Functions

Bash functions encapsulate functionality into re-usable code blocks you can call throughout your script. This Linuxize article explains them with good code examples.

The function keyword

When declaring a Bash function, the most common/accepted and compatible signature is:

function_name() {
  ...
}

You can also choose to prefix a function with the function keyword, like:

function function_name() {
  ...
}

There are mixed opinions on this because while the first option is compatible across more shells, the second option helps avoid a specific scenario where a function name can collide with a Bash alias.

Say you have an alias in your shell named ts, which returns a timestamp:

alias ts=$(date +"%Y-%m-%d_%H:%M")

And then in a script you create a function named ts using the first option with no function keyword:

#!/usr/bin/env bash

ts() { date +"%Y-%m-%d_%H:%M"; }

echo $(ts)

This will cause an error like:

bash: syntax error near unexpected token (

This collision occurs because the ts alias already exists, so writing the function like this tries to use the ts alias, and passes ( as the first arg, which is invalid.

Prefixing the script’s ts function with the function keyword prevents this error:

#!/usr/bin/env bash

function ts() { date +"%Y-%m-%d_%H:%M"; }

echo $(ts)