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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. === Manual loop === 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. Here's an example that will handle a combination of short (`-h`) and long (`--help`) options. |
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# Bash while [[ $1 == -* ]]; do case "$1" in -h|--help|-\?) show_help; exit 0;; -v) verbose=1; shift;; -f) output_file=$2; shift 2;; --) shift; break;; esac done |
# 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 exit 1 fi ;; --) shift; break;; -*) echo "invalid option: $1" 1>&2; show_help; exit 1;; esac done |
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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)!''') Here is a simplistic `getopts` example: |
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: $1" 1>&2; show_help; exit 1;; *) break;; esac done }}} What this example ''does not'' handle are: * You want things like `-xvf` to be handled as three separate flags (equivalent to `-x -v -f`). * You want to parse arguments out of `--file=bar`. It's certainly possible to do those things by changing the code, but at least in the first case, there's another approach that handles that automatically. === getopts === '''Never use getopt(1).''' `getopt` cannot handle empty arguments strings, or arguments with embedded whitespace. Please forget that it ever existed. The POSIX shell (and others) offer `getopts` which is safe to use instead. Here is a simplistic `getopts` example: |
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# POSIX x=1 # Avoids an error if we get no options at all. while getopts "abcf:g:h:" opt; do case "$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: $@" |
# POSIX x=1 # Avoids an error if we get no options at all. while getopts "h?vf:" opt; do case "$opt" in h|\?) show_help; exit 0;; v) verbose=1;; f) output_file=$OPTARG;; esac x=$OPTIND done shift $(($x-1)) if [ "$1" = -- ]; then shift; fi echo "verbose=$verbose, output_file='$output_file', Leftovers: $@" |
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The disadvantage of `getopts` is that it can only handle short options (`-h`), not long options. But it handles `-vf filename` in the expected Unix way, automatically. So, if one wishes to sacrifice `--help` to get `-vf filename`, then `getops` is a good candidate. | |
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If your prefer to check options with `if` statements, then a function like this one may be useful: | There is a [[http://bash-hackers.org/wiki/doku.php/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. === Silly repeated brute-force scanning === Another approach is to check options with `if` statements "on demand". A function like this one may be useful: |
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--) return 1; # by convention, -- is end of options | --) return 1;; # by convention, -- is end of options |
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*YES: ./script --quick *YES: ./script -other --quick |
* YES: ./script --quick * YES: ./script -other --quick |
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*NO: ./script -bar foo --quick *NO: ./script -bar -- --quick |
* NO: ./script -bar foo --quick * NO: ./script -bar -- --quick |
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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]] ---- 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
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.
Here's an example that will handle a combination of short (-h) and long (--help) options.
# 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 exit 1 fi ;; --) shift; break;; -*) echo "invalid option: $1" 1>&2; show_help; exit 1;; esac done
Now all of the remaining arguments are the filenames which followed the optional switches. You can process those with for i or "$@".
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: $1" 1>&2; show_help; exit 1;; *) break;; esac done
What this example does not handle are:
You want things like -xvf to be handled as three separate flags (equivalent to -x -v -f).
You want to parse arguments out of --file=bar.
It's certainly possible to do those things by changing the code, but at least in the first case, there's another approach that handles that automatically.
getopts
Never use getopt(1). getopt cannot handle empty arguments strings, or arguments with embedded whitespace. Please forget that it ever existed.
The POSIX shell (and others) offer getopts which is safe to use instead. Here is a simplistic getopts example:
# POSIX x=1 # Avoids an error if we get no options at all. while getopts "h?vf:" opt; do case "$opt" in h|\?) show_help; exit 0;; v) verbose=1;; f) output_file=$OPTARG;; esac x=$OPTIND done shift $(($x-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), not long options. But it handles -vf filename in the expected Unix way, automatically. So, if one wishes to sacrifice --help to get -vf filename, then getops is a good candidate.
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.
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
bhepple suggests the use of 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 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. -bhepple