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| == 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. |
== 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. |
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| === 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 making ''one pass'' over the argument list, examining each argument in turn, setting appropriate shell variables so that we remember which options are in effect, and ultimately ''discarding'' 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 processing. 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.) |
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| 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). | Manually parsing options is the most flexible approach, and is sufficient for most scripts. 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. |
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| This example will handle a combination of short and long options. Notice how both "--file" and "--file=FILE" are handled. | In this example, notice how both `--file FILE` and `--file=FILE` are handled. |
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| # (POSIX shell syntax) | # POSIX |
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| # Reset all variables that might be set | die() { printf '%s\n' "$1" >&2 exit 1 } # Initialize all the option variables. # This ensures we are not contaminated by variables from the environment. |
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| -h|-\?|--help) # Call a "show_help" function to display a synopsis, then exit. show_help |
-h|-\?|--help) show_help # Display a usage synopsis. |
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| -f|--file) # Takes an option argument, ensuring it has been specified. | -f|--file) # Takes an option argument; ensure it has been specified. |
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| shift 2 continue |
shift |
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| echo 'ERROR: Must specify a non-empty "--file FILE" argument.' >&2 exit 1 |
die 'ERROR: "--file" requires a non-empty option argument.' |
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| echo 'ERROR: Must specify a non-empty "--file FILE" argument.' >&2 exit 1 |
die 'ERROR: "--file" requires a non-empty option argument.' |
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| verbose=$((verbose + 1)) # Each -v argument adds 1 to verbosity. | verbose=$((verbose + 1)) # Each -v adds 1 to verbosity. |
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| *) # Default case: If no more options then break out of the loop. | *) # Default case: No more options, so break out of the loop. |
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| # Suppose --file is a required option. Check that it has been set. if [ ! "$file" ]; then echo 'ERROR: option "--file FILE" not given. See --help.' >&2 exit 1 |
# if --file was provided, open it for writing, else duplicate stdout if [ "$file" ]; then exec 3> "$file" else exec 3>&1 |
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| 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. | 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 this is left as an exercise for the reader. |
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| 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`. 1. Replace `case $1 in` with `case ${1+$1} in` (as well as bandaging all the other code that `set -u` breaks). 1. Replace `case $1 in` with `case ${1-} in` (every potentially undeclared variable could be written as ${variable-} to prevent `set -u` tripping). 1. Stop using `-u`. |
For the most part, shell scripts that you write will not need to worry about single-letter option combining, 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 [[https://linux.die.net/man/1/gcc|gcc(1)]] and [[https://linux.die.net/man/1/star|star(1)]] use. |
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| 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 only reason you would ever use `getopts` is to allow single-letter option combining (`-xvf` handled as `-x -v -f`). It has no other purpose. The trade-off for this is that you cannot use long arguments of any kind (GNU-style "--foo" or Tcl-style "-foo"). |
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| The POSIX shell (and others) offer `getopts` which is safe to use instead. Here is a simplistic `getopts` example: | '''Never use getopt(1).''' Traditional versions of `getopt` cannot handle empty argument strings, or arguments with embedded whitespace. There is a version of `getopt(1)` in util-linux, but you '''should not use it'''. Why not? Because you would need to write special safety-checking code to ensure that you've actually got this nonstandard `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 and getting no significant benefits for it. The POSIX shell, and others, offer `getopts` which is safe to use. Here is a simplistic `getopts` example: |
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| } | } |
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| 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. while getopts "hvf:" opt; do case "$opt" in |
OPTIND=1 # Resetting OPTIND is necessary if getopts was used previously in the script. # It is a good idea to make OPTIND local if you process options in a function. while getopts hvf: opt; do case $opt in |
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| '?') | *) |
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| shift "$((OPTIND-1))" # Shift off the options and optional --. | shift "$((OPTIND-1))" # Discard the options and sentinel -- |
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| # Everything that's left in "$@" is a non-option. In our case, a FILE to process. | |
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| The advantages of `getopts` are: | |
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| 1. It's portable, and will work in any POSIX shell e.g. dash. 1. It can handle things like `-vf filename` in the expected Unix way, automatically. 1. It understands `--` as the option terminator and more generally makes sure, options are parsed like for any standard command. |
There is a [[http://wiki.bash-hackers.org/howto/getopts_tutorial|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. 1. It makes sure options are parsed like any standard command (lowest common denominator), avoiding surprises. |
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| 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 [[http://wiki.bash-hackers.org/howto/getopts_tutorial|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. |
The disadvantages of `getopts`: 1. (Except for ksh93) it can only handle short options (`-h`, not `--help`). 1. It cannot handle options with optional arguments à la GNU. 1. 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. |
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| '' CategoryShell '' | CategoryShell |
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 making one pass over the argument list, examining each argument in turn, setting appropriate shell variables so that we remember which options are in effect, and ultimately discarding 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 processing.
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.)
Manual loop
Manually parsing options is the most flexible approach, and is sufficient for most scripts. 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.
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 this is left as an exercise for the reader.
For the most part, shell scripts that you write will not need to worry about single-letter option combining, 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 gcc(1) and star(1) use.
getopts
The only reason you would ever use getopts is to allow single-letter option combining (-xvf handled as -x -v -f). It has no other purpose. The trade-off for this is that you cannot use long arguments of any kind (GNU-style "--foo" or Tcl-style "-foo").
Never use getopt(1). Traditional versions of getopt cannot handle empty argument strings, or arguments with embedded whitespace. There is a version of getopt(1) in util-linux, but you should not use it. Why not? Because you would need to write special safety-checking code to ensure that you've actually got this nonstandard 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 and getting no significant benefits for it.
The POSIX shell, and others, offer getopts which is safe to use. 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
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 à la GNU.
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.