Introduction

I have occasionally been using a tool called ack for a

few years now. It’s billed as “an alternative to grep for

programmers”.

There are several features I find particularly useful:

It can restrict text searches to files of a particular

type

It uses Perl regular expressions which may be the most powerful

and feature rich types of RE’s available at present

You can limit the search area within a file if desired

It is a very comprehensive and useful tool, though maybe quite

complex to use. Personally I use it in special cases where I need its

power, and otherwise use the usual grep.

In this episode I will give you the flavour of its capabilities and

otherwise leave you to research more if it sounds interesting.

Installing ack

The tool can be found in repositories. I use Debian, and

ack is in the Debian repo and can be installed with:

sudo apt install ack

Installing it this way the version I have (and am describing here) is

3.6.0. There is a new version, 3.7.0 available from the website.

The documentation on the website suggests installing it as a Perl

module using CPAN, which is something I will do soon I

think.

Perl regular expressions

These are very sophisticated.

A project to convert the Perl regular expression capabilities into a

portable library form was undertaken by Philip Hazel of Cambridge

University in 1997, and was called Perl Compatible Regular

Expressions or PCRE.

Philip Hazel was the originator of the exim mail

transfer agent (MTA, or mail server), and wanted to use PCRE within

it.

Since then PCRE (and later PCRE2) is the way regular expressions are

implemented in a lot of other software, which shows how widespread use

of the Perl RE has become.

The ack documentation refers to the Perl manual for

details of this type of regular expression, and to a tutorial, if you

wish to gain a deeper understanding.

It should be noted that GNU grep can use Perl compatible

regular expressions when matching lines in files, but this feature is

marked as experimental.

File types

The ack command has rules for recognising file types. It

does this by looking at the name extensions ('.html' or

'.py' for example), and in some cases by examining their

contents. The complete list of types can be found by running:

ack --help-types

… or, for a more detailed but less readable list:

ack --dump

Some examples are:

cc for C files

haskell for Haskell files

lua for Lua files

python for Python files

shell for Bash, and other shell command files

These names can be used with the options -t TYPE and

--type=TYPE and also by simply preceding them with two

dashes (--TYPE). There are also ways of requesting files

not of a given type: -T TYPE, --type=noTYPE

and --noTYPE.

To check files in the current directory of type shell an

ack command like the following might be used and the

following type of output produced:

$ ack --shell declare

Bash_snippet__using_coproc_with_SQLite/examples/coproc_test.sh

11:declare -a com=('date +%F' 'whoami' 'id' 'echo "$BASH_VERSION"'

Note that ack reports the file path and numbered lines

within it that match.

You can