What are the parentheses used for in a bash shell script function definition like "f () {}"? Is it different than using the "function" keyword?

Question

I'v always wondered what they're used for? Seems silly to put them in every time if you can never put anything inside them.

function_name () {
    #statements
}

Also is there anything to gain/lose with putting the function keyword at the start of a function?

function function_name () {
    #statements
}

Answer 1

The keyword function has been deprecated in favor of function_name() for portability with the POSIX spec

A function is a user-defined name that is used as a simple command to call a compound command with new positional parameters. A function is defined with a "function definition command".

The format of a function definition command is as follows:

fname() compound-command[io-redirect ...]

Note that the { } are not mandatory so if you're not going to use the keyword function (and you shouldn't) then the () are necessary so the parser knows you're defining a function.

Example, this is a legal function definition and invocation:

$ myfunc() for arg; do echo "$arg"; done; myfunc foo bar
foo
bar

Answer 2

The empty parentheses are required in your first example so that bash knows it's a function definition (otherwise it looks like an ordinary command). In the second example, the () is optional because you've used function.

Answer 3

Without function, alias expansion happens at definition time. E.g.:

alias a=b
# Gets expanded to "b() { echo c; }" :
a() { echo c; }
b
# => c
# Gets expanded to b:
a
# => c

With function however, alias expansion does not happen at definition time, so the alias "hides" the definition:

alias a=b
function a { echo c; }
b
# => command not found
# Gets expanded to b:
a
# => command not found
unalias a
a
# => c