decorators / monkeypatching in bourne shell / bash

myscript.sh

#!/bin/sh
# usage:
#   ./myscript.sh foo bar
#   handler called: foo bar
#   i am foo

call(){ type on_$1 1>/dev/null 2>/dev/null && on_$1 "$@" || "$@"; }

on_foo(){
  echo before handler called: "$@"
  "$@"
  echo after handler called: "$@"
}

foo(){
  echo i am foo
}

call "$@"