u nix c omp -145 l ecture 3: t he unix f ile s ystem s ource : s. d as, “y our u nix : t he...

UNIX COMP-145

LECTURE 3: THE UNIX FILE SYSTEM

SOURCE: S. DAS, “YOUR UNIX: THE ULTIMATE GUIDE”, 2ND EDITION, MCGRAW HILL,

2006

9/21/2009 rwj BROOKDALE COMMUNITY COLLEGE 1

OBJECTIVES

• WHO IS LOGGED ON THE PATH & PRIVACY• FILE NAMING AND PROPERTIES• FILE ACCESS & NAVIGATION• TYPES & STRUCTURES OF FILES• FILE ACCESS PERMISSIONS• MOUNTING FILE SYSTEMS


BROOKDALE COMMUNITY COLLEGE 37/17/2009 rwj

WHO IS LOGGED ON & WHAT ARE THEY DOING?

• users– SHOWS A LIST OF ALL CURRENTLY LOGGED ON USERS

• who and whoami– DISPLAY A LIST OF ALL CURRENTLY LOGGED ON USERS

AND WHERE THEY ARE LOGGED ON FROM• W– SHOWS A LIST OF ALL CURRENTLY LOGGED ON USERS

AND WHAT THEY ARE RUNNING• finger– TELLS A LITTLE MORE INFORMATION ABOUT USERS


PRIVACY ISSUE?

• YOU CAN TELL WHEN SOMEONE ELSE IS LOGGED ON & EXACTLY WHAT THEY ARE DOING

• EVERYONE ELSE CAN SEE WHAT YOU ARE DOING

Is this acceptable?

THE UNIX FILE & NAMING

• A CONTAINER FOR STORING INFORMATION AND

DATA.• FILENAME - 1. LIMITED TO 255 CHARACTERS.

2. CAN’T CONTAIN / OR NULL.• FILENAMES ARE CASE-SENSITIVE, I.E.,

chap AND Chap ARE TWO DIFFERENT FILENAMES.

o Upper case “A” not same ASCII value as “a”.



THE UNIX FILE & NAMING (CONT’D)

• GROUP OF FILENAMES HELD TOGETHER IN A

DIRECTORY.

• DIRECTORY LISTING CONTAINS NAME OF THE FILE.

• BOTH FILES AND DIRECTORIES ARE SUBJECT TO

ACCESS CONTROL.

THE UNIX FILE (CONT’D)

• FILE CONTENT CAN BE ANY SEQUENCE OF ASCII CHARACTERS OR BINARY ENCODED CHARACTERS

• NEITHER FILE NAME OR SIZE IS STORED IN FILE

• FILENAMES CAN BE: Up to 255 char in length, file extensions are optional. Can’t contain “/” or NULL (ASCII value = 0 [i.e., zero] Avoid using $ ` ? * & in names


FILE TYPES

• ORDINARY OR REGULAR FILE: CONTAINS DATA AS A STREAM OF CHARACTERS. THIS FILE CAN BE A TEXT file (program sources, configuration files) – Contains only

printable and EOL characters visible with “od” command. BINARY file (executables, graphic and multimedia files).

• DIRECTORY: CONTAINS THE FILENAME AND A NUMBER (inode number).

• DEVICE FILE: CONTAINS NO DATA WHATSOEVER.

• SYMBOLIC LINK: CONTAINS THE LOCATION OF ANOTHER FILE.


PROPERTIES OF FILES

• ALL FILES HAVE THESE PROPERTIES– PERMISSIONS– LINKS

• For directories, this lists the number of subdirectories

– OWNER– OWNER GROUP– SIZE– TIMESTAMP– NAME

THE HIERARCHICAL STRUCTURE OF THE FILE SYSTEM

• A SINGLE HIERARCHICAL STRUCTURE THAT CONTAINS ALL FILES.

• TOP SIGNIFIED BY ROOT (/).• EXISTENCE OF A PARENT-CHILD

RELATIONSHIP.• PARENT OF ANY FILE MUST BE A DIRECTORY.• FILES ACCESSED WITH PATHNAMES (e.g.,

/etc/passwd).


HIERARCHICAL FILE STRUCTURE ILLUSTRATED


WHAT GOES WHERE?

• /– ROOT OF THE ENTIRE SYSTEM– COMPARABLE IN WINDOWS TO C:\

• WINDOWS, HOWEVER, HAS SEVERAL ROOTS DEPENDING ON WHAT PARTITION AND DEVICE YOU ARE LOOKING AT

• /bin– COMMONLY USED BINARIES (PROGRAMS)


WHAT GOES WHERE? (CONT’D)

