0)
execute all commands up to elif statement
elif condition2
then
condition2 is zero (true - 0)
execute all commands up to elif statement
else
None of the above condtion,condtion1,condtion2 are true (i.e.
all of the above nonzero or false)
execute all commands up to fi
fi
For multilevel if-then-else statement try the following script:
$ cat > fourth.sh
#
#!/bin/sh
# Script to test if..elif...else
#
if [ $1 -gt 0 ]; then
echo "$1 is positive"
elif [ $1 -lt 0 ]
then
echo "$1 is negative"
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elif [ $1 -eq 0 ]
then
echo "$1 is zero"
else
echo "Opps! $1 is not number, give number"
fi
Try above script as follows:
$ ./ fourth.sh
$ ./ fourth.sh -2
$ ./ fourth.sh 0
$ ./ fourth.sh a
Here o/p for last sample run:
./ fourth.sh: [: -gt: unary operator expected
./ fourth.sh: [: -lt: unary operator expected
./ fourth.sh: [: -eq: unary operator expected
Opps! a is not number, give number
Above program gives error for last run, here integer comparison is expected therefore error like
"./ fourth.sh: [: -gt: unary operator expected" occurs, but still our program notify this error to
user by providing message "Opps! a is not number, give number".
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Loops in Shell Scripts
Loop defined as:
"Computer can repeat particular instruction again and again, until particular condition satisfies.
A group of instruction that is executed repeatedly is called a loop."
Bash supports:
for loop
while loop
Note that in each and every loop,
(a) First, the variable used in loop condition must be initialized, then execution of the loop
begins.
(b) A test (condition) is made at the beginning of each iteration.
(c) The body of loop ends with a statement that modifies the value of the test (condition)
variable.
for Loop
Syntax:
for { variable name } in { list }
do
execute one for each item in the list until the list is
not finished (And repeat all statement between do and done)
done
Before try to understand above syntax try the following script:
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$ cat > for.sh
for i in 1 2 3 4 5
do
echo "Welcome $i times"
done
Run it above script as follows:
$ ./for.sh
The for loop first creates i variable and assigned a number to i from the list of number from 1 to
5, The shell execute echo statement for each assignment of i. (This is usually know as iteration)
This process will continue until all the items in the list were not finished, because of this it will
repeat 5 echo statements. To make you idea more clear try following script:
$ cat > table.sh
#!/bin/sh
#
#Script to test for loop
#
#
if [ $# -eq 0 ]
then
echo "Error - Number missing form command line argument"
echo "Syntax : $0 number"
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echo "Use to print multiplication table for given number"
exit 1
fi
n=$1
for i in 1 2 3 4 5 6 7 8 9 10
do
echo "$n * $i = `expr $i \* $n`"
done
Save above script and run it as:
$ ./table.sh 6
$ ./table.sh
For first run, above script print multiplication table of given number where i = 1,2 ... 10 is
multiply by given n (here command line argument 6) in order to produce multiplication table as
6* 1 = 6
6 * 2 = 12
...
..
6 * 10 = 60
And for second test run, it will print message –
Error - Number missing form command line argument
244
Syntax : ./table.sh number
Use to print multiplication table for given number
This happened because we have not supplied given number for which we want multiplication
table, Hence script is showing Error message, Syntax and usage of our script. This is good idea if
our program takes some argument, let the user know what is use of the script and how to used the
script.
Note that to terminate our script we used 'exit 1' command which takes 1 as argument (1
indicates error and therefore script is terminated)
Even you can use following syntax:
Syntax:
for (( expr1; expr2; expr3 ))
do
.....
...
repeat all statements between do and
done until expr2 is TRUE
Done
In above syntax BEFORE the first iteration, expr1 is evaluated. This is usually used to initialize
variables for the loop.
All the statements between do and done is executed repeatedly UNTIL the value of expr2 is
TRUE.
AFTER each iteration of the loop, expr3 is evaluated. This is usually use to increment a loop
counter.
245
$ cat > for1.sh
for (( i = 0 ; i <= 5; i++ ))
do
echo "Welcome $i times"
done
Run the above script as follows:
$ ./for1.sh
Welcome 0 times
Welcome 1 times
Welcome 2 times
Welcome 3 times
Welcome 4 times
Welcome 5 times
In above example, first expression (i = 0), is used to set the value variable i to zero.
