Diff for "BashFAQ/035"

How can I handle command-line arguments to my script easily?

Well, that depends a great deal on what you want to do with them. Here's a general template that might help for the simple cases:

# Bash
while [[ $1 == -* ]]; do
    case "$1" in
      -h|--help|-\?) show_help; exit 0;;
      -v|--verbose) verbose=1; shift;;
      -f) if (($# > 1)); then
            output_file=$2; shift 2
          else 
            printf "%s\n" "-f requires an argument"
            exit 1
          fi ;;
      --) shift; break;;
      -*) echo "invalid option: $1"; show_help; exit 1;;
    esac
done

Now all of the remaining arguments are the filenames which followed the optional switches. You can process those with for i or "$@".

A POSIX version of that same code:

# POSIX
while true; do
    case "$1" in
      -h|--help|-\?) show_help; exit 0;;
      -v|--verbose) verbose=1; shift;;
      -f) if [ $# -gt 1 ]; then
            output_file=$2; shift 2
          else 
            printf "%s\n" "-f requires an argument"
            exit 1
          fi ;;
      --) shift; break;;
      -*) echo "invalid option: $1"; show_help; exit 1;;
      *)  break;;
    esac
done

For more complex/generalized cases, or if you want things like "-xvf" to be handled as three separate flags, you can use getopts. (NEVER use getopt(1)!)

Here is a simplistic getopts example:

# POSIX
x=1         # Avoids an error if we get no options at all.
while getopts "abcf:g:h:" opt; do
  case "$opt" in
    a) echo "You said a";;
    b) echo "You said b";;
    c) echo "You said c";;
    f) echo "You said f, with argument $OPTARG";;
    g) echo "You said g, with argument $OPTARG";;
    h) echo "You said h, with argument $OPTARG";;
  esac
  x=$OPTIND
done
shift $(($x-1))
echo "Left overs: $@"

If you prefer to check options with if statements, then a function like this one may be useful:

# Bash
HaveOpt() {
  local needle=$1
  shift
  while [[ $1 == -* ]]; do
    case "$1" in
      --) return 1; # by convention, -- is end of options
      $needle) return 0;;
    esac
    shift
  done
  return 1
}
if HaveOpt --quick "$@"; then echo "Option quick is set"; fi

and it will work if script is run as:

• YES: ./script --quick
• YES: ./script -other --quick

but will stop on first argument with no "-" in front (or on --):

• NO: ./script -bar foo --quick
• NO: ./script -bar -- --quick

Of course, this approach (iterating over the argument list every time you want to check for one) is far less efficient than just iterating once and setting flag variables.