• /sbin– MORE PROGRAMS TO RUN

• STATICALLY LINKED• STILL SHOULD RUN IF YOU TINKER AROUND AND MESS THINGS UP

• /usr– USER RELATED COMMANDS AS WELL AS A WHOLE BUNCH OF

RANDOM STUFF


• /lib– LIBRARIES (RE-USED FILES/EXECUTABLES) GO IN HERE

• /dev– ALL DEVICES ARE LOCATED IN HERE

• /home– TRADITIONALLY, THIS IS WHERE USER ACCOUNTS ARE

STORED

• /etc– STARTUP FILES AND CONFIGURATION FILES FOR DAEMONS

AND OTHER PROGRAMS



• /var– VARIOUS FILES GO IN HERE– TRADITIONAL LOCATION OF MAILBOXES

• /var/spool/mail

• /proc– SPECIAL FILES THAT CONTAIN INFORMATION ABOUT THE SYSTEM OR

INFO FROM RUNNING PROGRAMS



NAVIGATING THE DIRECTORY STRUCTURE

• YOU ARE ALWAYS LOCATED AT ONE LOCATION IN THE TREE

• ls– PROVIDES A LISTING OF THE CURRENT DIRECTORY

• All files and directories are shown

• cd– CHANGE DIRECTORY– “cd /” MOVES YOUR CURRENT DIRECTORY TO /– cd WITHOUT ANY ARGUMENTS MOVES YOU TO YOUR

HOME DIRECTORY


SPECIAL NOTATION FOR TRAVERSING DIRECTORIES

• .– THIS STANDS FOR THE CURRENT DIRECTORY

• ..– THIS STANDS FOR THE DIRECTORY DIRECTLY ABOVE THE

CURRENT DIRECTORY• ~– YOUR HOME DIRECTORY

• THESE DIRECTORIES CAN BE “STACKED”– “../..” STANDS FOR TWO DIRECTORIES DIRECTLY ABOVE

THE CURRENT DIRECTORY


PATHNAMES: TWO TYPES

• ABSOLUTE PATHNAME: SPECIFIES LOCATION WITH REFERENCE TO THE FILE SYSTEM TOP

(e.g, cat /etc/passwd).• RELATIVE PATHNAME: SPECIFIES LOCATION

WITH REFERENCE TO THE USER’S CURRENT LOCATION

(e.g., cd ../include).• BOTH COMMANDS AND FILENAME

ARGUMENTS CAN BE REPRESENTED IN EITHER FORM.


PATHNAMES WITH RESPECT TO FILE HIERARCHY


/

bin dev home lib tmp etcsbin usr var

dsk fd0 fd1

austin juliebin lib sbin local

progs login.sql .profile

root


USEFUL PATHNAMES

• /bin & /usr/bin directories:o User accessible - commonly used command repositoryo bin indicates binary file store

• /sbin & /usr/sbin directories.o SuperUser or Adminsitrator accessible command repository

• /etc directory o Configuration file repository: e.g., /etc/passwd & /etc/shadow.

• /dev directoryo All device fileso May have sub-dirctories: pts, dsk, rdsk


USEFUL PATHNAMES (CONT’D)

• /lib & /usr/lib DIRECTORIES:o USER ACCESSIBLE – ALL REUSABLE FILES IN BINARY FORM.

• /usr/include DIRECTORY:o USER ACCESSIBLE – ALL STANDARD HEADER FILES USED IN C

PROGRAMSo NAMING: USUALLY HAVE A SUFFIX “.h”o C-CODE #include stdio.h WHICH INSTRUCTS

COMPILER TO INCLUDE THE FILE STDIO.H WHEN IT CREATES THE EXECUTABLE FOR C-PROG.

• /usr/share/man DIRECTORY o man PAGE REPOSITORY.

• /tmp DIRECTORY• USERS CREATE TEMPORARY FILES.


USEFUL PATHNAMES (CONT’D)

• USER SPECIFIC DIRECTORIES:o/tmp DIRECTORY

REPOSITORY FOR TEMPORARY FILES; DELETED REGULARLY BY SYSTEM

o/var DIRECTORY TEMPORARY REPOSITORY FOR PRINT JOBS & E-MAIL; TEMPORARY REPOSITORY FOR IN-COMING AND OUT-

GOING E-MAILo/home DIRECTORY

OFTEN USERS LOCATED AS CHILD DIRECTORIES OF /HOME;

ABSOLUTE PATHNAME

• BEGINS WITH a / (e.g., /etc/passwd).

• FIRST / SIGNIFIES THE ROOT DIRECTORY.• SYSTEM CONFIGURATION FILES THAT

NORMALLY DON’T CHANGE LOCATION SHOULD BE ADDRESSED IN ABSOLUTE MANNER.

• USED WITH A COMMAND THAT• DOESN’T FEATURE IN PATH.• RESIDES IN TWO OR MORE DIRECTORIES OF PATH.


RELATIVE PATHNAME

• TO SIGNIFY THE CURRENT DIRECTORY USE .• TO SIGNIFY THE PARENT DIRECTORY USE .. • RP USED TO REFER TO FILES THAT ARE (1)

IMPOSSIBLE OR (2) INCONVENIENT TO ACCESS IN AN ABSOLUTE MANNER.

• RP CAN BE A SYNONYM FOR A FILENAME ARGUMENT THAT DOESN’T HAVE A /

(e.g., cat foo IS THE SYNONYM FOR cat ./foo.)

• SAME SYNONYM DOESN’T AUTOMATICALLY EXIST FOR COMMANDS.

