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| #pragma section-numbers 3 | |
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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 without the use of a specialized function is the most flexible approach, and is sufficient for most simple scripts. |
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| This example will handle a combination of short and long options. Notice how both "--file" and "--file=FILE" are handled. | 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. |
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| # (POSIX shell syntax) | # POSIX |
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| file="" verbose=0 |
file= verbose=0 # Variables to be evaluated as shell arithmetic should be initialized to a default or validated beforehand. |
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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) # Call a "show_help" function to display a synopsis, then exit. show_help 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, ensuring it has been specified. if [ -n "$2" ]; then file=$2 shift else printf 'ERROR: "--file" requires a non-empty option argument.\n' >&2 exit 1 fi |
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| --file=*) file=${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= printf 'ERROR: "--file" requires a non-empty option argument.\n' >&2 exit 1 |
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| --) # End of all options | -v|--verbose) verbose=$((verbose + 1)) # Each -v argument 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' "$1" >&2 |
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| *) # no more options. Stop while loop | *) # Default case: If no more options then break out of the loop. |
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| ;; | |
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shift |
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| # Suppose some options are required. Check that we got them. if [ ! "$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 [ -n "$file" ]; then exec 3> "$file" else exec 3>&1 |
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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`. |
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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. | 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. |
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| # Usage info show_help() { cat << EOF Usage: ${0##*/} [-hv] [-f OUTFILE] [FILE]... Do stuff with FILE and write the result to standard output. With no FILE or when FILE is -, read standard input. -h display this help and exit -f OUTFILE write the result to OUTFILE instead of standard output. -v verbose mode. Can be used multiple times for increased verbosity. EOF } |
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| OPTIND=1 # Reset is necessary if getopts was used previously in the script. It is a good idea to make this local in a function. while getopts "h?vf:" opt; do case "$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 $((OPTIND-1)) # Shift off the options and optional --. | shift "$((OPTIND-1))" # Shift off the options and optional --. |
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| 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: 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. The disadvantage of `getopts` is that it can only handle short options (`-h`, not `--help`) without trickery. |
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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 advantages of `getopts`: |
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| ==== getopts 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. |
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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| {{{#!highlight bash #!/bin/bash # Uses bash extensions. Not portable as written. |
The disadvantages of `getopts`: 1. (Except for ksh93 `getopts`) it can only handle short options (`-h`, not `--help`) without trickery. 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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| 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 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.
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 # 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))" # Shift off the options and optional --.
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, which might be informative.
The advantages of getopts:
- 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 disadvantages of getopts:
(Except for ksh93 getopts) it can only handle short options (-h, not --help) without trickery.
- 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.