.. include:: ../header.txt

.. _stdin:
.. _stdout:
.. _stderr:
.. _io:

Standard Files and File Descriptors
=====================================

:t:`Linux for Programmers and Users`, Section 3.5

.. seealso:: :ref:`redirect`

.. index:: stdin, stdout, stderr

When each program begins, three file descriptors are open and ready for use.

.. describe:: stdin

   The primary input channel for the program.  Default source is the user's
   keyboard, but it can easily be switched to be from another process via a
   pipe ( | ) or a file via a file redirection ( < ).

.. describe:: stdout

   The primary output channel for program's data output.  Default destination
   is the user's screen, but it can easily be switched to another process via a
   pipe ( | ) or a file via a file redirection ( > ).

.. describe:: stderr

   Output channel for error messages.  Default destination is the user's
   screen, but it can be switched to another process via a pipe ( | ) or a file
   via a file redirection ( > ).  The details of how to do that will be covered
   later. See :ref:`redirect`.


.. figure:: file_descriptors.png
   :align: center

   Logical view of file descriptors