Second expression is condition i.e. all statements between do and done executed as long as
expression 2 (i.e continue as long as the value of variable i is less than or equel to 5) is TRUE.
Last expression i++ increments the value of i by 1 i.e. it's equivalent to i = i + 1 statement.
246
while loop
Syntax:
while [ condition ]
do
command1
command2
command3
..
....
done
Loop is executed as long as given condition is true. For e.g.. Above for loop program (shown in
last section of for loop) can be written using while loop as:
$cat > while.sh
#!/bin/sh
#
#Script to test while statement
#
#
if [ $# -eq 0 ]
then
echo "Error - Number missing form command line argument"
echo "Syntax : $0 number"
echo " Use to print multiplication table for given number"
exit 1
fi
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n=$1
i=1
while [ $i -le 10 ]
do
echo "$n * $i = `expr $i \* $n`"
i=`expr $i + 1`
done
Save it and try as
$./while.sh 6
Above loop can be explained as follows:
n=$1 Set the value of command line argument to
variable n. (Here it's set to 6 )
i=1 Set variable i to 1
while [ $i -le 10 ] This is our loop condition, here if value of i is
less than 10 then, shell execute all statements
between do and done
do Start loop
echo "$n * $i = `expr $i \* $n`" Print multiplication table as
6 * 1 = 6
6 * 2 = 12
....
6 * 10 = 60, Here each time value of variable n
is multiply be i.
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i=`expr $i + 1` Increment i by 1 and store result to i. ( i.e.
i=i+1)
Caution: If you ignore (remove) this
statement than our loop become infinite loop
because value of variable i always remain less
than 10 and program will only output
6 * 1 = 6
...
...
E (infinite times)
done Loop stops here if i is not less than 10 i.e.
condition of loop is not true. Hence
loop is terminated.
Note: For infinite loop with while you have to write while :
The case Statement
The case statement is good alternative to Multilevel if-then-else-fi statement. It enable you to
match several values against one variable. Its easier to read and write.
Syntax:
case $variable-name in
pattern1) command
...
..
command;;
pattern2) command
...
..
command;;
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patternN) command
...
..
command;;
*) command
...
..
command;;
esac
The $variable-name is compared against the patterns until a match is found. The shell then
executes all the statements up to the two semicolons that are next to each other. The default is *)
and its executed if no match is found. For e.g. write script as follows:
$ cat > case.sh
#
# if no name is given
# i.e. -z $1 is defined and it is NULL
#
# if no command line arg
if [ -z $1 ]
then
rental="*** I don‘t know the person ***"
elif [ -n $1 ]
then
# otherwise make first arg as name
name=$1
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fi
case $name in
"abc") echo "Hello abc, How are you?";;
"def") echo "Hello def, How are you?";;
"ghi") echo "Hello ghi, How are you?";;
"jkl") echo "Hello jkl, How are you?";;
*) echo "Sorry, I don‘t know who are you";;
Esac
Save it by pressing CTRL+D and run it as follows:
$ ./case.sh abc
$ ./case.sh def
$ ./case.sh ghi
First script will check, that if $1(first command line argument) is given or not, if NOT given set
value of name variable to "*** I don‘t know the person ***",if command line arg is
supplied/given set value of name variable to given value (command line arg). The $name is
compared against the patterns until a match is found.
For first test run its match with abc and it will show output " Hello abc, How are you?"
For second test run it print, " Hello def, How are you?".
And for last run, there is no match for xyz, hence default i.e. *) is executed and it prints, " Sorry,
I don‘t know who are you ".
Note that esac is always required to indicate end of case statement.
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ADVANCED SHELL SCRIPTING
Functions
When program gets complex we need to use divide and conquer technique. It means whenever
programs gets complicated, we divide it into small chunks/entities which is know as function.
Function is series of instruction/commands. Function performs particular activity in shell i.e. it
had specific work to do or simply say task. To define function use following syntax:
Syntax:
function-name ( )
{
command1
command2
.....
