Check Black List Words in Files by Bash Script

I wrote tiny bash script for checkin files contains black listed words or not.
The aim of this script is to automate verifying php or javascript files does not have any debugging code, such as "var_dump","alert" etc.

#!/bin/bash
PHP_BLACK_LIST_WORDS=("var_dump(")
JAVASCRIPT_BLACK_LIST_WORDS=("alert(")

verify_words_are_not_contained(){
  words="$1"
  len=${#words[@]}
  if [ $len -ge 0 ]
  then
    grep_words=""
    for word in "${words[@]}"
    do
      grep_words+="$word\|"
    done
  
    # remove last 2 characters
    grep_words=${grep_words%?}
    grep_words=${grep_words%?}

    grep_command="grep \"$grep_words\" $2 -nr --include=$3 > $OUTPUT_DIR/$4"
    echo "running $grep_command"
    eval $grep_command
  fi
}


setup_output_dir(){
  target_dir_name=${1##*/}
  OUTPUT_DIR=$target_dir_name"_detected_black_list_words"
  echo "$OUTPUT_DIR"
  rm -r -f "$OUTPUT_DIR"
  mkdir -p "$OUTPUT_DIR"
}

set -e
set -o pipefail

if [ $# -ne 1 ]
then
  echo "please specify target directory"
  exit 1
fi

TARGET_DIR=$1
setup_output_dir "$TARGET_DIR"
verify_words_are_not_contained $PHP_BLACK_LIST_WORDS $TARGET_DIR "*.php" "php_black_list_words.txt"
verify_words_are_not_contained $JAVASCRIPT_BLACK_LIST_WORDS $TARGET_DIR "*.js" "javascript_black_list_words.txt"