What is the preferred way to test if a command takes an option? What are the caveats?
As a motivating example, at login my shell aliases grep to add several --exclude-dir options but this option is not available on all the machines I access.
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4 Answers
Fortunately, there is a relatively standardized way to report potential options to programs. However, it is not always used, so this is not a fool-proof answer; just a poweruser-proof answer.
$prog --help | grep -e "\b$option\b" (note, \b means word boundary. That is so there will be no characters left after your $option, so, $option-f will not match)
That will return you the option, if it exists - but more importantly, return an exit status of 0 if it exists, or an exit status of 1 if it does not. You can test this with:
$ # we pipe to /dev/null to hide the output, and show that it doesn't matter $ grep --help | grep -e "\b--exclude-dir\b" > /dev/null $ echo $? 0 $ grep --help | grep -e "\b--exclude-dirf\b" > /dev/null $ echo $? 1 This will work on many programs, but not all have a --help that is... helpful. You may have also have luck with
man $progr | grep -e "\b$option\b" if the first fails
- Maybe use
\baround the $option, to avoid a false positive for--exclude-d?Paŭlo Ebermann– Paŭlo Ebermann2014-09-10 20:50:57 +00:00Commented Sep 10, 2014 at 20:50 - I like the simplicity of this but I ran into trouble with grep on OS X. grep --help sends text to stderr and it also doesn't list 'exclude-dir' in the grep --help output even though it is in the manpage.Praxeolitic– Praxeolitic2014-10-11 04:58:03 +00:00Commented Oct 11, 2014 at 4:58
- Adding this note to emphasis on how not all programs exit with non-zero exit code if unsupported option is used. Example with the
realpathprogram :realpath --foo="/tmp" "/tmp/bar" ; echo $?(I discovered this when using--relative-tooption).CDuv– CDuv2021-02-11 09:57:27 +00:00Commented Feb 11, 2021 at 9:57
You can do a test on dummy data which should succeed if and only if the option is available and working as expected:
trap 'if [ -e "$tmp" ]; then rm -rf -- "$tmp"; fi' EXIT tmp="$(mktemp -d)" cd -- "$tmp" mkdir exclude mkdir include echo foo > include/test.txt echo foo > exclude/test.txt [ "$(grep --exclude-dir exclude --recursive foo . | wc -l)" -eq 1 ] 
Given that most (all?) commands exit with return code greater than zero when called with unknown options, you can try
cmd --option-to-probe [other options and parameters] >/dev/null 2>/dev/null and check whether $? is 0 or not.
You could grep the man page of the command to see if the option is there, something like this:
CMD=grep OPTIONS="" for OPT in "--exclude-dir" "--text" "--not_an_option"; do if man $CMD | col -bx | egrep --quiet \\$OPT; then OPTIONS="$OPTIONS $OPT" fi done alias $CMD="$CMD $OPTIONS" 