.. include:: ../header.txt

.. _filtHoriz:

Filtering Horizontally
------------------------

:title:`Linux for Programmers and Users`, Section 4.1

Commands that filter data **horizontally** pass a subset of the **lines**
processed to standard output.

.. _grep:

grep
^^^^

.. index:: grep 
.. program:: grep 
.. describe:: grep

    Print selected parts of lines from each FILE to standard output.  The
    selection is based on matching of PATTERN in the line of input data.
    :command:`grep` can also read data from stdin (usually via a pipe).

SYNOPSIS
   grep [OPTIONS] PATTERN [FILE...]


.. option:: -i, --ignore-case

   Ignore case distinctions in both  the  PATTERN  and  the  input files.

.. option:: -v, --invert-match

   Invert the sense of matching, to select non-matching lines.



egrep, fgrep
^^^^^^^^^^^^^

.. index:: egrep, fgrep

.. describe:: fgrep 

    Fixed string only

.. describe:: grep 

    Regular expressions

.. describe:: egrep 

    Extended regular expressions


.. _uniq:

uniq
^^^^^

.. index:: uniq
.. program:: uniq 
.. describe:: uniq

   Remove repeating lines. The :command:`uniq` command is often used in
   combination with :ref:`sort` to remove duplicate items.

SYNOPSIS

   uniq [OPTIONS] [INPUT [OUTPUT]] 

.. option:: -f, --skip-fields=N

   avoid comparing the first N fields. A field is  a  run  of  whitespace,
   then  non-whitespace  characters.

::

    $ cat sample
    This is a test file for the uniq command.
    It contains some repeated and some nonrepeated lines.
    Some of the repeated lines are consecutive, like this.
    Some of the repeated lines are consecutive, like this.
    Some of the repeated lines are consecutive, like this.
    And, some are not consecutive, like the following.
    Some of the repeated lines are consecutive, like this.
    The above line, therefore, will not be considered a repeated
    line by the uniq command, but this will be considered repeated!
    line by the uniq command, but this will be considered repeated!
    $ uniq sample
    This is a test file for the uniq command.
    It contains some repeated and some nonrepeated lines.
    Some of the repeated lines are consecutive, like this.
    And, some are not consecutive, like the following.
    Some of the repeated lines are consecutive, like this.
    The above line, therefore, will not be considered a repeated
    line by the uniq command, but this will be considered repeated!
    $

.. _sort:

sort
^^^^

.. index:: sort
.. program:: sort 
.. describe:: sort

   sort lines of textual data

SYNOPSIS

   sort [OPTION] [FILE]

.. option:: -f, --ignore-case

   fold lower case to upper case characters

.. option:: -g, --general-numeric-sort

   compare according to general numerical value

.. option:: -k, --key=POS1[,POS2]

   start a key at POS1, end it at POS 2 (origin 1)

.. option:: -t, --field-separator=SEP 

   use SEP instead of non-blank to blank transition

The following example sorts data from the system's database of users
according to their UID (third field) of colon (:) separated fields.
Note the data is sort numerically rather than alphabetically by using the
-g option.

::

    $ getent passwd | sort -gt: -k 3,4

awk
^^^^

See :ref:`horiz_awk`

.. note::

    If awk is to be considered for filtering data, so should perl and python.