I'll engage in some expectation management: ag doesn't cover all uses of Ack. I made ag because I loved Ack, but it wasn't fast enough for my needs. For the features of Ack that I use, ag is interchangeable. For features I don't use... not so much. Also, since the release of Ack 2.0, some behaviors and command line options have differed slightly.

If you want to know why ag is so fast, there's a summary in the GitHub README.[1] I've also written a few blog posts on the subject.[2]

1. https://github.com/ggreer/the_silver_searcher#how-is-it-so-f...

2. http://geoff.greer.fm/ag/#related-posts

* The command name is 33% shorter than ack, and all keys are on the home row!

Sold!

There's also The Platinum Searcher (pt) which add support for searching SHIFT_JIS and EUC-JP files and about as fast as ag.

https://github.com/monochromegane/the_platinum_searcher

I'm not sure how valid the comparison benchmark is:

  Ack and Ag found the same results, but Ag was 34x faster
  (3.2 seconds vs 110 seconds). My ~/code directory is about
  8GB. Thanks to git/hg/svn-ignore, Ag only searched 700MB of that.

Searching less data definitely is going to take less time!

This one is fast. Really fast.

It even better than ack. This is very useful on large codebases.