Check if a function exists before executing it in shell

Question

I want to check if a function exist or not before executing it in the shell script.

Does script shell support that? and how to do it?

Catalan

See #86380 - but you mention ash, not sure if that solution works in ash as well. – Criticize

Answer 1

As read in this comment, this should make it:

type -t function_name

this returns function if it is a function.

Test

$ type -t f_test
$ 
$ f_test () { echo "hello"; }
$ type -t f_test
function

Note that type provides good informations:

$ type -t ls
alias
$ type -t echo
builtin

Halfhour

Is -t available in ash? It's not POSIX, so I wonder if it is a bash extension. – Hasseman

To be honest, I don't know it, @chepner. – Halfhour

Answer 2

POSIX does not specify any arguments for the type built-in and leaves its output unspecified. Your best bet, apart from a shell-specific solution, is probably

if type foo | grep -i function > /dev/null; then
   # foo is a function
fi

Selfrestraint

Do not use this. Depending on the language of the user the word "function" may not exists in the output. – Ephesus

Answer 3

You can always just try executing the function. If it doesn't exist, the exit status of the command will be 127:

$ myfunction arg1 arg2 || {
>  if [ $? -ne 127 ]; then
>    echo Error with my function
>  else
>    echo myfunction undefined
>  fi; }

Of course, the function may have the same name as another binary, and you may not want to run that program if it is not shadowed by the function.

Hasseman

What if the function is destructive (think make clean)? What if the function itself returns 127 or its last command does? I wouldn't want to run a reboot function just to see if it exists... – Selfrestraint

The first point is valid. I would argue that if the function itself returns 127, that's a bug in the function, as it is defined by POSIX to mean "command not found". I was working on the assumption that if he's going to call it if it exists, there's no harm in just trying to call it first. – Hasseman