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

Manually parsing options without the use of a specialized function is the most flexible approach, and is sufficient for most simple scripts.

This example will handle a combination of short (POSIX) and long "GNU style" options with option arguments. Notice how both --file FILE and --file=FILE are handled. Typical scripts may also use functions and local variables, which can greatly improve your code. This example however illustrates a strictly POSIX conforming script.

   1 #!/bin/sh
   2 # POSIX
   3 
   4 # Reset all variables that might be set
   5 file=
   6 verbose=0 # Variables to be evaluated as shell arithmetic should be initialized to a default or validated beforehand.
   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 [ -n "$2" ]; then
  16                 file=$2
  17                 shift
  18             else
  19                 printf 'ERROR: "--file" requires a non-empty option argument.\n' >&2
  20                 exit 1
  21             fi
  22             ;;
  23         --file=?*)
  24             file=${1#*=} # Delete everything up to "=" and assign the remainder.
  25             ;;
  26         --file=)         # Handle the case of an empty --file=
  27             printf 'ERROR: "--file" requires a non-empty option argument.\n' >&2
  28             exit 1
  29             ;;
  30         -v|--verbose)
  31             verbose=$((verbose + 1)) # Each -v argument adds 1 to verbosity.
  32             ;;
  33         --)              # End of all options.
  34             shift
  35             break
  36             ;;
  37         -?*)
  38             printf 'WARN: Unknown option (ignored): %s\n' "$1" >&2
  39             ;;
  40         *)               # Default case: If no more options then break out of the loop.
  41             break
  42     esac
  43 
  44     shift
  45 done
  46 
  47 # if --file was provided, open it for writing, else duplicate stdout
  48 if [ -n "$file" ]; then
  49     exec 3> "$file"
  50 else
  51     exec 3>&1
  52 fi
  53 
  54 # Rest of the program here.
  55 # If there are input files (for example) that follow the options, they
  56 # will remain in the "$@" 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.

External utilities

You can use the manual loop, but it is more convenient to do so if somebody else writes it. There are two possibilities: Either you use a bash library, or you use a code generator that generates the library for you.

Code generators

Argbash

Argbash is a feature-rich code generator that can either generate a library for you, or that can put your definitions straight into your script. The project features extensive documentation.

The sample project from above (script accepting a -f|--file|--verbose|...) would use the following template:

   1 #!/bin/bash
   2 
   3 # ARG_OPTIONAL_SINGLE([file],[f],[input file])
   4 # ARG_VERBOSE()
   5 # ARG_POSITIONAL_DOUBLEDASH()
   6 # ARG_LEFTOVERS([other args])
   7 # ARGBASH_GO()
   8 
   9 # [ <-- needed because of Argbash
  10 
  11 if [ $_ARG_VERBOSE = on ]; then
  12         echo "Input file: $_ARG_FILE"
  13         echo "Other args: ${_ARG_LEFTOVERS[@]}"
  14 fi
  15 
  16 # ] <-- needed because of Argbash

Then, executing the result script as

./script.sh -f my-file --verbose -- one two three --file foo

would yield

Input file: my-file
Other args: one two three --file foo

Libraries

An nice overview of libraries is part of the Argbash project's documentation: http://argbash.readthedocs.io/en/latest/others.html.

getopts

getopts can be considered as a special case of a widely-available argument parsing library. 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=$((verbose+1))
  29             ;;
  30         f)  output_file=$OPTARG
  31             ;;
  32         '?')
  33             show_help >&2
  34             exit 1
  35             ;;
  36     esac
  37 done
  38 shift "$((OPTIND-1))" # Shift off the options and optional --.
  39 
  40 printf 'verbose=<%d>\noutput_file=<%s>\nLeftovers:\n' "$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