Mastering Shell Special Parameters: $*, $@, $#, $$, $! with Real Examples

In this guide we introduce several special parameters used in Bash scripts: $*, $@, $#, $$ and $!. Each parameter’s behavior is demonstrated with a concrete example.

Example 1 – Expanding positional parameters with $* and $@

The script expan.sh sets IFS='-' and iterates over the arguments using both "$*" and "$@". The output shows that $* treats all arguments as a single string separated by the internal field separator, while $@ yields each argument individually.

#!/bin/bash
export IFS='-'
cnt=1
echo "Values of \"$*\":"
for arg in "$*"
do
  echo "Arg #$cnt= $arg"
  let "cnt+=1"
done

cnt=1
echo "Values of \"$@\":"
for arg in "$@"
do
  echo "Arg #$cnt= $arg"
  let "cnt+=1"
done

Running ./expan.sh "Hello world" 2 3 4 produces:

Values of "$*":
Arg #1= Hello world-2-3-4
Values of "$@":
Arg #1= Hello world
Arg #2= 2
Arg #3= 3
Arg #4= 4

Example 2 – Counting arguments with $#

The script count.sh checks whether the number of supplied arguments is less than two. If so, it prints a usage message; otherwise it displays the first two arguments and performs basic arithmetic on them.

#!/bin/bash
if [ $# -lt 2 ]
then
  echo "Usage: $0 arg1 arg2"
  exit
fi

echo -e "$1=$1"
echo -e "$2=$2"

let add=$1+$2
let sub=$1-$2
let mul=$1*$2
let div=$1/$2

echo -e "Addition=$add
Subtraction=$sub
Multiplication=$mul
Division=$div"

Sample execution:

$ ./count.sh 2314 15241
$1=2314
$2=15241
Addition=17555
Subtraction=-12927
Multiplication=35267674
Division=0

Example 3 – Process‑related parameters $$ and $!

The script proc.sh prints its own process ID using $$, starts a background sleep command, and then prints the background job’s PID via $!.

#!/bin/bash
echo -e "Process ID=$$"

sleep 1000 &
echo -e "Background Process ID=$!"

Typical output:

Process ID=14625
Background Process ID=14626

These examples illustrate how $* concatenates all positional parameters into a single string, $@ treats each parameter separately, $# reports the count of arguments, $$ gives the script’s own PID, and $! provides the PID of the most recent background command.