...
commandN
return
}
Where function-name is name of you function, that executes series of commands. A return
statement will terminate the function. Example:
Type Hello() at $ prompt as follows
$ Hello()
{
echo "Hello $LOGNAME, Have nice computing"
return
}
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To execute this Hello() function just type it name as follows:
$ Hello
Hello demo, Have nice computing.
This way you can call function. Note that after restarting your computer you will loss this Hello()
function, since its created for current session only. To overcome this problem and to add you
own function to automat some of the day today life task, add your function to /etc/bashrc file. To
add function to this file you must logon as root.
# vi /etc/bashrc
# At the end of file add following in /etc/bashrc file
#
# today() to print formatted date
#
# To run this function type today at the $ prompt
#
today()
{
echo This is a `date +"%A %d in %B of %Y (%r)"`
return
}
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DIALOG UTILITY
Before programming using dialog utility you need to install the dialog utility, since dialog utility
in not installed by default.
For Red Hat Linux user install the dialog utility as follows
# mount /mnt/cdrom
# cd /mnt/cdrom/RHEL-6…./Packages
# rpm -ivh dialog-0.6-16.i686.rpm
After installation you can start to use dialog utility. Before understanding the syntax of dialog
utility try the following script:
$ cat > dialog.sh
dialog --title "Linux Dialog Utility Infobox" --backtitle "Linux Shell Script\
Tutorial" --infobox "This is dialog box called infobox, which is used \
to show some information on screen, Thanks to Savio Lam and\
Stuart Herbert to give us this utility. Press any key. . . " 7 50 ; read
Save the shell script and run it as:
$ sh dialog.sh
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After executing this dialog statement you will see box on screen with titled as "Welcome to
Linux Dialog Utility" and message "This is dialog....Press any key. . ." inside this box. The title
of box is specified by --title option and infobox with --infobox "Message" with this option. Here
7 and 50 are height-of-box and width-of-box respectively. "Linux Shell Script Tutorial" is the
backtitle of dialog show on upper left side of screen and below that line is drawn. Use dialog
utility to Display dialog boxes from shell scripts.
Figure 33 Linux Dialog Utility Infobox
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Syntax:
dialog --title {title} --backtitle {backtitle} {Box options}
where Box options can be any one of following
--yesno {text} {height} {width}
--msgbox {text} {height} {width}
--infobox {text} {height} {width}
--inputbox {text} {height} {width} [{init}]
--textbox {file} {height} {width}
--menu {text} {height} {width} {menu} {height} {tag1} item1}...
Message box (msgbox) using dialog utility
$cat > dialog1.sh
dialog --title "Linux Dialog Utility Msgbox" --backtitle "Linux Shell Script\
Tutorial" --msgbox "This is dialog box called msgbox, which is used\
to show some information on screen which has also Ok button, Thanks to Savio Lam\
and Stuart Herbert to give us this utility. Press any key. . . " 9 50
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Save it and run as
$ sh dialog1.sh
yesno box using dialog utility
$ cat > dialog2.sh
dialog --title "Alert : Delete File" --backtitle "Linux Shell Script\
Tutorial" --yesno "\nDo you want to delete '/usr/letters/jobapplication'\
file" 7 60
Figure 34 Linux Dialog Utility Msgbox
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sel=$?
case $sel in
0) echo "User select to delete file";;
1) echo "User select not to delete file";;
255) echo "Canceled by user by pressing [ESC] key";;
Esac
Save the script and run it as:
$ chmod +x dialog2.sh
$ ./dialog2.sh
Figure 35 Alert Dialog box
258
Above script creates yesno type dialog box, which is used to ask some questions to the user , and
answer to those question either yes or no. After asking question how do we know, whether user
has press yes or no button ? The answer is exit status, if user press yes button exit status will be
zero, if user press no button exit status will be one and if user press Escape key to cancel dialog
box exit status will be one 255. That is what we have tested in our above shell script as
Statement Meaning
sel=$? Get exit status of dialog utility
case $sel in
0) echo "You select to delete file";;
1) echo "You select not to delete file";;
255) echo "Canceled by you by pressing [Escape]
key";;
esac
Now take action according to exit
status of dialog utility, if exit status is
0 , delete file, if exit status is 1 do not
delete file and if exit status is 255,
means Escape key is pressed.
