Get parent function argument length in bash?

Question

You're able to access the parent function name with ${FUNCNAME[1]} (In BASH, is it possible to get the function name in function body?). Is there a way to also access the parent function argument length (arity)?

So instead of this:

get() {
  [ "$#" != 2 ] && exit 1
}

You could do something like this:

get() {
  assert_arity 2
}

assert_arity() {
  local arity=${parent_function_arity}
  local value=$1
  [ "${arity}" != "${value}" ] && exit 1
}

Is that possible?

score 4 · Accepted Answer · answered Sep 12 '15 at 17:47

4

It is possible:

shopt -s extdebug

assert_arity() {
    local arity=${BASH_ARGC[1]}
    local value=$1
    [ "${arity}" != "${value}" ] && echo "nope"
}

BASH_ARGC is set only in extdebug mode. If it is, it is an array of the number of arguments of the whole call stack, with the current frame at index 0.

answered Sep 12 '15 at 17:47

Renaud Pacalet

25,260
3
34
51

So you just add that `shopt -s extdebug` to the top of any bash script to use it? – Lance Sep 12 '15 at 17:51
Yes, or just before using it (I mean, just before you want to stack the argc). – Renaud Pacalet Sep 12 '15 at 17:57
any idea how to do this in zsh? – Chris May 24 '21 at 00:34

score 2 · Answer 2 · answered Sep 12 '15 at 20:06

2

If all you want is a cleaner code you can pass the actual arguments to the assert_arity() function like this:

get() {
  assert_arity 2 "$@"
}

assert_arity() {
  local arity=$(( $# - 1 ))
  local value=$1
  [ "${arity}" != "${value}" ] && exit 1
}

answered Sep 12 '15 at 20:06

vlp

7,811
2
23
51

score 0 · Answer 3 · answered Sep 12 '15 at 17:46

If you set the extdebug shell option, the arities of the current call stack are stored in the array BASH_ARGC.

shopt -s extdebug

get () {
  assert_arty 2
}

assert_arity () {
  local arity=${BASH_ARGC[1]}  # 0 is us, 1 is the parent
  local value=$1
  (( arity != value )) && exit 1
}

Get parent function argument length in bash?

3 Answers3