I do wish the author described which 'ack' is being lauded. When I attempt to use 'ack' on my linux workstation the results are confusing.

    moses@deunan:~$ </etc/mime.types grep application |grep x-ruby
    application/x-ruby				rb

    moses@deunan:~$ </etc/mime.types ack application x-ruby
    application: No such file or directory
    x-ruby: No such file or directory

    moses@deunan:~$ ack -h
    ack v1.39 Copyright 1993,94 Ogasawara Hiroyuki (COR.)
    usage: ack [-{e|s|j|c[c]}] [-{a|A|o<file>}] [-zCntud] [-{E|S}] [<file>..]