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How can I handle command-line options and arguments in my script easily?

Well, that depends a great deal on what you want to do with them. There are two standard approaches, each with its strengths and weaknesses.

Overview

A Unix command generally has an argument syntax like this:

tar -x -f archive.tar -v -- file1 file2 file3

Please note the conventions and the ordering here, because they are important. They actually matter. This command has some arguments (file1, file2, file3), and some options (-x -f archive.tar -v), as well as the traditional end of options indicator "--".

The options appear before the non-option arguments. They do not appear afterward. They do not appear at just any old random place in the command.

Some options (-x, -v) are standalones. They are either present, or not. Some options (-f) take an argument of their own.

In all cases, option processing involves writing a loop. Ideally, this loop will make one pass over the argument list, examining each argument in turn, and setting appropriate shell variables so that the script remembers which options are in effect. Ultimately, it will discard all of the options, so that the argument list is left holding only the non-option arguments (file1 file2 file3). The rest of the script, then, can simply begin processing those, referring as needed to the variables that were set up by the option processor.

The option processor recognizes the end of options when it finds a -- argument, or when it finds an argument that doesn't start with a hyphen. (The option argument archive.tar does not signal the end of options, because it is processed along with the -f option.)

There are two basic approaches to writing an option processing loop: either write the loop yourself from scratch (we'll call this a "manual loop"), or use the shell's getopts command to assist with option splitting. We'll cover both of these cases.

Never use getopt(1). Traditional versions of getopt cannot handle empty argument strings, or arguments with embedded whitespace. There is a version of getopt in util-linux which supports these, but you should not use it. Why not? Because:

1. You would need to write special safety-checking code to ensure that you've actually got the util-linux version of getopt, and then you would still need to write a fallback option processor for when you don't have it. So you're doing twice as much work.

2. getopt doesn't even simplify your option parsing loop. getopt only does single-letter option splitting. Everything else, you still have to write by hand. You can live without single-letter option splitting. It's not worth the effort.

Manual loop

Manually parsing options is the most flexible approach. It is the best way, really, because it allows you to do anything you like: you can handle both single-letter and long options, with or without option arguments. That's why we're showing it first.

If you want to handle GNU-style --long-options or Tcl-style -longopts, a manual loop is your only choice. getopts does not support these.

In this example, notice how both --file FILE and --file=FILE are handled.

   1 #!/bin/sh
   2 # POSIX
   3 
   4 die() {
   5     printf '%s\n' "$1" >&2
   6     exit 1
   7 }
   8 
   9 # Initialize all the option variables.
  10 # This ensures we are not contaminated by variables from the environment.
  11 file=
  12 verbose=0
  13 
  14 while :; do
  15     case $1 in
  16         -h|-\?|--help)
  17             show_help    # Display a usage synopsis.
  18             exit
  19             ;;
  20         -f|--file)       # Takes an option argument; ensure it has been specified.
  21             if [ "$2" ]; then
  22                 file=$2
  23                 shift
  24             else
  25                 die 'ERROR: "--file" requires a non-empty option argument.'
  26             fi
  27             ;;
  28         --file=?*)
  29             file=${1#*=} # Delete everything up to "=" and assign the remainder.
  30             ;;
  31         --file=)         # Handle the case of an empty --file=
  32             die 'ERROR: "--file" requires a non-empty option argument.'
  33             ;;
  34         -v|--verbose)
  35             verbose=$((verbose + 1))  # Each -v adds 1 to verbosity.
  36             ;;
  37         --)              # End of all options.
  38             shift
  39             break
  40             ;;
  41         -?*)
  42             printf 'WARN: Unknown option (ignored): %s\n' "$1" >&2
  43             ;;
  44         *)               # Default case: No more options, so break out of the loop.
  45             break
  46     esac
  47 
  48     shift
  49 done
  50 
  51 # if --file was provided, open it for writing, else duplicate stdout
  52 if [ "$file" ]; then
  53     exec 3> "$file"
  54 else
  55     exec 3>&1
  56 fi
  57 
  58 # Rest of the program here.
  59 # If there are input files (for example) that follow the options, they
  60 # will remain in the "$@" positional parameters.

This parser does not handle single-letter options concatenated together (like -xvf being understood as -x -v -f). This could be added with effort, but it's left as an exercise for the reader. In practice, it's exceptionally rare for shell scripts that handle long options to handle single-letter option splitting as well. It's simply not worth the effort.

For the most part, shell scripts that you write will not need to worry about single-letter option splitting, because you are the only person using them. Fancy option processing is only desirable if you are releasing the program for general use, and that is almost never going to be the case in real life. Single-letter option combining also precludes the use of Tcl-style long arguments (-foo), which some commands like find(1), gcc(1) and star(1) use.

getopts

The main benefit of getopts is to allow single-letter option splitting (-xvf handled as -x -v -f). The trade-off for this is that you cannot use long arguments of any kind (GNU-style --foo or Tcl-style -foo), or options with a optional argument (like mysql's -p[password] option).

getopts is suitable for simple scripts. The more complex your option parsing needs are, the less likely it is that you'll be able to make use of getopts.

Here is a getopts example:

   1 #!/bin/sh
   2 
   3 # Usage info
   4 show_help() {
   5 cat << EOF
   6 Usage: ${0##*/} [-hv] [-f OUTFILE] [FILE]...
   7 Do stuff with FILE and write the result to standard output. With no FILE
   8 or when FILE is -, read standard input.
   9 
  10     -h          display this help and exit
  11     -f OUTFILE  write the result to OUTFILE instead of standard output.
  12     -v          verbose mode. Can be used multiple times for increased
  13                 verbosity.
  14 EOF
  15 }
  16 
  17 # Initialize our own variables:
  18 output_file=""
  19 verbose=0
  20 
  21 OPTIND=1
  22 # Resetting OPTIND is necessary if getopts was used previously in the script.
  23 # It is a good idea to make OPTIND local if you process options in a function.
  24 
  25 while getopts hvf: opt; do
  26     case $opt in
  27         h)
  28             show_help
  29             exit 0
  30             ;;
  31         v)  verbose=$((verbose+1))
  32             ;;
  33         f)  output_file=$OPTARG
  34             ;;
  35         *)
  36             show_help >&2
  37             exit 1
  38             ;;
  39     esac
  40 done
  41 shift "$((OPTIND-1))"   # Discard the options and sentinel --
  42 
  43 # Everything that's left in "$@" is a non-option.  In our case, a FILE to process.
  44 printf 'verbose=<%d>\noutput_file=<%s>\nLeftovers:\n' "$verbose" "$output_file"
  45 printf '<%s>\n' "$@"
  46 
  47 # End of file

There is a getopts tutorial which explains what all of the syntax and variables mean. In bash, there is also help getopts.

The advantages of getopts over a manual loop:

1. It can handle things like -xvf filename in the expected Unix way, automatically.

2. It makes sure options are parsed like any standard command (lowest common denominator), avoiding surprises.
3. With some implementations, the error messages will be localised in the language of the user.

The disadvantages of getopts:

1. (Except for ksh93) it can only handle short options (-h, not --help).

2. It cannot handle options with optional arguments like mysql's -p[password].

3. It doesn't exist in the Bourne shell.

4. It only allows options to be parsed in the "standard way" (lowest common denominator).

5. Options are coded in at least 2, probably 3 places -- in the call to getopts, in the case statement that processes them, and in the help/usage message that documents them.

For other, more complicated ways of option parsing, see ComplexOptionParsing.

CategoryShell