Input Box (inputbox) using dialog utility
$ cat > dialog3.sh
dialog --title "Inputbox - To take input from you" --backtitle "Linux Shell\
Script Tutorial" --inputbox "Enter your name please" 8 60 2>/tmp/input.$$
sel=$?
na=`cat /tmp/input.$$`
case $sel in
0) echo "Hello $na" ;;
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1) echo "Cancel is Press" ;;
255) echo "[ESCAPE] key pressed" ;;
esac
rm -f /tmp/input.$$
Run it as follows:
$ sh dialog3.sh
Inputbox is used to take input from user, In this example we are taking Name of user as input.
But where we are going to store inputted name, the answer is to redirect inputted name to file via
statement 2>/tmp/input.$$ at the end of dialog command, which means send screen output to file
called /tmp/input.$$, letter we can retrieve this inputted name and store to variable as follows
na=`cat /tmp/input.$$`.
For input box's exit status refer the following table:
Exit Status for
Input box
Meaning
0 Command is successful
1 Cancel button is pressed by user
255 Escape key is pressed by user
Its time to write script to create menus using dialog utility, following are menu items
Date/time
Calendar
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Editor
and action for each menu-item is follows :
MENU-ITEM ACTION
Date/time Show current date/time
Calendar Show calendar
Editor Start vi Editor
$ cat > dialogmenu.sh
#
#How to create small menu using dialog
#
dialog --backtitle "Linux Shell Script Tutorial " --title "Main\
Menu" --menu "Move using [UP] [DOWN],[Enter] to\
Select" 15 50 3\
Date/time "Shows Date and Time"\
Calendar "To see calendar "\
Editor "To start vi editor " 2>/tmp/choice.$$
choice=`cat /tmp/choice.$$`
opt=$?
case $choice in
Date/time) date;;
Calendar) cal;;
Editor) vi;;
Esac
Save it and run as:
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$ rm -f /tmp/choice.$$
$ sh dialogmenu.sh
--menu option is used of dialog utility to create menus, menu option take
--menu options Meaning
"Move using [UP] [DOWN],[Enter]
to Select"
This is text show before menu
15 Height of box
50 Width of box
3 Height of menu
Date/time "Shows Date and Time" First menu item called as tag1 (i.e. Date/time) and
description for menu item called
Figure 36 Menu Dialog Box
262
as item1 (i.e. "Shows Date and Time")
Calendar "To see calendar "
First menu item called as tag2 (i.e. Calendar) and
description for menu item called as item2 (i.e. "To
see calendar")
Editor "To start vi editor " First menu item called as tag3 (i.e. Editor) and
description for menu item called as item3 (i.e."To
start vi editor")
2>/tmp/choice.$$ Send selected menu item (tag) to this temporary file
After creating menus, user selects menu-item by pressing the ENTER key, selected choice is
redirected to temporary file, Next this menu-item is retrieved from temporary file and following
case statement compare the menu-item and takes appropriate step according to selected menu
item. As you see, dialog utility allows more powerful user interaction then the older read and
echo statement. The only problem with dialog utility is it work slowly.
TRAP COMMAND
Controls the action to be taken by the shell when a signal is received.T rap command is used to
catch a signal that is sent to a process. An action is taken based on the signal by using the action
which is defined in the trap command instead of taking the default effect on the process.
Trap [OPTIONS] [ [arg] signspec..]
Arg is the action to be taken or executed on receiving a signal specified in signspec.
e.g. trap ―rm $FILE; exit‖ // exit (signal) and remove file (action)
Signal Number When occurs
0 shell exit
1 Hangup
2 interrupt (CTRL+C)
3 quit
9 kill (cannot be caught)
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Example:
$ vi trap.sh
#
# signal is trapped to delete temporary file , version 2
#
del_file()
{
echo "* * * CTRL + C Trap Occurs (removing temporary file)* * *"
rm -f /tmp/input0.$$
exit 1
}
Take_input1()
{
recno=0
clear
echo "Appointment Note keeper Application for Linux"
echo -n "Enter your database file name : "
read filename
if [ ! -f $filename ]; then
echo "Sorry, $filename does not exit, Creating $filename database"
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echo "Appointment Note keeper Application database file" > $filename
fi
echo "Data entry start data: `date`" >/tmp/input0.$$
#
# Set a infinite loop
#
while :
do
echo -n "Appointment Title:"
read na
echo -n "time :"
read ti
echo -n "Any Remark :"
read remark
echo -n "Is data okay (y/n) ?"
read ans
if [ $ans = y -o $ans = Y ]; then
recno=`expr $recno + 1`
echo "$recno. $na $ti $remark" >> /tmp/input0.$$
fi
echo -n "Add next appointment (y/n)?"
