Minimal parser and interpreter

minimal-parser-example.bash

#!/usr/bin/env bash

: ${DEBUG:=} # 0 ... 2

shopt -s extglob
debug(){ if [[ $DEBUG -ge $1 ]]; then shift 1; eval "${@}"; fi; }
debug 2 shopt -p extglob

acceptArg () {
  local spec="$1" cb="$2" arg="$3";
  declare -A status=([args]=$(($# < 3)) [matched]=0);
  shift 3;

  # https://unix.stackexchange.com/a/234415/61350  # how-can-i-use-a-variable-as-a-case-condition
  matcher="@($spec)"
  case "$arg" in
    $matcher) status[matched]=1; eval $cb ;;
    *) debug 1 echo "spec: '$spec' failed to match arg: '$arg'";;
  esac
  debug 2 declare -p status
  (( status[matched] ))
  return $?
}

count=0
incr(){ echo "$((++count)) $1"; }
handleFirst(){ incr first; }
handleSecond(){ incr second; }

acceptArg "f|first" handleFirst "$1" && shift 1;
acceptArg "s|second|*2" handleSecond "$1" && shift 1;
[[ $? ]] && echo done || echo failed

I'm writing a bit at odds with bash convention here.
Re: DEBUG, [[ <unset> ]] is false. status[matched]=1, and then checking == 1

Rather bash has 2 conflicting conventions, (( <non-zero> )) -> 0, [[ <zero/null/undef/empty-string> ]] -> 0.