Defining and Calling Functions

Defining a Simple Function

A function is defined with name() { commands }, where the curly braces wrap the block of commands that make up the function's body. The function keyword can optionally precede the name as an alternative style, but is not required in Bash.

Example: Defining a Simple Function

#!/bin/bash
greet() {
    echo "Hello from the greet function!"
}
greet

Calling a Function Multiple Times

Once defined, a function can be called as many times as needed throughout the rest of the script, exactly like any built-in command. This is the core benefit of functions: writing the logic once and reusing it wherever it's needed.

Example: Calling a Function Multiple Times

#!/bin/bash
show_divider() {
    echo "------------------------"
}
show_divider
echo "Section one"
show_divider
echo "Section two"
show_divider

Definitions Must Come Before Use

Bash reads and executes a script top to bottom, so a function must already be defined by the time the script reaches a line that calls it. Calling a function before its definition appears results in a 'command not found' error.

Warning: It's common practice to define all functions near the top of a script, followed by the main sequence of calls at the bottom.

Example: Definitions Must Come Before Use

#!/bin/bash
say_hi() {
    echo "Hi there!"
}
say_hi

The function Keyword Alternative

Bash also accepts function name { commands; } as an alternative syntax to name() { commands; }; both define a function identically. The parenthesis-based form is more portable to other POSIX-compatible shells, while function is a Bash/ksh-style extension.

Example: The function Keyword Alternative

#!/bin/bash
function farewell {
    echo "Goodbye!"
}
farewell