.. include:: ../header.txt

.. _auto_var:

Environment Variables
===========================

:title:`Linux for Programmers and Users`, Section 6.3.10

The Bash shell maintains several environment variables that may be useful
in shell script programs.  Some of these are discussed in :ref:`variables`.
The list below represents some of the most commonly used environment
variables.

===================  =======================================================
  Name                Meaning
===================  =======================================================
:envvar:`$$`          The process ID of this shell.

:envvar:`$!`          The process ID of the last background command.

:envvar:`$#`          The number of positional parameters.

:envvar:`$?`          The exit value of the last command.

:envvar:`$@`          An individually quoted list of all the positional
                      parameters.

:envvar:`$HOSTNAME`   Hostname of the computer

:envvar:`$OLDPWD`     The previous working directory of the shell.

:envvar:`$OSTYPE`     The operating system of the computer.

:envvar:`$PPID`       The process ID of the shell's parent.

:envvar:`$PS1`        The setting for the command prompt. See :ref:`ps1`.

:envvar:`$PWD`        The current working directory of the shell.

:envvar:`$UID`        The user ID of the user.

===================  =======================================================