One of the most useful things about running software in unusual configurations is that sometimes it exposes unexpected flaws you never knew you had.

The which utility is one of those commandline niceties you never really think about until it’s not there anymore. While sometimes implemented as a shell builtin¹, it’s also frequently shipped as a standalone utility. Apple’s modern version, which is part of the shell_utils package and crystallized around Snow Leopard, works like this:

• If the specified tool is found on the path, prints the path to the first version found (e.g., the one the shell would execute), and exits 0.
• If the specified tool isn’t found, prints a newline and exits 1.

This version of the tool is really useful in shell scripts to determine a) if a program is present, and b) where it’s located, and until fairly recently Homebrew used it extensively. Unfortunately, early on in my work on Tigerbrew, I discovered that Tiger’s version was… deficient. It works like this:

• If the specified tool is found on the path, prints the path to the first version found, and exits 0.
• If the specified tool isn’t found, prints a verbose message to stdout, and exits 0.

The lack of a meaningful exit status and the error message on stdout are both pretty poor behaviour for a CLI app, and broke Homebrew’s assumptions about how it should work.

To work around this, I replaced Homebrew’s wrapper function with a two-line native Ruby method for Tigerbrew, like so:

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def which cmd
  dir = ENV['PATH'].split(':').find {|p| File.executable? File.join(p, cmd)}
  Pathname.new(File.join(dir, cmd)) unless dir.nil?
end

As it turns out, not only does it work better on Tiger, but this method is actually faster² than shelling out like Homebrew did; process spawning is relatively expensive. As a result, I ended up using the new helper in Homebrew even though it wasn’t strictly necessary.

(And as for the commandline utility, Tigerbrew has a formula for the shell_cmds collection of utilities.)