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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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| verbose=0 | verbose=0 # Variables to be evaluated as shell arithmetic should be initialized to a default or validated beforehand. |
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| if [ "$2" ]; then | if [ -n "$2" ]; then |
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| echo 'ERROR: Must specify a non-empty "--file FILE" argument.' >&2 | printf 'ERROR: "--file" requires a non-empty option argument.\n' >&2 |
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| echo 'ERROR: Must specify a non-empty "--file FILE" argument.' >&2 | printf 'ERROR: "--file" requires a non-empty option argument.\n' >&2 |
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| # Suppose --file is a required option. Check that it has been set. if [ ! "$file" ]; then echo 'ERROR: option "--file FILE" not given. See --help.' >&2 |
# Suppose --file is a required option. Ensure the variable "file" has been set and exit if not. if [ -z "$file" ]; then printf 'ERROR: option "--file FILE" not given. See --help.\n' >&2 |
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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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| '' 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=$2
17 shift 2
18 continue
19 else
20 printf 'ERROR: "--file" requires a non-empty option argument.\n' >&2
21 exit 1
22 fi
23 ;;
24 --file=?*)
25 file=${1#*=} # Delete everything up to "=" and assign the remainder.
26 ;;
27 --file=) # Handle the case of an empty --file=
28 printf 'ERROR: "--file" requires a non-empty option argument.\n' >&2
29 exit 1
30 ;;
31 -v|--verbose)
32 verbose=$((verbose + 1)) # Each -v argument adds 1 to verbosity.
33 ;;
34 --) # End of all options.
35 shift
36 break
37 ;;
38 -?*)
39 printf 'WARN: Unknown option (ignored): %s\n' "$1" >&2
40 ;;
41 *) # Default case: If no more options then break out of the loop.
42 break
43 esac
44
45 shift
46 done
47
48 # Suppose --file is a required option. Ensure the variable "file" has been set and exit if not.
49 if [ -z "$file" ]; then
50 printf 'ERROR: option "--file FILE" not given. See --help.\n' >&2
51 exit 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.