52.Shell_Variables.md

Shell Variables

Variables act as placeholders to hold values and are defined as key-value pairs in Shell programming. Shell variables do not have data types; all values are treated as text or string type.

Types of Variables
Variables in Shell are divided into two types:

1. Environment Variables / Predefined Variables
2. User-Defined Variables

Environment Variables

• Predefined variables used internally by the system; also referred to as System Variables.
• Can be utilized in user scripts as per requirement.
• Use env or set commands to display all environment variables:

$ env
LANG=en_IN  
USERNAME=durgasoft  
USER=durgasoft  
PWD=/home/durgasoft  
HOME=/home/durgasoft  
SHELL=/bin/bash  
LOGNAME=durgasoft  
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/snap/bin:/home/durgasoft/scripts  
...  

Changing the Prompt

• The PS1 environment variable determines the terminal prompt display.
• Modify PS1 to change the terminal prompt:

$ PS1=DURGA$  
DURGA$ ls  
abc.txt Documents d.txt Pictures scripts Videos  

Demo Script Using Environment Variables
Create and execute the following script to use some environment variables:

#!/bin/bash  
echo "User Name: $USER"  
echo "User Home Directory: $HOME"  
echo "Default Shell Name: $SHELL"  
echo "PATH: $PATH"  

Execute with the following commands:

$ chmod 755 env.sh  
$ ./env.sh  

User-Defined Variables

• Define custom variables based on programming requirements.

Example:

$ GUEST=Dhoni  
$ echo "Hello $GUEST You are my Hero"  
Hello Dhoni You are my Hero  

Rules for Defining Variables

1. Use UPPERCASE characters for variable names.
2. Separate multiple words with underscores _.
3. Variable names should not begin with a digit.

   $ 123A=40  
   123A=40: command not found  

4. Avoid special symbols like -, @, #, etc.

Read-Only Variables

• Use readonly keyword to define variables that cannot be reassigned.

Example:

$ A=100  
$ readonly A  
$ A=300  
bash: A: readonly variable