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. There are several approaches, each with its strengths and weaknesses.

Manual loop

This approach handles any arbitrary set of options, because you're writing the parser yourself. For 90% of programs, this is the simplest approach (because you rarely need fancy stuff).

This example will handle a combination of short and long options. Notice how both "--file" and "--file=FILE" are handled.

   1 #!/bin/sh
   2 # (POSIX shell syntax)
   3 
   4 # Reset all variables that might be set
   5 file=
   6 verbose=0
   7 
   8 while :; do
   9     case $1 in
  10         -h|-\?|--help)   # Call a "show_help" function to display a synopsis, then exit.
  11             show_help
  12             exit
  13             ;;
  14         -f|--file)       # Takes an option argument, ensuring it has been specified.
  15             if [ "$2" ]; then
  16                 file=$2
  17                 shift 2
  18                 continue
  19             else
  20                 echo 'ERROR: Must specify a non-empty "--file FILE" argument.' >&2
  21                 exit 1
  22             fi
  23             ;;
  24         --file=?*)
  25             file=${1#*=} # Delete everything up to "=" and assign the remainder.
  26             ;;
  27         --file=)         # Handle the case of an empty --file=
  28             echo 'ERROR: Must specify a non-empty "--file FILE" argument.' >&2
  29             exit 1
  30             ;;
  31         -v|--verbose)
  32             verbose=$((verbose + 1)) # Each -v argument adds 1 to verbosity.
  33             ;;
  34         --)              # End of all options.
  35             shift
  36             break
  37             ;;
  38         -?*)
  39             printf 'WARN: Unknown option (ignored): %s\n' "$1" >&2
  40             ;;
  41         *)               # Default case: If no more options then break out of the loop.
  42             break
  43     esac
  44 
  45     shift
  46 done
  47 
  48 # Suppose --file is a required option. Check that it has been set.
  49 if [ ! "$file" ]; then
  50     echo 'ERROR: option "--file FILE" not given. See --help.' >&2
  51     exit 1
  52 fi
  53 
  54 # Rest of the program here.
  55 # If there are input files (for example) that follow the options, they
  56 # will remain in the "$@" positional parameters.

This parser does not handle separate options concatenated together (like -xvf being understood as -x -v -f). This could be added with effort, but this is left as an exercise for the reader.

Some Bash programmers like to write this at the beginning of their scripts to guard against unused variables:

    set -u     # or, set -o nounset

The use of this breaks the loop above, as "$1" may not be set upon entering the loop. There are four solutions to this issue:

1. Stop using -u.

2. Replace case $1 in with case ${1+$1} in (as well as bandaging all the other code that set -u breaks).

3. Replace case $1 in with case ${1-} in (every potentially undeclared variable could be written as ${variable-} to prevent set -u tripping).

4. Stop using -u.

getopts

Unless it's the version from util-linux, and you use its advanced mode, never use getopt(1). Traditional versions of getopt cannot handle empty argument strings, or arguments with embedded whitespace.

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

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

The advantages of getopts are:

1. It's portable, and will work in any POSIX shell e.g. dash.

2. It can handle things like -vf filename in the expected Unix way, automatically.

3. It understands -- as the option terminator and more generally makes sure, options are parsed like for any standard command.

4. With some implementations, the error messages will be localised in the language of the user.

The disadvantage of getopts is that (except for ksh93 getopts) it can only handle short options (-h, not --help) without trickery and cannot handle options with optional arguments à la GNU.

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

There is also 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. This can be avoided by using callback functions, but this approach kind of defeats the purpose of using getopts at all.

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

CategoryShell