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 $!

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

This script reads the arguments passed to it.

sample_script.sh

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

The following example demonstrates this:

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

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

sample_check_args.sh

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

Run the following command:

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

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

sample_exit_status.sh

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

ls /tmp > /dev/null 2>&1
echo "ls exit status: $?"

Run the following command:

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.

sample_pid.sh

echo "Current PID: $$"

sleep 10 &
echo "Background PID: $!"

Run the following command:

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.

sample_at_vs_star.sh

args=("Ayanami Rei" "Ikari Shinji")
for a in "${args[@]}"; do echo "[$a]"; done
for a in "${args[*]}"; do echo "[$a]"; done

Run the following command:

bash at_vs_star.sh
[Ayanami Rei]
[Ikari Shinji]
[Ayanami Rei Ikari Shinji]

Common Mistake 1: $? is overwritten before it is checked

$? holds the exit status of the most recent command, but it is overwritten by every subsequent command — including echo. Read it immediately or save it to a variable before using it.

ls /nonexistent 2>/dev/null
echo "doing something else"
echo "exit status: $?"
exit status: 0
（$? reflects echo's status, not ls's）

Save the exit status to a variable before anything else.

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

Common Mistake 2: Using $* instead of $@ loses argument boundaries

"$*" joins all arguments into a single string. When arguments contain spaces, they are merged together and their boundaries are lost. Use "$@" to keep each argument separate.

sample_at_vs_star_mistake.sh

show_args() {
    echo "--- using \$*"
    for a in "$*"; do echo "  [$a]"; done
    echo "--- using \$@"
    for a in "$@"; do echo "  [$a]"; done
}
show_args "Ayanami Rei" "Ikari Shinji"

Run the following command:

bash at_vs_star_mistake.sh
--- using $*
  [Ayanami Rei Ikari Shinji]
--- using $@
  [Ayanami Rei]
  [Ikari Shinji]

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