What is the 'function' keyword used in some bash scripts?

Question

For example: Bash-Prog-Intro-HOWTO

function foo() {}

I make search queries in info bash and look in releted chapters of POSIX for

Answer 1

The reserved word function is optional. See the section 'Shell Function Definitions' in the bash man page.

Answer 2

The function keyword is necessary in rare cases when the function name is also an alias. Without it, Bash expands the alias before parsing the function definition -- probably not what you want:

alias mycd=cd
mycd() { cd; ls; }  # Alias expansion turns this into cd() { cd; ls; }
mycd                # Fails. bash: mycd: command not found
cd                  # Uh oh, infinite recursion.

With the function keyword, things work as intended:

alias mycd=cd
function mycd() { cd; ls; }  # Defines a function named mycd, as expected.
cd                           # OK, goes to $HOME.
mycd                         # OK, goes to $HOME.
\mycd                        # OK, goes to $HOME, lists directory contents.

Answer 3

The function keyword is optional when defining a function in Bash, as documented in the manual:

Functions are declared using this syntax:

name () compound-command [ redirections ]

or

function name [()] compound-command [ redirections ]

The first form of the syntax is generally preferred because it's compatible with Bourne/Korn/POSIX scripts and so more portable.
That said, sometimes you might want to use the function keyword to prevent Bash aliases from colliding with your function's name. Consider this example:

$ alias foo="echo hi"
$ foo() { :; }
bash: syntax error near unexpected token `('

Here, 'foo' is replaced by the text of the alias of the same name because it's the first word of the command. With function the alias is not expanded:

 $ function foo() { :; }