Unix/Linux

cs3353

The Shell

• The shell is a program that acts as the interface between the user and the kernel.– The shell is fully programmable and will

interpret lists of commands entered by the user, or read cmds from a file (known as a script).

– A prompt is displayed by the shell program indicating it is ready to accept input from the user.

Shell Types

• Bourne Shell (ALGOL syntax)– sh,– bash– ksh– zsh

• C-shell (based on C language syntax)– csh– tcsh

Unix/Linux Commands

• commandName options files

• man commandName – to read about the command

Example: man find

accounts

• Root– Has absolute control over the OS

• Regular– User login id number, string, and password.

Shell Startup

• The .bash_profile (in place of the .profile) and .bashrc are used at TU.

• Ubuntu, .bashrc

Linux Files

• Everything in the OS is organized into files.

• There is a i-node for every file on the file system (i = information)

Files

• Regular files includes (aka ordinary)– executable (*) and – hidden files (.).

• Special files:– Symbolic link (@)– Directory (d)– Others: block special, named pipe, character special

Use the cmd: file filename

Use the ls command.

The File System

• Navigate with cd, pwd, ls• Organized by directories

– mkdir, rmdir– cp, mv, rm

• / is known as root• /boot• /home• /var• /tmp

Directory Pathnames

• Absolute – always begins with root /.

• Relative – does not begin with root

./aFile

bFile

/home/user/cFile

../data/dFile

File names

• See pg 57

• 255 characters, no / allowed.

ls –l

• Type-user-group-other

• -rwxr-xr—

• Token 1

• Token 2-4

• Token 5-7

• Token 8-10

• Access control is through the chmod cmd.

chmod

• Octal:– chmod [ugo]+|-[rwx]– Octal r=4, w=2, x=1, the complement of

umask– X is required for a script to run (pg 29)

• Examples

The Shell Env

• Variables– Local are only visible in the current process.– Environment variables are inherited by sub-

processes, Example: PATH– PATH is used by the shell to find commands

entered by the user.

export PATH=$PATH:.:~/bin

echo $PATH

Input and Output

• STDIN = 0 the standard input

• STDOUT = 1 the standard output

• STDERR = 2 the standard error

• Most cmds are designed to display output to the stdout

• Many cmds are designed to read input from the stdin.

• Error messages go to stderr.

Input & Output

• The default location for output is the terminal screen.

• It is possible to redirect the output of a command to:– Another command using a pipe |– A file using the output redirect symbol >

Example: date > aFile

ps | wc –l

Input & Output

• It is possible to control the input to a command• The read command is designed to capture input from the

standard input:

cat << stop > file>Steve>stopgreeting < file

echo “What is your name?”read Nameecho “Greetings $Name”

Processes

• Init is process id # 1

• Dummy processes during system startup do not have a process ID.

• All other processes have a process ID.

Example

• Use ps and sleep to introduce background processes using the &

• ps will show processes running on a system.

• A process can be stopped (killed) by using the PID.

kill pid

Substitution (Meta Characters)

• *, ?, [range]

• !, ^, $