Diff for "BashFAQ/035"

How can I handle command-line arguments (options) 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, or if you want to handle GNU-style long options, you need a different approach.

Never use getopt(1). getopt cannot handle empty arguments strings, or arguments with embedded whitespace. Please forget that it ever existed.

The POSIX shell (and others) offer getopts which is safe to use. Here is a simplistic getopts example:

# POSIX
x=1         # Avoids an error if we get no options at all.
while getopts "h?vf:" opt; do
  case "$opt" in
    h|\?) show_help; exit 0;;
    v) verbose=1;;
    f) output_file=$OPTARG;;
  esac
  x=$OPTIND
done
shift $(($x-1))
if [ "$1" = -- ]; then shift; fi
echo "verbose=$verbose, output_file='$output_file', Leftovers: $@"

There is still the disadvantage that options are coded in at least 2, probably 3 places - in the call to getopts, in the case statement that processes them and presumably in the help message that you are going to get around to writing one of these days. This is a classic opportunity for errors to creep in as the code is written and maintained - often not discovered till much, much later.

Another approach is to check options with if statements "on demand". 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. But it does offer a consolidation of the option-handling code into a single place (or two places if you count the help message).

bhepple suggests the use of process-getopt (GPL licensed) and offers this example code:

PROG=$(basename $0)
VERSION='1.2'
USAGE="A tiny example using process-getopt(1)"

# call process-getopt functions to define some options:
source process-getopt

SLOT=""
SLOT_func()   { [ "${1:-""}" ] && SLOT="yes"; }      # callback for SLOT option
add_opt SLOT "boolean option" s "" slot

TOKEN=""
TOKEN_func()  { [ "${1:-""}" ] && TOKEN="$2"; }      # callback for TOKEN option
add_opt TOKEN "this option takes a value" t n token number

add_std_opts     # define the standard options --help etc:

TEMP=$(call_getopt "$@") || exit 1
eval set -- "$TEMP" # just as with getopt(1)

# remove the options from the command line
process_opts "$@" || shift "$?"

echo "SLOT=$SLOT"
echo "TOKEN=$TOKEN"
echo "args=$@"

Here, all information about each option is defined in one place making for much easier authoring and maintenance. A lot of the dirty work is handled automatically and standards are obeyed as in getopt(1) - because it calls getopt for you.

• Actually, what the author forgot to say was that it's actually using getopts semantics, rather than getopt. I ran this test:

   wooledg@wooledg:~/process-getopt-1.6$ set -- one 'rm -rf /' 'foo;bar' "'"
   wooledg@wooledg:~/process-getopt-1.6$ call_getopt "$@"
    -- 'rm -rf /' 'foo;bar' ''\'''

• It appears to be intelligent enough to handle null options, whitespace-containing options, and single-quote-containing options in a manner that makes the eval not blow up in your face. But this is not an endorsement of the process-getopt software overall; I don't know it well enough. -GreyCat

It's written and tested on Linux where getopt(1) supports long options. For portability, it tests the local getopt(1) at runtime and if it finds an non-GNU one (ie one that does not return 4 for getopt --test) it only processes short options. It does not use the bash builtin getopts(1) command. -bhepple

CategoryShell