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<h3 class="section">25.4 Environment Variables</h3>

<p><a name="index-environment-variable-3098"></a>When a program is executed, it receives information about the context in
which it was invoked in two ways.  The first mechanism uses the
<var>argv</var> and <var>argc</var> arguments to its <code>main</code> function, and is
discussed in <a href="Program-Arguments.html#Program-Arguments">Program Arguments</a>.  The second mechanism uses
<dfn>environment variables</dfn> and is discussed in this section.

   <p>The <var>argv</var> mechanism is typically used to pass command-line
arguments specific to the particular program being invoked.  The
environment, on the other hand, keeps track of information that is
shared by many programs, changes infrequently, and that is less
frequently used.

   <p>The environment variables discussed in this section are the same
environment variables that you set using assignments and the
<code>export</code> command in the shell.  Programs executed from the shell
inherit all of the environment variables from the shell. 
<!-- !!! xref to right part of bash manual when it exists -->

   <p><a name="index-environment-3099"></a>Standard environment variables are used for information about the user's
home directory, terminal type, current locale, and so on; you can define
additional variables for other purposes.  The set of all environment
variables that have values is collectively known as the
<dfn>environment</dfn>.

   <p>Names of environment variables are case-sensitive and must not contain
the character &lsquo;<samp><span class="samp">=</span></samp>&rsquo;.  System-defined environment variables are
invariably uppercase.

   <p>The values of environment variables can be anything that can be
represented as a string.  A value must not contain an embedded null
character, since this is assumed to terminate the string.

<ul class="menu">
<li><a accesskey="1" href="Environment-Access.html#Environment-Access">Environment Access</a>:           How to get and set the values of
				 environment variables. 
<li><a accesskey="2" href="Standard-Environment.html#Standard-Environment">Standard Environment</a>:         These environment variables have
                		 standard interpretations. 
</ul>

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