List of Special Parameters/Variables in Bash Shell Scripting

There is a list of Special Parameters or Variables that are available when working with Bash Shell that can be used with scripts or command line arguments.

Bash Parameter	Description	Usage
$0	Name of the script or the shell	To Identify the script or shell that is currently running
$1, $2, $3, ...	Positional parameters or arguments passed to the script	To access individual command-line arguments
$*	All positional parameters as a single string	To represent all command-line arguments as a single string
$@	All positional parameters as separate strings	To represent all command-line arguments as separate strings
$#	Number of positional parameters	To count the total number of command-line arguments
$?	Exit status of the last executed command	To indicate the success or failure of the last command
$$	Process ID (PID) of the current script or shell	To represent the unique identifier of the running process
$!	Process ID (PID) of the last background command	To store the PID of the most recent background process
$-	Current options set for the shell	To show the currently active options for the shell
$IFS	Internal Field Separator	To determine how Bash splits words into fields
$PWD	Current working directory	To represent the directory in which the script is running
$OLDPWD	Previous working directory	To hold the path of the previous working directory
$HOME	Home directory of the current user	To point to the user's home directory
$USER	Username of the current user	To store the name of the user executing the script
$HOSTNAME	Hostname of the system	To represent the name of the system
$RANDOM	A random number	To provide a random number each time it is accessed

Let's see examples of each of them

#!/bin/bash

# Special Parameters Example Script

# $0 - Name of the script or the shell
echo "Script name: $0"

# $1, $2, $3, ... - Positional parameters or arguments passed to the script
echo "First argument: $1"
echo "Second argument: $2"
echo "Third argument: $3"

# $* - All positional parameters as a single string
echo "All arguments as a single string: $*"

# $@ - All positional parameters as separate strings
echo "All arguments as separate strings:"
for arg in "$@"; do
    echo "$arg"
done

# $# - Number of positional parameters
echo "Total number of arguments: $#"

# $? - Exit status of the last executed command
echo "Exit status of last command: $?"

# $$ - Process ID (PID) of the current script or shell
echo "Process ID: $$"

# $! - Process ID (PID) of the last background command
echo "Process ID of last background command: $!"

# $- - Current options set for the shell
echo "Current options: $-"

# $IFS - Internal Field Separator
echo "Internal Field Separator: $IFS"

# $PWD - Current working directory
echo "Current working directory: $PWD"

# $OLDPWD - Previous working directory
echo "Previous working directory: $OLDPWD"

# $HOME - Home directory of the current user
echo "Home directory: $HOME"

# $USER - Username of the current user
echo "Username: $USER"

# $HOSTNAME - Hostname of the system
echo "Hostname: $HOSTNAME"

# $RANDOM - A random number
echo "Random number: $RANDOM"

Output:

 ./special_parameters_example.sh 

Script name: ./special_parameters_example.sh
First argument: hello
Second argument: 1
Third argument: 2
All arguments as a single string: hello 1 2 3
All arguments as separate strings:
hello
1
2
3
Total number of arguments: 4
Exit status of last command: 0
Process ID: 4662
Process ID of last background command: 
Current options: hB
Internal Field Separator:  	

Current working directory: /Users/c2ctech/Desktop
Previous working directory: 
Home directory: /Users/c2ctech
Username: c2ctech
Hostname: C2C-Tech-Mac.local
Random number: 14105

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