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Comment: Manual loop and getopts is enough to answer this faq. Moved the rest to a separate page.
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| Well, that depends a great deal on what you want to do with them. There are several approaches, each with its strengths and weaknesses. | Well, that depends a great deal on what you want to do with them. There are several approaches, each with its strengths and weaknesses. <<TableOfContents>> |
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| This approach handles any arbitrary set of options, because you're writing the parser yourself. For 90% of programs, this turns out to be the simplest and most direct approach, since very few scripts need complicated option processing. | 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). |
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| Here's an example that will handle a combination of short (`-h`) and long (`--help`) options. | This example will handle a combination of short and long options. Notice how both "--file" and "--file=FILE" are handled. {{{#!highlight bash #!/bin/sh # (POSIX shell syntax) # Reset all variables that might be set file="" verbose=0 while : do case $1 in -h | --help | -\?) # Call your Help() or usage() function here. exit 0 # This is not an error, User asked help. Don't do "exit 1" ;; -f | --file) file=$2 # You might want to check if you really got FILE shift 2 ;; --file=*) file=${1#*=} # Delete everything up till "=" shift ;; -v | --verbose) # Each instance of -v adds 1 to verbosity verbose=$((verbose+1)) shift ;; --) # End of all options shift break ;; -*) echo "WARN: Unknown option (ignored): $1" >&2 shift ;; *) # no more options. Stop while loop break ;; esac done # Suppose some options are required. Check that we got them. if [ ! "$file" ]; then echo "ERROR: option '--file FILE' not given. See --help" >&2 exit 1 fi # Rest of the program here. # If there are input files (for example) that follow the options, they # 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: |
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| # Bash while [[ $1 == -* ]]; do case "$1" in -h|--help|-\?) show_help; exit 0;; -v|--verbose) verbose=1; shift;; -f) if (($# > 1)); then output_file=$2; shift 2 else echo "-f requires an argument" 1>&2 |
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 ${variable-} to prevent `set -u` tripping). 1. Stop using `-u`. === getopts === Unless it's the version from util-linux, and you use its advanced mode, '''never use getopt(1).''' Traditional versions of `getopt` cannot handle empty argument strings, or arguments with embedded whitespace. The POSIX shell (and others) offer `getopts` which is safe to use instead. Here is a simplistic `getopts` example: {{{#!highlight bash #!/bin/sh # Initialize our own variables: output_file="" verbose=0 OPTIND=1 # Reset is necessary if getopts was used previously in the script. It is a good idea to make this local in a function. while getopts "hvf:" opt; do case "$opt" in h) show_help exit 0 ;; v) verbose=1 ;; f) output_file=$OPTARG ;; '?') show_help >&2 |
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| fi ;; --) shift; break;; -*) echo "invalid option: $1" 1>&2; show_help; exit 1;; |
;; |
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| shift $((OPTIND-1)) # Shift off the options and optional --. echo "verbose=$verbose, output_file='$output_file', Leftovers: $@" # End of file |
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| Now all of the remaining arguments are the filenames which followed the optional switches. You can process those with `for i` or `"$@"`. | The advantages of `getopts` are: |
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| A POSIX version of that same code: {{{ # POSIX while true; do case "$1" in -h|--help|-\?) show_help; exit 0;; -v|--verbose) verbose=1; shift;; -f) if [ $# -gt 1 ]; then output_file=$2; shift 2 else echo "-f requires an argument" 1>&2 exit 1 fi ;; --) shift; break;; -*) echo "invalid option: $1" 1>&2; show_help; exit 1;; *) break;; esac done }}} |
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. 1. With some implementations, the error messages will be localised in the language of the user. |
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| Some Bash programmers write this at the beginning of their scripts: {{{ set -u # or, set -o nounset }}} This way Bash stops if it's forced to work with the value of an unset variable. If you use `set -o nounset`, the Bash version of the "manual loop" shown above may break, if there are no additional non-option arguments. It can be fixed thus: {{{ # Bash (with set -u) while [[ ${1+defined} && $1 == -* ]]; do case "$1" in -h|--help|-\?) show_help; exit 0;; -v|--verbose) verbose=1; shift;; -f) if (($# > 1)); then output_file=$2; shift 2 else echo "-f requires an argument" 1>&2 exit 1 fi ;; --) shift; break;; -*) echo "invalid option: $1" 1>&2; show_help; exit 1;; esac done }}} Of course, a simpler fix would be ''not to use'' `set -u` in the first place; or at least to use it only after the option processing is finished. What these examples ''do not'' handle are: * You want things like `-xvf` to be handled as three separate flags (equivalent to `-x -v -f`). * You want to parse arguments out of `--file=bar`. It's certainly possible to do those things by changing the code, but at least in the first case, there's another approach