Functions

Functions allow you to reuse code and much more. Here are some examples:

1. Modularize

function say_hello() {
    echo "Hello, $1"
}

say_hello "world"

Function say_hello takes one argument ($1). When you call the function, it will print “Hello, world”.

2. Return values

Functions in BASH can do more than just encapsulate code. They can also take arguments and return values. Here’s an example of a function that calculates the factorial of a number:

factorial() {
    if [[ "$1" -le '1' ]]; then
        echo '1'
    else
        last=$( factorial "$(( $1 - 1 ))" )
        echo "$(( $1 * last ))"
    fi
}

echo "Factorial of 5 is $( factorial 5 )"

This function uses recursion to calculate the factorial. Note how it calls itself with the expression factorial "$(( $1 - 1 ))", and how it uses echo to return a value.

3. Global variables

Here’s an example where a function modifies a global variable:

my_var="Original value"

change_var() {
    my_var="Changed value"
}

echo $my_var
change_var
echo $my_var

4. Local variables

Bash also supports local variables within functions, which won’t affect the global scope:

my_var="Original value"

change_var() {
    local my_var="Changed value"
    echo $my_var
}

echo $my_var
change_var
echo $my_var

In this script, my_var within the function is a local variable. Its change doesn’t affect the global my_var, so the second echo $my_var outside the function still prints “Original value”.