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 function keyword but nothing found.

What is function keyword used in some bash scripts? Is that some deprecated syntax?

Answer 1

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() { :; }