Pure bash tolower example

tolower

if (( BASH_VERSINFO >= 4 )); then
  tolower() {
    # If parameter is a file, or stdin is used, action that first
    if [[ -r "${1}" ]]||[[ ! -t 0 ]]; then
      # We structure our while read loop to handle no newline at EOF
      eof=
      while [[ -z "${eof}" ]]; do
        read -r || eof=true
        printf -- '%s\n' "${REPLY,,}"
      done < "${1:-/dev/stdin}"
    # Otherwise, if a parameter exists, modify it
    elif [[ "${1}" ]]; then
      printf -- '%s\n' "${*,,}"
    # Otherwise we print our usage
    else
      printf -- '%s\n' "Usage: tolower [FILE|STDIN|STRING]"
      return 1
    fi
  }
else
  # This is the magic sauce - we convert the input character to a decimal (%d)
  # Then add 32 to move it 32 places on the ASCII table
  # Then we print it in unsigned octal (%o)
  # And finally print the char that matches the octal representation (\\)
  # Example: printf '%d' "'A" => 65 (+32 = 97)
  #          printf '%o' "97" => 141
  #          printf \\141 => a
  lc(){
    # shellcheck disable=SC2059
    case "${1}" in
      ([[:upper:]])
        printf \\"$(printf '%o' "$(( $(printf '%d' "'${1}") + 32 ))")"
      ;;
      (*)
        printf "%s" "${1}"
      ;;
    esac
  }
  tolower() {
    if [[ -r "${1}" ]]||[[ ! -t 0 ]]; then
      eof=
      while [[ -z "${eof}" ]]; do
        read -r || eof=true
        for ((i=0;i<${#REPLY};i++)); do
          lc "${REPLY:$i:1}"
        done
        printf -- '%s\n' ""
      done < "${1:-/dev/stdin}"
    elif [[ "${1}" ]]; then
      output="$*"
      for ((i=0;i<${#output};i++)); do
        lc "${output:$i:1}"
      done
      printf -- '%s\n' ""
    else
      printf -- '%s\n' "Usage: tolower [FILE|STDIN|STRING]"
      return 1
    fi
  }
fi