.. index:: path names, absolute pathname, relative pathname
.. _pathnames:

Pathnames: Two Types
----------------------

:t:`Linux for Programmers and Users`, Section 3.12

.. describe:: Absolute pathname 

   Specifies the path with reference to the root of the file system.  (Always
   begins with /, for the root directory).

   System configuration files that normally do not change location should
   be addressed with an absolute pathname.

.. describe:: Relative pathname 

    Specifies the path with reference to the user's current location. 
    (``cd ../include``).

    * Uses **.** to signify the current directory.

    * Uses **..** to signify the parent directory.

    * Used to refer to files that are either impossible or inconvenient to
      access in an absolute manner.


Both commands and filename arguments can be represented in either form.

.. note:: 

    A command is also a file and many commands also use filenames as arguments.
    Absolute and relative pathnames can be used with both.

.. note:: 

    Consider the grep command which occurs as two versions in /usr/bin and
    /usr/xpg4/bin on Solaris. If PATH is set up like this::

        PATH=/usr/bin:/usr/dt/bin:/usr/xpg4/bin:.

    We need to use an absolute pathname to access the version in /usr/xpg4/bin.



.. note:: 

    A relative pathname may require fewer keystrokes compared to an absolute
    pathname. For instance, ``cd ..`` is often more convenient to use than
    :command:`cd` used with an absolute pathname. But should we use relative
    pathnames in our programs?

    We can do that only if the program runs from a specific directory. For
    instance, the command ``cp *.c ../backup`` may not work properly or not at
    all unless we are positioned in a specific directory. We can't always
    ensure that, in which case we need to use an absolute pathname.

    If a command named cat exists both in /bin and in the current directory,
    and this directory occurs after /bin in PATH, then ``cat foo`` is not the
    same as ``./cat foo``. They run two different programs.