Bash allows to define functions, which is very good way to reuse parts of shell scripts. This tutorial shows how to define Bash functions and apply in scripts.

Function can be defined using the function keyword, like so:

#!/bin/bash

# $0 is a Bash variable that resolves to script's name
function print_usage {
    echo "Usage: $0 [message]"
    echo "Prints message and exits."
    # the following line ends the script
    exit 1
}

# function parameters are accessible as variables $1, $2, ...
function my_echo {
    echo "Number of params passed to the my_echo function: $#"
    echo "${1}"
}

echo "Number of params passed to the script: $#"

if [ $# -eq 0 ]; then
    # function without arguments can be called like that:
    print_usage
else
    # all parameters passed to the script are joined
    # and passed as one to the function:
    my_echo "$*"
fi

Calling the script without parameters:

$> ./echo.sh
Number of params passed to the script: 0
Usage: ./echo.sh [message]
Prints message and exits.

Calling the script with one parameter:

$> ./echo.sh Hello
Number of params passed to the script: 1
Number of params passed to the my_echo function: 1
Hello

Calling the script with more than one parameter (note the number of parameters!):

$> ./echo.sh Hello World
Number of params passed to the script: 2
Number of params passed to the my_echo function: 1
Hello World

Remember to define function before it’s use. Otherwise you will receive the following error:

$> ./echo.sh
./echo.sh: line 4: print_usage: command not found

Defining Bash functions is basic of advanced shell scripting. From now on possibilities are endless! :-)