that handles that automatically. === getopts === '''Never use getopt(1).''' `getopt` cannot handle empty arguments strings, or arguments with embedded whitespace. Please forget that it ever existed. The POSIX shell (and others) offer `getopts` which is safe to use instead. Here is a simplistic `getopts` example: {{{ # POSIX OPTIND=1 # Reset in case getopts has been used previously in the shell. while getopts "h?vf:" opt; do case "$opt" in h|\?) show_help; exit 0;; v) verbose=1;; f) output_file=$OPTARG;; esac done shift $((OPTIND-1)) if [ "$1" = -- ]; then shift; fi echo "verbose=$verbose, output_file='$output_file', Leftovers: $@" }}} The disadvantage of `getopts` is that it can only handle short options (`-h`) without trickery. It handles `-vf filename` in the expected Unix way, automatically. `getopts` is a good candidate because it is portable and e.g. also works in dash. |
The disadvantage of `getopts` is that (except for ksh93 `getopts`) it can only handle short options (`-h`, not `--help`) without trickery and cannot handle options with optional arguments à la GNU. |
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| Here is an example which claims to parse long options with `getopts`. The basic idea is quite simple: just put "-:" into the optstring. This trick requires a shell which permits the option-argument (i.e. the filename in "-f filename") to be concatenated to the option (as in "-ffilename"). The [[http://pubs.opengroup.org/onlinepubs/9699919799/utilities/getopts.html|POSIX standard]] says there must be a space between them; bash and dash permit the "-ffilename" variant, but one should not rely on this leniency if attempting to write a portable script. ''I do not quite understand the point, can you give an example? The example script below accepts --loglevel 5 and --loglevel=5. Support for the second form may be removed. Maybe the usage message is not quite correctly formatted? Anyway there is no concatenation necessary nor supported, neither does the script affect the treatment of short options in any way, does it?'' -- the author of the example |
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| {{{#!highlight bash #!/bin/bash # Uses bash extensions. Not portable as written. optspec=":h-:" while getopts "$optspec" optchar; do case "${optchar}" in -) case "${OPTARG}" in loglevel) 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 }}} ''Even if we ignore the portability issue, are you sure this is an ''improvement'' over the manual loop in the first example? The manual loop is much simpler. Also, why is it checking `OPTERR` which is something the programmer sets, not something set by `getopts`?'' - GreyCat ''Thre is nore than one way to do a thing :-) Since this is section is titled "getopts", I guess it fits here. Why not let people decide what suits their needs, without bias. The example works in the more stricly POSIX-compliant Debian Almquist shell (apart from the variable indirection ${!OPTIND}), so I guessed that portability would be ok. The OPTERR-conditional behavior was just non-essential sugar, I removed it for clarity of the example.'' -- the author of the example === Silly repeated brute-force scanning === Another approach is to check options with `if` statements "on demand". A function like this one may be useful: {{{ # Bash HaveOpt() { local needle=$1 shift while [[ $1 == -* ]]; do case "$1" in --) return 1;; # by convention, -- is end of options $needle) return 0;; esac shift done return 1 } if HaveOpt --quick "$@"; then echo "Option quick is set"; fi }}} 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=$@" }}} Here, all information about each option is defined in one place making for much easier authoring and maintenance. A lot of the dirty work is handled automatically and standards are obeyed as in getopt(1) - because it calls getopt for you. . ''Actually, what the author forgot to say was that it's actually using `getopts` semantics, rather than `getopt`. I ran this test:'' {{{ wooledg@wooledg:~/process-getopt-1.6$ set -- one 'rm -rf /' 'foo;bar' "'" wooledg@wooledg:~/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 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.
Contents
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:
Stop using -u.
Replace case $1 in with case ${1+$1} in (as well as bandaging all the other code that set -u breaks).
Replace case $1 in with case ${1-} in (every potentially undeclared variable could be written as ${variable-} to prevent set -u tripping).
Stop using -u.
getopts
Unless it's the version from util-linux, and you use its advanced mode, never use getopt(1). Traditional versions of getopt cannot handle empty argument strings, or arguments with embedded whitespace.
The POSIX shell (and others) offer getopts which is safe to use instead. Here is a simplistic getopts example:
1 #!/bin/sh
2
3 # Initialize our own variables:
4 output_file=""
5 verbose=0
6
7 OPTIND=1 # Reset is necessary if getopts was used previously in the script. It is a good idea to make this local in a function.
8 while getopts "hvf:" opt; do
9 case "$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:
- It's portable, and will work in any POSIX shell e.g. dash.
It can handle things like -vf filename in the expected Unix way, automatically.
It understands -- as the option terminator and more generally makes sure, options are parsed like for any standard command.
- 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.