.. include:: ../header.txt

.. _filtering:

Filtering Data 
===========================

:title:`Linux for Programmers and Users`, Section 4.2 - 4.3.

We use commands that filter data to select only the portion of data
that we wish to view or operate on.  Filtering commands are usually run with
either a filename as a command line argument or they read data from `stdin`,
usually from a pipe.  Some programs, such as :ref:`awk` and :ref:`sed`,
which are really programming languages, can filter data horizontally,
vertically or inline, while other tools filter data either horizontally or
vertically.  

-------------------------------

.. centered:: The Pipe (`|`)

.. index:: pipe, |

A fundamental concept that will be used in filtering data and for the rest of
the semester is that of the **pipe**.  A pipe is a facility of the shell
that makes it very to chain together multiple commands.  A is called upon
with the vertical bar character (**\ |**).  When used between two Unix
commands, it means that output from the first command should become the
input to the second command.  For example, to count how many files
underneath a directory have been modified in the last day, the :command:`find`
and :command:`wc` commands may be used along with a pipe (:ref:`find`, 
:ref:`wc`).  The :command:`find`
command will list the modified files and :command:`wc` can count them::

    find . -type f -mtime -1 | wc -l

-------------------------------

**Contents**

.. toctree::
   :maxdepth: 2

   filtHoriz
   filtVert
   inline
   awk