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| [[Anchor(faq35)]] == 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: |
<<Anchor(faq35)>> == 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. |
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| {{{ while [[ $1 == -* ]]; do case "$1" in -h|--help) show_help; exit 0;; -v) verbose=1; shift;; -f) output_file=$2; shift 2;; esac done # Now all of the remaining arguments are the filenames which followed # the optional switches. You can process those with "for i" or "$@". |
<<TableOfContents>> === 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. {{{#!highlight bash #!/bin/sh # POSIX # Reset all variables that might be set file= verbose=0 # Variables to be evaluated as shell arithmetic should be initialized to a default or validated beforehand. while :; 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 [ -n "$2" ]; then file=$2 shift else printf 'ERROR: "--file" requires a non-empty option argument.\n' >&2 exit 1 fi ;; --file=?*) file=${1#*=} # Delete everything up to "=" and assign the remainder. ;; --file=) # Handle the case of an empty --file= printf 'ERROR: "--file" requires a non-empty option argument.\n' >&2 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' "$1" >&2 ;; *) # Default case: If no more options then break out of the loop. break esac shift done # if --file was provided, open it for writing, else duplicate stdout if [ -n "$file" ]; then exec 3> "$file" else exec 3>&1 fi # Rest of the program here. # If there are input files (for example) that follow the options, they # will remain in the "$@" positional parameters. |
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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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| 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)!''') | === 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. |
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| Here is a simplistic getopts example: | The POSIX shell (and others) offer `getopts` which is safe to use instead. Here is a simplistic `getopts` example: |
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| {{{ 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: $@" |
{{{#!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 ;; esac done 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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| The advantages of `getopts` are: 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. 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. For other, more complicated ways of option parsing, see ComplexOptionParsing. ---- 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: ${0##*/} [-hv] [-f OUTFILE] [FILE]...
7 Do stuff with FILE and write the result to standard output. With no FILE
8 or when FILE is -, read standard input.
9
10 -h display this help and exit
11 -f OUTFILE write the result to OUTFILE instead of standard output.
12 -v verbose mode. Can be used multiple times for increased
13 verbosity.
14 EOF
15 }
16
17 # Initialize our own variables:
18 output_file=""
19 verbose=0
20
21 OPTIND=1 # Reset is necessary if getopts was used previously in the script. It is a good idea to make this local in a function.
22 while getopts "hvf:" opt; do
23 case "$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.