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

The article introduces several special parameters used in Bash scripting: $*, $@, $#, $$, and $!, and explains how each one works.

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

#!/bin/bash

export IFS='-'
cnt=1
# Printing the data available in $*
echo "Values of \"$*\":"
for arg in "$*"
do
  echo "Arg #$cnt= $arg"
  let "cnt+=1"
done

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

Running the script with ./expan.sh "Hello world" 2 3 4 shows that $* produces a single string "Hello world-2-3-4" (joined by the IFS delimiter), while $@ treats each argument separately, yielding four distinct lines.

Example 2 – Using $# to count arguments and perform arithmetic

#!/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
"

The script first checks that at least two positional parameters are supplied; if not, it prints a usage message. It then displays the two arguments and calculates their sum, difference, product, and integer division using the let builtin.

Example 3 – Accessing process IDs with $$ and $!

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

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

When executed, $$ prints the PID of the running script, and $! prints the PID of the most recent background command (the sleep process). The article also shows a sample ps output confirming the IDs.

Key takeaways

$*

expands all positional parameters into a single word, separated by the current IFS value. $@ expands each positional parameter as a separate word, preserving quoting. $# provides the count of positional parameters passed to the script. $$ returns the PID of the current shell process. $! returns the PID of the most recently executed background command.

Understanding these parameters enables more precise control over argument handling, script flow, and process management in Bash scripts.