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 :
   9 do
  10     case $1 in
  11         -h | --help | -\?)
  12             #  Call your Help() or usage() function here.
  13             exit 0      # This is not an error, User asked help. Don't do "exit 1"
  14             ;;
  15         -f | --file)
  16             file=$2     # You might want to check if you really got FILE
  17             shift 2
  18             ;;
  19         --file=*)
  20             file=${1#*=}        # Delete everything up till "="
  21             shift
  22             ;;
  23         -v | --verbose)
  24             # Each instance of -v adds 1 to verbosity
  25             verbose=$((verbose+1))
  26             shift
  27             ;;
  28         --) # End of all options
  29             shift
  30             break
  31             ;;
  32         -*)
  33             echo "WARN: Unknown option (ignored): $1" >&2
  34             shift
  35             ;;
  36         *)  # no more options. Stop while loop
  37             break
  38             ;;
  39     esac
  40 done
  41 
  42 # Suppose some options are required. Check that we got them.
  43 
  44 if [ ! "$file" ]; then
  45     echo "ERROR: option '--file FILE' not given. See --help" >&2
  46     exit 1
  47 fi
  48 
  49 # Rest of the program here.
  50 # If there are input files (for example) that follow the options, they
  51 # 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 # Initialize our own variables:
   4 output_file=""
   5 verbose=0
   6 
   7 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.
   8 while getopts "hvf:" opt; do
   9     case "$opt" in
  10         h)
  11             show_help
  12             exit 0
  13             ;;
  14         v)  verbose=1
  15             ;;
  16         f)  output_file=$OPTARG
  17             ;;
  18         '?')
  19             show_help >&2
  20             exit 1
  21             ;;
  22     esac
  23 done
  24 shift $((OPTIND-1)) # Shift off the options and optional --.
  25 
  26 echo "verbose=$verbose, output_file='$output_file', Leftovers: $@"
  27 
  28 # 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