May 07 2008

Unambiguous command abbreviation

When using RubyGems from the command line, I almost always type sudo gem i synthesis as opposed to sudo gem install rails, the emphasis targeted at using "i" instead of "install", of course. The gem executable happily understands what command it is being asked to execute when provided with the first few letters of the command, as long as those letters are not ambiguous, i.e. don't clash with the names of other commands. So even though sudo gem u foo complains that Ambiguous command u matches [uninstall, unpack, update], sudo gem uni foo will uninstall the specified gem.

Here's how this is implemented in RubyGems.

def find_command(cmd_name)
  possibilities = find_command_possibilities(cmd_name)
  if possibilities.size > 1
    raise "Ambiguous command #{cmd_name} matches [#{possibilities.join(', ')}]"
  if possibilities.size < 1
    raise "Unknown command #{cmd_name}"


def find_command_possibilities(cmd_name)
  len = cmd_name.length { |n| cmd_name == n[0,len] }

In the same vein, although not strictly a command abbreviation, Danilo pointed out git understands abbreviated revision hashes, so it's possible to use something like git diff d0a..HEAD even with the hash's complete representation being d0aa7dd4aa9a95090df1e0b9d0f426d5a5bd56ae.

Less typing is almost always a good option to have. The easy to implement Unambiguous command abbreviation trick adds a subtle usability improvement to command line interfaces and holds a nice treat to the utility's power users.