recursive md5 implementation with md5sum fallback, bash only

md5.sh

#!/bin/bash

if [ "$#" -eq 0 ]; then
  md5sum "$@"
  exit 0
fi

recursive=0
exclude=0
sum=0
excludes=()
root=()
skip=0

for param in "$@"; do
  if [ $skip -ne 1 ]; then
    if [ $exclude -eq 1 ]; then
      excludes+=('(' '-name' "$param" '-prune' ')' '-o')
      exclude=0
    elif [ "${param,,}" == '-r' ]; then
      if [ $recursive -eq 1 ]; then
        echo "unknown option -r" 1>&2
        exit 1
      fi
      recursive=1
    elif [ "${param,,}" == '-e' ]; then
      exclude=1
    elif [ "${param,,}" == '-s' ]; then
      sum=1
    elif [ "$param" == '--' ]; then
      skip=1
    elif [ "${param:0:1}" == '-' ]; then
      echo "unknown option $param" 1>&2
      exit 1
    else
      # TODO: combine with outer else clause
      root+=("$param")
    fi
  else
    root+=("$param")
  fi
done

if [ $exclude -eq 1 ]; then
  echo "expected exclude string" 1>&2
  exit 1
fi

if [ $recursive -eq 0 ]; then
  md5sum "$@"
  exit 0
fi

if [ "${#root[@]}" -eq 0 ]; then
  root+=('.')
fi

find "${root[@]}" "${excludes[@]}" -type d -o -print0 | xargs -0 md5sum | (if [[ "$sum" -eq 1 ]]; then md5sum | awk '{ print $1 }'; else cat; fi)