Bash vs. Which

The Unix which command is useful for searching your $PATH to find which version of an executable is going to run. But did you know that it doesn't always tell you the truth? For instance, it doesn't tell you when the command you're asking about is hidden by a shell builtin (or function, keyword, or alias):

  $ which echo
  /usr/bin/echo

which found /usr/bin/echo on your path, but echo is a shell builtin (at least in bash), so that's what will get run, not the one on the path. The GNU man page for which presents workarounds that will pick up functions and aliases, but shell builtins still slip through the cracks. The situation is even weirder on Solaris, where which is a csh shell-script, that sources your .cshrc file as part of its operation -- very strange if your shell is bash.

Clearly, which is not to be trusted. The easiest workaround, if you use bash, is the type builtin:

  $ type -a echo
  echo is a shell builtin
  echo is /bin/echo

A slightly nicer workaround is to make a bash function for which, that uses type to do the right thing. I also like which to do an ls -l where applicable. Here's what I have in my .bashrc:

  if alias which > /dev/null 2>&1; then unalias which; fi

  function which {
      hash -r
      t=`type -t $@`

      if [ -z "${t%%file*}" ]; then
          for p in `type -p $@`; do
              ls -l ${p};
          done
      else
          type $@;
      fi
  }

Now which gives you much more information:

  $ which -a wish
  lrwxrwxrwx  1 root  root  13 Dec 23 2008 /usr/local/bin/wish -> /usr/bin/wish*
  lrwxrwxrwx  1 root  root   7 Jan  8 2007 /usr/bin/wish -> wish8.3*

The unalias protects you in case some systemwide .bashrc or .profile aliases which in some way (unlikely, but good hygiene anyway). The call to hash clears the hashed function list, so that any changes to your PATH are reflected. The reason for the if statement in the function, is that we certainly want to report builtins, functions, etc., but if there are none, we can use type -p to make it easier to pass the filenames to ls -l. If there are builtins or such, we just punt and give the unaltered type output.

You can pass -a to this which if you want to see all the matches for the command, in order of precedence.

Note that this has to be a function, not a script, because running the script will source your .bashrc, which might change the PATH (all PATH changes should be in .profile, but sometimes people do it in .bashrc).