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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 use 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 file="" |
die() { printf '%s\n' "$1" >&2 exit 1 } # Initialize all the option variables. # This ensures we are not contaminated by variables from the environment. file= |
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| while : do |
while :; do |
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| -h | --help | -\?) # Call your Help() or usage() function here. exit 0 # This is not an error, User asked help. Don't do "exit 1" |
-h|-\?|--help) show_help # Display a usage synopsis. exit |
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| -f | --file) file=$2 # You might want to check if you really got FILE shift 2 |
-f|--file) # Takes an option argument; ensure it has been specified. if [ "$2" ]; then file=$2 shift else die 'ERROR: "--file" requires a non-empty option argument.' fi |
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| --file=*) file=${1#*=} # Delete everything up till "=" shift |
--file=?*) file=${1#*=} # Delete everything up to "=" and assign the remainder. |
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| -v | --verbose) # Each instance of -v adds 1 to verbosity verbose=$((verbose+1)) shift |
--file=) # Handle the case of an empty --file= die 'ERROR: "--file" requires a non-empty option argument.' |
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| --) # End of all options | -v|--verbose) verbose=$((verbose + 1)) # Each -v adds 1 to verbosity. ;; --) # End of all options. |
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| -*) echo "WARN: Unknown option (ignored): $1" >&2 shift |
-?*) printf 'WARN: Unknown option (ignored): %s\n' "$1" >&2 |
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| *) # no more options. Stop while loop | *) # Default case: No more options, so break out of the loop. |
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| ;; | |
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shift |
|
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| # Suppose some options are required. Check that we got them. if [ ! "$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. | |
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| Some Bash programmers like to write this at the beginning of their scripts to guard against unused variables: | 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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| {{{ 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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| '''Never use getopt(1).''' `getopt` cannot handle empty arguments strings, or arguments with embedded whitespace. Please forget that it ever existed. | 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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| # A POSIX variable OPTIND=1 # Reset in case getopts has been used previously in the shell. |
# Usage info show_help() { cat << EOF Usage: ${0##*/} [-hv] [-f OUTFILE] [FILE]... Do stuff with FILE and write the result to standard output. With no FILE or when FILE is -, read standard input. -h display this help and exit -f OUTFILE write the result to OUTFILE instead of standard output. -v verbose mode. Can be used multiple times for increased verbosity. EOF } |
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| while getopts "h?vf:" opt; do case "$opt" in h|\?) |
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 $opt in h) |
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| v) verbose=1 | v) verbose=$((verbose+1)) |
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| *) show_help >&2 exit 1 ;; |
|
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| shift "$((OPTIND-1))" # Discard the options and sentinel -- | |
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| shift $((OPTIND-1)) [ "$1" = "--" ] && shift echo "verbose=$verbose, output_file='$output_file', Leftovers: $@" |
# Everything that's left in "$@" is a non-option. In our case, a FILE to process. printf 'verbose=<%d>\noutput_file=<%s>\nLeftovers:\n' "$verbose" "$output_file" printf '<%s>\n' "$@" |
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| The advantages of `getopts` are: | |
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| 1. It's portable, and will work in e.g. dash. 1. It can handle things like `-vf filename` in the expected Unix way, automatically. |
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`. |
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| The disadvantage of `getopts` is that it can only handle short options (`-h`, not `--help`) without trickery. | The advantages of `getopts` over a manual loop: |
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| 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. | 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. 1. With some implementations, the error messages will be localised in the language of the user. |
