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| == How can I handle command-line arguments to my script easily? == Well, that depends a great deal on what you want to do with them. Here's a general template that might help for the simple cases: |
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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=$2; shift 2 else printf "%s\n" "-f requires an argument" |
== How can I handle command-line arguments (options) to my script easily? == Well, that depends a great deal on what you want to do with them. There are several approaches, each with its strengths and weaknesses. <<TableOfContents>> === Manual loop === This approach handles any arbitrary set of options, because you're writing the parser yourself. For 90% of programs, this is the simplest approach (because you rarely need fancy stuff). This example will handle a combination of short and long options. Notice how both "--file" and "--file=FILE" are handled. {{{#!highlight bash #!/bin/sh # (POSIX shell syntax) set -o nounset # Reset all variables that might be set file= verbose=0 while [ "$#" -gt 0 ]; do case $1 in -h|-\?|--help) # Call a "show_help" function to display a synopsis, then exit. show_help exit ;; -f|--file) # Takes an option argument, ensuring it has been specified. if [ "$#" -gt 1 ]; then file=$2 shift 2 continue else echo 'ERROR: Must specify a non-empty "--file FILE" argument.' >&2 exit 1 fi ;; --file=?*) file=${1#*=} # Delete everything up to "=" and assign the remainder. ;; --file=) # Handle the case of an empty --file= echo 'ERROR: Must specify a non-empty "--file FILE" argument.' >&2 |
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| fi ;; --) shift; break;; -*) echo "invalid option: $1"; show_help; exit 1;; |
;; -v|--verbose) verbose=$((verbose + 1)) # Each -v argument adds 1 to verbosity. ;; --) # End of all options. shift break ;; -?*) printf 'WARN: Unknown option (ignored): %s\n' "$1" >&2 ;; *) # Default case: If no more options then break out of the loop. break esac shift done # Suppose --file is a required option. Check that it has been set. if [ ! "$file" ]; then echo 'ERROR: option "--file FILE" not given. See --help.' >&2 exit 1 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. === 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 # Usage info show_help() { cat << EOF Usage: ${0##*/} [-hv] [-f OUTFILE] [FILE]... Do stuff with FILE and write the result to standard output. With no FILE or when FILE is -, read standard input. -h display this help and exit -f OUTFILE write the result to OUTFILE instead of standard output. -v verbose mode. Can be used multiple times for increased verbosity. EOF } # Initialize our own variables: output_file="" verbose=0 OPTIND=1 # Reset is necessary if getopts was used previously in the script. It is a good idea to make this local in a function. while getopts "hvf:" opt; do case "$opt" in h) show_help exit 0 ;; v) verbose=$((verbose+1)) ;; f) output_file=$OPTARG ;; '?') show_help >&2 exit 1 ;; |
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| shift "$((OPTIND-1))" # Shift off the options and optional --. printf 'verbose=<%d>\noutput_file=<%s>\nLeftovers:\n' "$verbose" "$output_file" printf '<%s>\n' "$@" # 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 printf "%s\n" "-f requires an argument" exit 1 fi ;; --) shift; break;; -*) echo "invalid option: $1"; show_help; exit 1;; *) break;; esac done }}} |
1. It's portable, and will work in any POSIX shell e.g. dash. 1. It can handle things like `-vf filename` in the expected Unix way, automatically. 1. It understands `--` as the option terminator and more generally makes sure, options are parsed like for any standard command. 1. With some implementations, the error messages will be localised in the language of the user. |
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| For more complex/generalized cases, or if you want things like "-xvf" to be handled as three separate flags, you can use `getopts`. ('''NEVER use getopt(1)!''') | The disadvantage of `getopts` is that (except for ksh93 `getopts`) it can only handle short options (`-h`, not `--help`) without trickery and cannot handle options with optional arguments à la GNU. |
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| Here is a simplistic `getopts` example: | There is a [[http://wiki.bash-hackers.org/howto/getopts_tutorial|getopts tutorial]] which explains what all of the syntax and variables mean. In bash, there is also `help getopts`, which might be informative. |
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| {{{ # POSIX x=1 # Avoids an error if we get no options at all. while getopts "abcf:g:h:" opt; do case "$opt" in a) echo "You said a";; b) echo "You said b";; c) echo "You said c";; f) echo "You said f, with argument $OPTARG";; g) echo "You said g, with argument $OPTARG";; h) echo "You said h, with argument $OPTARG";; esac x=$OPTIND done shift $(($x-1)) echo "Left overs: $@" }}} If you prefer to check options with `if` statements, then a function like this one may be useful: |
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. |
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| {{{ # 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: |
For other, more complicated ways of option parsing, see ComplexOptionParsing. |
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| * YES: ./script --quick * YES: ./script -other --quick but will stop on first argument with no "-" in front (or on --): |
---- '' CategoryShell '' |
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| * 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. |
---- CategoryBashguide CategoryBashguide |
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 set -o nounset
5 # Reset all variables that might be set
6 file=
7 verbose=0
8
9 while [ "$#" -gt 0 ]; do
10 case $1 in
11 -h|-\?|--help) # Call a "show_help" function to display a synopsis, then exit.
12 show_help
13 exit
14 ;;
15 -f|--file) # Takes an option argument, ensuring it has been specified.
16 if [ "$#" -gt 1 ]; then
17 file=$2
18 shift 2
19 continue
20 else
21 echo 'ERROR: Must specify a non-empty "--file FILE" argument.' >&2
22 exit 1
23 fi
24 ;;
25 --file=?*)
26 file=${1#*=} # Delete everything up to "=" and assign the remainder.
27 ;;
28 --file=) # Handle the case of an empty --file=
29 echo 'ERROR: Must specify a non-empty "--file FILE" argument.' >&2
30 exit 1
31 ;;
32 -v|--verbose)
33 verbose=$((verbose + 1)) # Each -v argument adds 1 to verbosity.
34 ;;
35 --) # End of all options.
36 shift
37 break
38 ;;
39 -?*)
40 printf 'WARN: Unknown option (ignored): %s\n' "$1" >&2
41 ;;
42 *) # Default case: If no more options then break out of the loop.
43 break
44 esac
45
46 shift
47 done
48
49 # Suppose --file is a required option. Check that it has been set.
50 if [ ! "$file" ]; then
51 echo 'ERROR: option "--file FILE" not given. See --help.' >&2
52 exit 1
53 fi
54
55 # Rest of the program here.
56 # If there are input files (for example) that follow the options, they
57 # 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: ${0##*/} [-hv] [-f OUTFILE] [FILE]...
7 Do stuff with FILE and write the result to standard output. With no FILE
8 or when FILE is -, read standard input.
9
10 -h display this help and exit
11 -f OUTFILE write the result to OUTFILE instead of standard output.
12 -v verbose mode. Can be used multiple times for increased
13 verbosity.
14 EOF
15 }
16
17 # Initialize our own variables:
18 output_file=""
19 verbose=0
20
21 OPTIND=1 # Reset is necessary if getopts was used previously in the script. It is a good idea to make this local in a function.
22 while getopts "hvf:" opt; do
23 case "$opt" in
24 h)
25 show_help
26 exit 0
27 ;;
28 v) verbose=$((verbose+1))
29 ;;
30 f) output_file=$OPTARG
31 ;;
32 '?')
33 show_help >&2
34 exit 1
35 ;;
36 esac
37 done
38 shift "$((OPTIND-1))" # Shift off the options and optional --.
39
40 printf 'verbose=<%d>\noutput_file=<%s>\nLeftovers:\n' "$verbose" "$output_file"
41 printf '<%s>\n' "$@"
42
43 # 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.