ack - better than grep

Ack is a program meant to be a friendly replacement for the famous grep. Many of its most common flags and options are the same as grep (and here’s a separate overview guide on grep), which makes it a “friendly” replacement.

But there are two key advantages to ack:

  1. It supports the full flavor of regular expressions (often referred to as PCRE), which is the basically the same flavor of regex we’ll be using in Python and most other languages.
  2. It has an --output flag, which is a great way to combine with capturing groups for custom output.

Like grep and other grep-likes, you can use ack to match lines in a given file:

$ ack '\w{5}' words.txt

But the most frequent usage is to have ack be part of a pipeline of filtering programs:

$ curl | ack '\w{5}'


Outputting only where the pattern matches

One of the kind of confusing things about all grep-like programs is that, by default, output any line that contains a specified pattern:

$ curl | ack '\w{5}'

Use the -o flag to return only the matches made by the specified regex pattern:

$ curl | ack -o '\w{5}'

Outputting captured groups

This feature (which requires understanding the regex notion of capturing groups) is the killer feature that makes ack worth installing no matter how many other grep-like programs you might already have. Use the --output flag (not to be mistaken for the -o flag), which takes as an argument the kind of string you would put in the “Replace” field, when doing a “Find-and-Replace”.

Here’s a toy example:

$ echo 'Call Jenny at 8675309' | ack '(\d{3})(\d{4})' --output '$1-$2'