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| 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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| ==== getops long option trickery ==== Here is an example which claims to parse long options with `getopts`. The basic idea is quite simple: just put "-:" into the optstring. This trick requires a shell which permits the option-argument (i.e. the filename in "-f filename") to be concatenated to the option (as in "-ffilename"). The [[http://pubs.opengroup.org/onlinepubs/9699919799/utilities/getopts.html|POSIX standard]] says there must be a space between them; bash and dash permit the "-ffilename" variant, but one should not rely on this leniency if attempting to write a portable script. {{{#!highlight bash #!/bin/bash # Uses bash extensions. Not portable as written. optspec=":h-:" while getopts "$optspec" optchar do case "${optchar}" in -) case "${OPTARG}" in loglevel) eval val="\$${OPTIND}"; OPTIND=$(( $OPTIND + 1 )) echo "Parsing option: '--${OPTARG}', value: '${val}'" >&2 ;; loglevel=*) val=${OPTARG#*=} opt=${OPTARG%=$val} echo "Parsing option: '--${opt}', value: '${val}'" >&2 ;; esac ;; h) echo "usage: $0 [--loglevel[=]<value>]" >&2 exit 2 ;; esac done # End of file }}} In practice, this example is so obfuscated that it may be preferable to add concatenated option support (like `-vf filename`) to a manual parsing loop instead, if that was the only reason for using `getopts`. Here's an improved and more generalized version of above attempt to add support for long options when using `getopts`: {{{#!highlight bash #!/bin/bash # Uses bash extensions. Not portable as written. declare -A longoptspec longoptspec=( [loglevel]=1 ) #use associative array to declare how many arguments a long option expects, in this case we declare that loglevel expects/has one argument, long options that aren't listed in this way will have zero arguments by default optspec=":h-:" while getopts "$optspec" opt; do while true; do case "${opt}" in -) #OPTARG is name-of-long-option or name-of-long-option=value if [[ "${OPTARG}" =~ .*=.* ]] #with this --key=value format only one argument is possible then opt=${OPTARG/=*/} OPTARG=${OPTARG#*=} ((OPTIND--)) else #with this --key value1 value2 format multiple arguments are possible opt="$OPTARG" OPTARG=(${@:OPTIND:$((longoptspec[$opt]))}) fi ((OPTIND+=longoptspec[$opt])) continue #now that opt/OPTARG are set we can process them as if getopts would've given us long options ;; loglevel) loglevel=$OPTARG ;; h|help) echo "usage: $0 [--loglevel[=]<value>]" >&2 exit 2 ;; esac break; done done # End of file }}} With this version you can have long and short options side by side and you shouldn't need to modify the code from line 10 to 22. This solution can also handle multiple arguments for long options, just use ${OPTARG} or ${OPTARG[0]} for the first argument, ${OPTARG[1]} for the second argument, ${OPTARG[2]} for the third argument and so on. It has the same disadvantage of its predecessor in not being portable and specific to bash. Additionally, it breaks arguments with whitespace (line 18) for long options, when given using the syntax that does not include equal sign (=). === Silly repeated brute-force scanning === Another approach is to check options with `if` statements "on demand". A function like this one may be useful: {{{#!highlight bash #!/bin/bash HaveOpt () { local needle=$1 shift while [[ $1 == -* ]] do # By convention, "--" means end of options. case "$1" in --) return 1 ;; $needle) return 0 ;; esac shift done return 1 } HaveOpt --quick "$@" && echo "Option quick is set" # End of file }}} and it will work if script is run as: * YES: ./script --quick * YES: ./script -other --quick but will stop on first argument with no "-" in front (or on --): * NO: ./script -bar foo --quick * NO: ./script -bar -- --quick Of course, this approach (iterating over the argument list every time you want to check for one) is far less efficient than just iterating once and setting flag variables. It also spreads the options throughout the program. The literal option `--quick` may appear a hundred lines down inside the main body of the program, nowhere near any other option name. This is a nightmare for maintenance. === Complex nonstandard add-on utilities === [[http://bhepple.freeshell.org/oddmuse/wiki.cgi/process-getopt|bhepple]] suggests the use of [[http://sourceforge.net/projects/process-getopt/|process-getopt]] (GPL licensed) and offers this example code: {{{ PROG=$(basename $0) VERSION='1.2' USAGE="A tiny example using process-getopt(1)" # call process-getopt functions to define some options: source process-getopt SLOT="" SLOT_func() { [ "${1:-""}" ] && SLOT="yes"; } # callback for SLOT option add_opt SLOT "boolean option" s "" slot TOKEN="" TOKEN_func() { [ "${1:-""}" ] && TOKEN="$2"; } # callback for TOKEN option add_opt TOKEN "this option takes a value" t n token number add_std_opts # define the standard options --help etc: TEMP=$(call_getopt "$@") || exit 1 eval set -- "$TEMP" # just as with getopt(1) # remove the options from the command line process_opts "$@" || shift "$?" echo "SLOT=$SLOT" echo "TOKEN=$TOKEN" echo "args=$@" }}} Here, all information about each option is defined in one place making for much easier authoring and maintenance. A lot of the dirty work is handled automatically and standards are obeyed as in getopt(1) - because it calls getopt for you. . ''Actually, what the author forgot to say was that it's actually using `getopts` semantics, rather than `getopt`. I ran this test:'' {{{ ~/process-getopt-1.6$ set -- one 'rm -rf /' 'foo;bar' "'" ~/process-getopt-1.6$ call_getopt "$@" -- 'rm -rf /' 'foo;bar' ''\''' }}} . ''It appears to be intelligent enough to handle null options, whitespace-containing options, and single-quote-containing options in a manner that makes the [[BashFAQ/048|eval]] not blow up in your face. But this is not an endorsement of the process-getopt software overall; I don't know it well enough. -GreyCat '' ''It's written and tested on Linux where getopt(1) supports long options. For portability, it tests the local getopt(1) at runtime and if it finds an non-GNU one (ie one that does not return 4 for {{{getopt --test}}}) it only processes short options. It does not use the bash builtin getopts(1) command. -[[http://bhepple.freeshell.org/oddmuse/wiki.cgi/process-getopt|bhepple]] '' |
For other, more complicated ways of option parsing, see ComplexOptionParsing. |
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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 use 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.