Jozef Goetz, 2007

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expanded by Jozef Goetz, 2006

Credits: Parts of the slides are based on slides created by textbook authors, Syed M. Sarwar, Robert Koretsky, Syed A. Sarwar

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Objectives

To describe what a UNIX/LINUX shell is

To describe briefly some commonly used shells

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what a UNIX/LINUX shell acts as an interface between the user and

the UNIX/LINUX kernel goal: interpret the user’s cmds

Shell = Command Interpreter Shell command can be

internal - built-in cmd (part of the shell process - bg, cd, continue, echo, exec) or

external cmd (grep, more ,cat, mkdir, rmdir, ls)

a file in the form of a binary executable program file or

a shell script to terminate a shell press <^D>

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how it works after reading the user’s cmd, it

determines whether cmd is internal

own code is executed or external cmd

search for a file that has the name of the cmd by searching several directories

the search path is determine in the shell variables

PATH (or path in the TC shell) You can view the search path by using

echo $PATH

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PATH and path variables search PATH variable defined in a hidden file called

.profile or .login in a system start-up file or start-up file (ha dot file - .bashrc for Bash and .cshrc file

for TC shell) in your home directory.

in TC shell (separated by “ “ )

in Bourne, Korn, and Bash shell (separated by “:” )

// set path, the search starts with ~/bin and then with .// ~ represents a home directory

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PATH and path variables

To change the search path temporary

- change the value of PATHPATH=~/bin:$PATH:.- The search path has been modified by 2 directories,

~/bin and . (current directory)

• To change the search path permanently you need to change it in the corresponding “.” dot file

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Intro to UNIX Shell The UNIX/LINUX shell is a program

that starts running when you log on and interprets the commands that you type.

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Which Shell Suits Your Needs?

reach common-level interface

good programming language

good features of sh and csh and is a superset of sh

more adv programming features than tcshequally powerful in their interactive use

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Table 4.1  Shell Locations and Program Names

chsh – change the default shell

chsh –l available shells with a path

[jgoetz jgoetz]$ chsh -l/bin/bash/bin/sh/bin/bash2/bin/tcsh/bin/csh/bin/badsh

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Shell Similarities

exec /bin/csh - the same as issuing command csh

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11Table 4.2 Shell Similarities and Dissimilarities

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12Table 4.2 Shell Similarities and Dissimilarities

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13Table 4.3 Some Useful Shell Built-In Commands

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14Ways to Change Your Shell

When you 1st logon your default shell will be shown by typing: $ echo $SHELL

/bin/csh $ chsh –l // gives you a list of all shells available

You can change your shell one of 2 ways:1. You can change to a new default for every subsequent

login session on your system Changing the default shell:

$ chsh // may not working on your system ask you for your login password then type complete path shell: e.g. /bin/tcsh

2. You can create additional shell sessions running on top of, or concurrently with, the default shell

Create or run additional shells on top of your default shell$ echo $ SHELL/usr/bin/sh$ csh // new one% // use ps to test it

or exec /bin/csh

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Start-up Files Initial system start-up file

has initial settings of important variables for the shell and some other utilities

/etc/profile

When you start particular shell the corresponding shell start-up file found in the user’s home dir initially configured by the administrator

executes:

.profile // Bourne, Korn

.cshrc

.kshrc // Korn

.bashrc

.zshrc

.tcshrc

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16Table 4.5 Some important Shell Startup Files for Bash and TC Shells

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3 bash4 bash

1. When you log on and your login shell is Bash, it first executes commands in the /etc/profile file, if this file exists.

2. It then searches for the ~/.bash_profile, ~/.bash_login, or ~/.profile file, in this order, and executes commands in the first of these that is found and is

readable. 3. When you start an interactive Bash shell, it executes commands in the

~/.bashrc file, if this file exists and is readable.

4. When a login Bash exits, it executes commands in the ~/.bash_logout file.

When started non-interactively to run a shell script (see Chapters 15 and 16), Bash looks for the environment variable BASH ENV to find out the name of the file to be executed.

Numbers show possible sequence of command execution from the corresponding file

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17Table 4.5 Some important Shell Startup Files for Bash and TC Shells

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3 bash4 bash

1 for tcsh2 for tcsh3 for tcsh4 for tcsh

If your shell is a TC shell, it executes commands in the /etc/csh.cshrc or /etc/.cshrc file, if it exists and is readable.

A login shell then executes commands in the /etc/csh.login file, if it exists.

1. Every shell (login or non-login) then executes commands in the ~/.tcshrc file (or the ~/.cshrc file if ~/.tcshrc does not exist), followed by reading the ~/.history file.

2. and 3. A login shell then executes commands in the ~/.login and ~/.cshdirs files.

4. When a login TC shell exits, it executes commands in the /etc/csh.logout

and ~/.logout files, if they exist and are readable

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set | more gives you shell variables values to set a variable

set history = 10

for csh (to get it do: exec /bin/csh) setenv | more or setenv variable settings

Shell Start-up Files and Environment Variables

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19Environment Variables seen in shell start-up files

C shell are in lowercase, others are Bourne and Korn shells

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Displaying files:

display all contents of f1, f2:

cat f1 f2

more f1 f2

pg f1 f2 // for some shells

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Communication Commands

Communication Commands:

$ who // check the user name to whom you want to talk

$ mesg [y|n] // permit execution write or talk$ write user [tty]$ talk user [tty] - shows 2 sections of the screen: sender and receiver

$ biff [y|n] // notification on/off

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Utility Commands (Contd)

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23Command Aliases p.55[3] The alias command can be used to create

pseudonyms (nicknames) for commands

they can be placed in ~/.profile or ~/.login executes when you log on

but typically in a shell start files .bashrc ( .cshrc for TC shell)

every time you start Tcsh or Bash

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24Command Aliases

Syntax for the alias command is: alias [name [ = string ] …] // for Bourne, Korn, Bash

shells alias l=‘ls –la’

alias [name [ string ] ] // C shell alias l ‘ls –la’ alias

list all aliases

remove it

unalias ls // remove ls

unalias –a // remove all

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25alias Command Examples

The \!* string is substituted by the actual parameter passed to the given command

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Displaying System Up Time

uptime

1:16pm up 28 days, 10:44, 2 users, load average: 0.04, 0.01, 0.00

displays the duration of time the system has been running since it was last booted , # of users and some additional info

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Shell Metacharacters

These are the characters other than letters and digits that have special meaning to the shell. They cannot be used in shell

commands without specifying them in particular way

Allow you to specify multiple files in multiple directories

in one command line.

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30Examples of Shell Metacharacters

$ lpr -Pspr [0-9][a-zA-Z].html print on the spr printer, P specifies the printer name,

then the names of all the files as follows: 2 chr file names with the first chr digit and 2nd being an uppercase or lowercase letter

$

$ ls lab[0-9]??.c lab11a.c lab1a1.c lab123.c lab4ab.c displays 6 chr long files with .c extension

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31Examples of Shell Metacharacters

$ lpr -Pspr [0-9][a-zA-Z].html print on the spr printer, P specifies the printer name,

then the names of all the files as follows: 2 chr file names with the first chr digit and 2nd being an uppercase or lowercase letter

$

$ ls lab[0-9]??.c lab11a.c lab1a1.c lab123.c lab4ab.c displays 6 chr long files with .c extension