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.
Contents
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.
Do not use getopt(1). Do not even discuss getopt on this page. Go to ComplexOptionParsing to learn more about it.
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 an 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:
It can handle things like -xvf filename in the expected Unix way, automatically.
- It makes sure options are parsed like any standard command (lowest common denominator), avoiding surprises.
- With some implementations, the error messages will be localised in the language of the user.
The disadvantages of getopts:
(Except for ksh93) it can only handle short options (-h, not --help).
It cannot handle options with optional arguments like mysql's -p[password].
- It doesn't exist in the Bourne shell.
It only allows options to be parsed in the "standard way" (lowest common denominator).
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.