Diff for "BashFAQ/081"

Anchor(faq81)

How can I determine whether a command exists anywhere in my PATH?

In BASH, there are a couple builtins that are suitable for this purpose: hash and type. Here's an example using hash:

# This one works in bash
if hash qwerty 2>/dev/null; then
  echo qwerty exists
else
  echo qwerty does not exist
fi

Or, if you prefer type:

# Also a bash solution
if type -P qwerty >/dev/null; then
  echo qwerty exists
else
  echo qwerty does not exist
fi

Korn shell has whence instead:

# Here's a ksh solution
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.

# 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.

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. That should tell you how unreliable which(1) is. Here's a simplified version of such a test:

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.