Special Variables

$0, $1, $2... and $#

$0 holds the name the script was invoked with, while $1, $2, and so on hold the individual arguments passed to it in order. $# tells you how many positional arguments were supplied, which is useful for validating input before using it.

Warning: set -- alpha beta gamma is used here to simulate command-line arguments inside the script for demonstration.

Example: $0, $1, $2... and $#

#!/bin/bash
set -- alpha beta gamma
echo "Script name: $0"
echo "First arg: $1, second arg: $2"
echo "Total args: $#"

$@ vs $* Unquoted

Unquoted, both $@ and $* expand to all positional parameters separated by spaces and are then subject to word splitting, so they behave identically in that form. The difference only shows up once you quote them.

Example: $@ vs $* Unquoted

#!/bin/bash
set -- one two three
for arg in $@; do
    echo "unquoted \$@: $arg"
done

"$@" vs "$*" Quoted

Quoted as "$@", each positional parameter expands as its own separate, fully preserved word (spaces inside an argument stay intact). Quoted as "$*", all parameters are joined into a single string separated by the first character of IFS (a space by default).

Warning: When forwarding arguments to another command, "$@" is almost always the correct choice.

Example: "$@" vs "$*" Quoted

#!/bin/bash
set -- "first arg" "second arg"
echo "Using \"\$@\":"
for arg in "$@"; do
    echo "  [$arg]"
done
echo "Using \"\$*\": [$*]"

$?, $$, and Other Specials

$? gives the exit status of the most recently completed command, and $$ gives the process ID of the currently running shell itself, which is often used to build unique temporary filenames. These specials are read-only and update automatically as the script runs.

Example: $?, $$, and Other Specials

#!/bin/bash
echo "This script's PID is $$"
true
echo "Exit status after true: $?"
tmpfile="/tmp/scratch_$$.txt"
echo "Would use temp file: $tmpfile"