Special Variables

Display the script name.

echo $0

Display positional parameters (arguments).

echo $1 $2 $3

Reference all arguments.

echo "$@"   # Treats each argument as a separate string
echo "$*"   # Treats all arguments as a single string

Display the number of arguments.

echo $#

Display the exit status of the last command.

echo $?

Display the PID of the current shell.

echo $$

Display the PID of the last process run in the background.

echo $!

Variable	Description
$0	The filename of the script (or the shell name).
$1 〜 $9	The 1st through 9th arguments passed to the script.
${10} and beyond	Arguments from the 10th onward require curly braces.
$@	Expands all arguments as separate strings (can be safely quoted with double quotes).
$*	Expands all arguments as a single string, separated by IFS.
$#	The number of arguments passed.
$?	The exit status of the last command (0 = success, 1 or more = failure).
$$	The PID of the current shell process.
$!	The PID of the last process run in the background.
$_	The last argument of the previous command.
$LINENO	The line number currently being executed. Useful for debugging.
$FUNCNAME	An array of the currently executing function names.
$RANDOM	Returns a random integer between 0 and 32767 each time it is referenced.
$SECONDS	The number of seconds elapsed since the script started.

Save the following script as script.sh and run it with $ ./script.sh Hello World 3.

This script reads the arguments passed to it.

script.sh

#!/bin/bash
echo "Script name: $0"
echo "1st argument: $1"
echo "2nd argument: $2"
echo "Number of arguments: $#"
echo "All arguments: $@"

bash script.sh Hello World 3
Script name: script.sh
1st argument: Hello
2nd argument: World
Number of arguments: 3
All arguments: Hello World 3

Print an error message and exit when required arguments are missing.

check_args.sh

if [ $# -lt 2 ]; then
    echo "Usage: $0 arg1 arg2" >&2
    exit 1
fi

bash check_args.sh one
Usage: check_args.sh arg1 arg2

Use $? to check the exit status of the previous command. 0 means success; 1 or greater means failure.

exit_status.sh

ls /nonexistent 2>/dev/null
echo "ls exit status: $?"

ls /tmp 2>/dev/null
echo "ls exit status: $?"

bash exit_status.sh
ls exit status: 2
ls exit status: 0

Use $$ to get the PID of the current shell, and $! to get the PID of a background process.

pid.sh

echo "Current PID: $$"

sleep 10 &
echo "Background PID: $!"

bash pid.sh
Current PID: 12345
Background PID: 12346

$RANDOM returns a random integer between 0 and 32767 each time it is referenced.

echo "Random: $RANDOM"
Random: 27483

The difference between "$@" and "$*" becomes clear when arguments contain spaces.

at_vs_star.sh

args=("hello world" "foo")
for a in "${args[@]}"; do echo "[$a]"; done
for a in "${args[*]}"; do echo "[$a]"; done

bash at_vs_star.sh
[hello world]
[foo]
[hello world foo]

$? is essential for checking whether the previous command succeeded. However, it is immediately overwritten after being read, so save it to another variable first for safe use (e.g., ret=$?; if [ $ret -ne 0 ]; then ...).

Wrapping "$@" in double quotes safely handles arguments that contain spaces. Always use "$@" when passing script arguments directly to another command.

For how arguments work inside functions, see Function Arguments and Return Values.