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| <<Anchor(faq81)>> == How can I determine whether a command exists anywhere in my PATH? == The POSIX shell specifies a builtin called {{{command}}} which can be used for this purpose: {{{ # POSIX if command -v qwerty >/dev/null; then echo qwerty exists else echo qwerty does not exist fi }}} In BASH, there are a couple more builtins that may also be used: {{{hash}}} and {{{type}}}. Here's an example using {{{hash}}}: {{{ # Bash if hash qwerty 2>/dev/null; then echo qwerty exists else echo qwerty does not exist fi }}} Or, if you prefer {{{type}}}: {{{ # Bash if type -P qwerty >/dev/null; then echo qwerty exists else echo qwerty does not exist fi }}} KornShell has {{{whence}}} instead: {{{ # ksh if whence -p qwerty >/dev/null; then echo qwerty exists else echo qwerty does not exist fi }}} If these builtins are not available (because you're in a Bourne shell, or whatever), then you may have to rely on the external command {{{which}}} (which is often a csh script, although sometimes a compiled binary). Unfortunately, {{{which}}} may not set a useful exit code, and it may not even write errors to stderr. In those cases, one must parse its output. {{{ # Bourne. Last resort -- using which(1) tmpval=`LC_ALL=C which qwerty 2>&1` if test $rc -ne 0; then # FOR NOW, we'll assume that if this machine's which(1) sets a nonzero # exit status, that it actually failed. I've yet to see any case where # which(1) sets an erroneous failure -- just erroneous "successes". echo "qwerty is not installed. Please install it." else case "$tmpval" in *no\ *\ in\ *|*not\ found*|'') echo "qwerty is not installed. Please install it." ;; *) echo "Congratulations -- it seems you have qwerty (in $tmpval)." ;; esac fi }}} Note that `which(1)`'s output when a command is not found is ''not'' consistent across platforms. On HP-UX 10.20, for example, it prints {{{no qwerty in /path /path /path ...}}}; on OpenBSD 4.1, it prints {{{qwerty: Command not found.}}}; on Debian (3.1 and 4.0 at least) and SuSE, it prints nothing at all; on Red Hat 5.2, it prints {{{which: no qwerty in (/path:/path:...)}}}; on Red Hat 6.2, it writes the same message, but on standard error instead of standard output; and on Gentoo, it writes something on stderr. So our best portable solution is to match the words `no` and `in`, or the phrase `not found`, in the combined stdout + stderr and pray. Note to the person who tried to put a function in this FAQ as a legacy Bourne shell example: legacy Bourne shells ''don't have functions'', at all. POSIX shells have them, but the syntax is: {{{ foo() { commands } }}} You may ''not'' use the word `function`, and you may ''especially not'' use the combination of the word `function` and the `()` symbols. No shell but bash accepts that. Also see BashPitfalls. The approach used in `configure` scripts is usually to iterate over the components of `$PATH` and explicitly test for the presence of the command in each directory. The fact that this is ''preferred'' over `which` should tell you how unreliable `which` is. Here's a ''simplified'' version of such a test: {{{ # Bourne save_IFS=$IFS IFS=: found=no for dir in $PATH; do if test -x "$dir/qwerty"; then echo "qwerty is installed (in $dir)" found=yes break fi done IFS=$save_IFS if test $found = no; then echo "qwerty is not installed" fi }}} Real `configure` scripts are generally much more complicated than this, since they may deal with systems where `$PATH` is not delimited by colons; or systems where executable programs may have optional extensions like `.EXE`; or `$PATH` variables that have the current working directory included in them as an empty string; etc. If you're interested in such things, I suggest reading an actual GNU autoconf-generated `configure` script. They're far too large and complicated to include in this FAQ. |
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