265
read isnext
if [ $isnext = n -o $isnext = N ]; then
cat /tmp/input0.$$ >> $filename
rm -f /tmp/input0.$$
return # terminate loop
fi
done # end_while
}
#
# Set trap to for CTRL+C interrupt i.e. Install our error handler
# When occurs it first it calls del_file() and then exit
#
trap del_file 2
#
# Call our user define function : Take_input1
#
Take_input1
Run the script as:
$ ./trap.sh
After giving database file name and after giving appointment title press CTRL+C, Here we have
already captured this CTRL + C signal (interrupt), so first our function del_file() is called, in
which it gives message as "* * * CTRL + C Trap Occurs (removing temporary file)* * * " and
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then it remove our temporary file and then exit with exit status 1. Now check /tmp directory as
follows
The shift Command
The shift command moves the current values stored in the positional parameters (command line
args) to the left one position. For example, if the values of the current positional parameters are:
$1 = -a $2 = abc $3 = xyz
and you executed the shift command the resulting positional parameters would be as follows:
$1 = abc $2 = xyz
e.g. Write the following shell script to clear you idea:
$ vi shift.sh
echo "Current command line args are: \$1=$1, \$2=$2, \$3=$3"
shift
echo "After shift command the args are: \$1=$1, \$2=$2, \$3=$3"
Excute above script as follows:
$ ./shift.sh -a abc xyz
Current command line args are: $1=-a, $2=abc, $3=xyz
After shift command the args are: $1=abc, $2=xyz, $3=
You can also move the positional parameters over more than one place by specifying a number
with the shift command. The following command would shift the positional parameters two
places:
shift 2
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Troubleshooting
Password Recovery
Breaking root password:
When system boots it ask to select OS,select linux OS and then press ‗e‘.
Choose the second option ‗Kernel/vmlinuz-2.6.i8, again press ‗e‘.
Write ‗1‘ at the last of the line,
Then press esc key and then ‗b‘.
Now system will reboot and stops on single user mode
sh#
here type passwd , then enter
type new password
now reboot & give root and new password.
Password Security (adding password on grub after installation):
{on graphical terminal}
# grub-md5-crypt
Password:
Confirm password:
(Here you see some digits, copy those digits)
Now edit file grub.conf
# vi /boot/grub/grub.conf
Here you see a line where written hidden menu, come under that line and type
password --md5 {paste those digits}
Save file.
Now your password cannot be broken on startup.
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Grub recovery
Booting partition problem, when grub is destroyed:
Now when you boot your system, it does‘nt boot and came to grub>
Then we have to do the following step
grub> root (hd0, #use tab see all partitions
grub> root (hd0,1)
grub> kernel /vmlinuz-2.6.262.8.el6 root=/dev/sda3
grub> initrd /initrd-2.6.262-8.el6.img
grub> boot
Now it will boot your linux PC .
Grub installation:
Now if grub of our linux pc is destroyed and cannot be recovered, then we have to install grub
For installing grub boot from your linux DVD and write linux rescue to boot your PC in rescue
mode
# chroot /mnt/sysimage
# grub-install /dev/sda,hda
#exit
Finally it installs your grub and now your system will boot with linux OS.
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INITTAB REPAIR
If your inittab file is missed or moved and now your PC is unable to boot and it stops on grub,
then
first check the from where you have to copy the inittab file here we are taking pendrive
(/dev/sdb) drive as source.
grub> kernel /vmlinuz-2.6.18-8.el5 rw root=lable=/ init=/bin/sh
sh-3.1# cd /etc/rc.d
sh-3.1# ./rc.sysinit
sh-3.1# mkdir /dir
sh-3.1# mount /dev/sdb /dir
sh-3.1# cp inittab /etc
now reboot your PC.