( e.g., cat foo MAY NOT be the same as ./cat foo.)9/21/2009 rwj BROOKDALE COMMUNITY COLLEGE 24

THE HOME DIRECTORY

• DIRECTORY WHERE USER IS PLACED ON LOGIN.• DETERMINED BY SIXTH FIELD IN /etc/passwd:

romeo:x:500:100:romeo vincent:/home/romeo:/bin/bash

• CAN ALSO BE REFERRED TO BY• the shell variable $HOME (e.g. cat $HOME/foo).• tilde (~) expansion in most shells: (e.g. cat ~/foo).

• cd COMMAND USED WITHOUT ARGUMENTS RETURNS USER TO HOME DIRECTORY.

• USER CAN CREATE AND REMOVE FILES IN THEIR HOME DIRECTORY BUT NOT IN OTHER DIRECTORIES.



FILE ACCESS PERMISSIONS

-rwxrwxrwx

- rwx rwx rwx

File Type Owner Group World

- = filed = directory

OWNER, GROUP, THE WORLD

• OWNER– THE CREATOR OF THE FILE

• GROUP– A SET OF USERS GROUPED TOGETHER

• THE WORLD– EVERY OTHER ACCOUNT NOT IN THE GROUP



READ, WRITE, EXECUTE

• READ– FILE CAN BE READ, BUT NOT MODIFIED

• WRITE– PERMISSION IS GRANTED TO MODIFY THE FILE

• EXECUTE– RUN DIRECTLY AS IF THE FILE IS A PROGRAM– ALL PROGRAMS SHOULD BE EXECUTABLE (/bin)


WHAT PERMISSIONS MEAN ON A DIRECTORY

• READ– USERS CAN GET A LISTING OF THAT DIRECTORY

• WRITE– USERS CAN CREATE AND REMOVE FILES IN THAT

DIRECTORY

• EXECUTE– USERS CAN EXAMINE FILES IN THAT DIRECTORY


CHANGING PERMISSIONS WITH CHMOD

chmod <SETTINGS> <FILE>

chmod a-x testFile

u = user

g = group

o = other

a = all

+ (add)

- (remove)

= (set)

r = read

w = write

x = execute


MORE EXAMPLES

• SET READ AND WRITE ACCESS FOR ALL?– chmod a=rw <FILE>

• ADD EXECUTABLE ACCESS FOR OTHERS?– chmod o+x <FILE>

• REMOVE ALL ACCESS FOR OWNER?– chmod u-rwx <FILE>

• SET READ, WRITE & EXECUTE ACCESS FOR ALL?– chmod a=rwx <FILE>

• 777 PERMISSIONSo Octal Representation of -rwxrwxrwxo Three binary digits or bits corresponds to one octal digit:

Read = 4, Write = 2 Execute =1 i.e., rwx = 4+2+1 = 7 chmod 750 <file> same as chmod u=rwx,g=rx,o= <file>

ADVANCED chmod USAGE

• MOST UNIX HACKERS DON’T USE THIS FORM• THEY PREFER THE MORE DIRECT APPROACH

– SET PERMISSIONS FOR OWNER, GROUP, AND OTHERS ALL WITH ONE NUMBER

• UNFORTUNATELY, THIS APPROACH REQUIRES A LITTLE BIT OF INFORMATION


MOUNT & Un-mount

• COMMAND LINE UTILITIES.• REQUIRE ROOT USER PRIVILEGE OR THE

CORRESPONDING FINE-GRAINED PRIVILEGE, EXCEPT IF THE FILE SYSTEM IS DEFINED AS "user mountable" IN /etc/fstabfile FILE (ONLY BE MODIFIABLE BY THE ROOT USER).• mount

o INSTRUCTS THE OS THAT A FILE SYSTEM IS READY TO USE, o ASSOCIATES IT WITH A PARTICULAR POINT IN THE SYSTEM'S FILE

SYSTEM HIERARCHY (ITS MOUNT POINT).NOTE: EACH MOUNT POINT IS MAPPED TO A REMOTE FILE

• unmount o INSTRUCTS THE OS THAT THAT THE FILE SYSTEM SHOULD BE

DISASSOCIATED FROM ITS MOUNT POINT, I.E., MAKE IT NO LONGER ACCESSIBLE.



Backup Slides


man Page Sections & What They Mean

• 1 – USER COMMANDS• 2 – SYSTEM CALLS• 3 – C LIBRARY ROUTINES• 4 – ADMINISTRATIVE FILES• 5 – MISCELLANEOUS• 6 – GAMES• 7 – I/O AND SPECIAL FILES• 8 – SYSTEM ADMINISTRATION COMMANDS


More on man

• OTHER SECTIONS EXIST (SUCH AS THE PERL REFERENCE MANUAL AND THE TCL REFERENCE MANUAL)

• -A WILL SHOW ALL THE MAN PAGES THAT MATCH– First is shown by default

• HOW WOULD YOU FIND OUT MORE ABOUT MAN?– man man

u nix c omp -145 l ecture 3: t he unix f ile s ystem s ource : s. d as, “y our u nix : t he...

Documents

directory file

file filenames

file attributes

regular file

file extensions

device file

unix file contd file

sensitive file etcshadow