Shell Positional and Predefined Variables

This article explains common shell positional and predefined variables such as $#, $*, $$, $!, $@, $-, and $?, describing their meanings and typical uses.

It then provides two example scripts. The first script ( 01.sh) demonstrates adding two arguments using expr and printing the result.

Example script content:

#!/bin/bash

SUM=$(expr $1 + $2)

echo "$1 + $2 = $SUM"

The second script ( param.sh) shows how to display the script’s path, name, and each positional parameter using echo and the special variables described earlier.

Example script content:

#!/bin/sh

# $0: file full path name

echo "path of script : $0"

# $0 basename

echo "name of script : $(basename $0)"

# $1 … $5

echo "parameter 1 : $1"

echo "parameter 2 : $2"

echo "parameter 3 : $3"

echo "parameter 4 : $4"

echo "parameter 5 : $5"

# $# and $*

echo "The number of arguments passed : $#"

echo "Show all arguments : $*"

# $$ and $?

echo "Process ID : $$"

echo "errors : $?"