Parameters in bash script

Positional Parameters

Arguments passed to a script are processed in the same order in which they’re sent. The indexing of the arguments starts at one, and the first argument can be accessed inside the script using $1. Similarly, the second argument can be accessed using $2, and so on. The positional parameter refers to this representation of the arguments using their position.

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Let’s take an example of the following script, test.sh, which prints username, age, and full name in that order:

> ./test.sh one two "three four"

Output:

$1 = one
$2 = two
$3 = three four

# Size of the input array
$# = 3

Example script:

echo "Using \"\$*\":"
for a in "$*"; do
    echo $a;
done

echo -e "\nUsing \$*:"
for a in $*; do
    echo $a;
done

echo -e "\nUsing \"\$@\":"
for a in "$@"; do
    echo $a;
done

echo -e "\nUsing \$@:"
for a in $@; do
    echo $a;
done

The explanation and the results for the four cases are below.

In the first case, the parameters are regarded as one long quoted string:

Using "$*":
one two three four

Case 2 (unquoted) - the string is broken into words by the for loop:

Using $*:
one
two
three
four

Case 3 - it treats each element of $@ as a quoted string:

Using "$@":
one
two
three four

Last case - it treats each element as an unquoted string, so the last one is again split by what amounts to for three four:

Using $@:
one
two
three
four

Flags

Another way of passing input to a script is using flags.

sh test.sh -f 'John Smith' -a 25 -u john

We’ll modify the earlier script to use flags instead of relying on positional parameters. The getopts function reads the flags in the input, and OPTARG refers to the corresponding values:

`#!/bin/bash

while getopts u:a:f: flag
do
    case "${flag}" in
        u) username=${OPTARG};;
        a) age=${OPTARG};;
        f) fullname=${OPTARG};;
    esac
done

echo "Username: $username";
echo "Age: $age";
echo "Full Name: $fullname";

Let’s run this script with the same input as before, only this time, we’ll add flags to the input:

sh test.sh -f 'John Smith' -a 25 -u john

The output is the same as before, though we have shifted the positions of the username and full name arguments:

Username : john
Age: 25
Full Name: John Smith

Tags: bash

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