This document discusses customizing the Unix environment. It covers shells, environment variables, command aliases, command history, and initialization scripts. Environment variables store information like the current user and shell. Aliases create shorthand names for commands. Command history allows reusing previous commands. Initialization scripts customize a user's environment when they log in and open new shells.

1. Introduction to Unix and OS
Module 2
Dr. Girisha G S
Dept. of CSE
SoE,DSU, Bengaluru
Customizing the Unix Environment

2. Agenda
 The Shells
 Environment Variables
 Command Aliases- Shorthand names for commands
 Command History
 The Initialization Scripts

3. - You can find out what shell you are using like this:
$ echo $SHELL
/bin/bash
- Shell features are customizable
- The default prompt for the Bourne, Bourne Again, and Korn shells is the
dollar sign ($).
- The default prompt for the C shell is the percent sign (%).
- The default prompt for root in either shell is the pound sign (#).

4. $ env
Common Environment Variables
$ set
DISPLAY=:0.0
EDITOR=/usr/bin/vi
SHELL=/bin/bash
TERM=xterm-256color
USER=geek
- Environment variables are variables that are set up in your shell when you log in.
- The environment variables are managed by the shell.

5. Customizing Environment Variable: PS1
- The shell primary prompt string is $ stored in the shell variable PS1, and you can
customize it according to your preference.
Example : change the primary prompt string to present working directory
$ PS1=”$PWD -> “
/home/user->
Customizing Environment Variable : PATH
- The PATH variable contains a list of directory path names, separated by colons for
executing commands and scripts and you can customize it according to your
preference.
Example: Include the home directory in the PATH variable
$ PATH=$PATH:/home/user
- You can display a single variable with a simple echo command.
Example: $ echo $PWD
/home/user

6. Aliases
- Aliases let you assign shorthand names to frequently used commands
alias: This command assign shorthand names to command
The syntax is as follows:
$ alias shortname=“command [options]”
Example: Create an alias called list, which will use the ls command to print a long-style
listing of all files in the current directory
$ alias list=‘ls –la’
$ list
unalias: this command unset an alias
Example: To unset alias list
$ unalias list

7. Command history
- Lets you recall previous commands and edit/ re-execute them
- Every command is treated as an event and assigned an event number
- Using this number you can recall previous command, edit them if
required and re-execute them
history: this command displays the history list showing the event number
of every previously executed command
Example: $ history

8. Example : display the last two commands
$ history 2
- The symbol ! Is used to repeat previous commands
Example: re-execute the command with event number 12
$ !12
- To repeat the previous command, use !!
Example : $ !!
Accessing previous commands by event numbers
Accessing previous commands by context
- When you don’t remember the event number of a command but
know that the command started with a specific letter of a string,
you can use the history facility with context
Example: repeats the last command beginning with v
$ !v
- You can specify a numeric argument to specify the number of previous commands to display

9. The Initialization Scripts
- The start up scripts are executed when the user logs in
- The initialization scripts in different shells are:
The Profile
- If you'd like to set the environment variables permanently, add your settings to
the initialization file
In the bash login shell, the startup scripts are executed in the following order
/etc/profile
~/.bash_profile
~/.bash_login
~/.profile
- reads and executes commands from the first one that exists
- If none exists, /etc/bashrc applied
- When a login shell exits, bash reads and executes commands from the file,
~/.bash_logout, if it exists.

10. - Every time you change the profile file, You can execute it by using a special command
(called dot).
Example : $ . .profile
The specific settings which an unix user usually does is:
― Setting of any environment variable
― Setting of any alias
― Setting of PATH variable or any other path variables
$cat $HOME/.profile
export PATH=$PATH:~blogger/bin
alias l="ls -lrt"
A typical profile file will look as shown below
The rc File
- This type of file is run every time a new shell is opened up even if it is not a login
shell.
- This file is ~/.bashrc for the bash shell
- The rc file is used to define command aliases, variable settings, and shell options.
- The rc file will be executed after the profile.