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<title>Environment Variables in Apache - Apache HTTP Server</title>
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<p class="apache">Apache HTTP Server Version 2.0</p>
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<a href="http://www.apache.org/">Apache</a> > <a href="http://httpd.apache.org/">HTTP Server</a> > <a href="http://httpd.apache.org/docs/">Documentation</a> > <a href="./">Version 2.0</a></div><div id="page-content"><div id="preamble"><h1>Environment Variables in Apache</h1>
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<p>The Apache HTTP Server provides a mechanism for storing
information in named variables that are called <em>environment
variables</em>. This information can be used to control various
operations such as logging or access control. The variables are
also used as a mechanism to communicate with external programs
such as CGI scripts. This document discusses different ways to
manipulate and use these variables.</p>
<p>Although these variables are referred to as <em>environment
variables</em>, they are not the same as the environment
variables controlled by the underlying operating system.
Instead, these variables are stored and manipulated in an
internal Apache structure. They only become actual operating
system environment variables when they are provided to CGI
scripts and Server Side Include scripts. If you wish to
manipulate the operating system environment under which the
server itself runs, you must use the standard environment
manipulation mechanisms provided by your operating system
shell.</p>
</div>
<div id="quickview"><ul id="toc"><li><img alt="" src="./images/down.gif" /> <a href="#setting">Setting Environment Variables</a></li>
<li><img alt="" src="./images/down.gif" /> <a href="#using">Using Environment Variables</a></li>
<li><img alt="" src="./images/down.gif" /> <a href="#special">Special Purpose Environment Variables</a></li>
<li><img alt="" src="./images/down.gif" /> <a href="#examples">Examples</a></li>
</ul></div>
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<div class="section">
<h2><a name="setting" id="setting">Setting Environment Variables</a></h2>
<table class="related"><tr><th>Related Modules</th><th>Related Directives</th></tr><tr><td><ul><li><code class="module"><a href="./mod/mod_env.html">mod_env</a></code></li><li><code class="module"><a href="./mod/mod_rewrite.html">mod_rewrite</a></code></li><li><code class="module"><a href="./mod/mod_setenvif.html">mod_setenvif</a></code></li><li><code class="module"><a href="./mod/mod_unique_id.html">mod_unique_id</a></code></li></ul></td><td><ul><li><code class="directive"><a href="./mod/mod_setenvif.html#browsermatch">BrowserMatch</a></code></li><li><code class="directive"><a href="./mod/mod_setenvif.html#browsermatchnocase">BrowserMatchNoCase</a></code></li><li><code class="directive"><a href="./mod/mod_env.html#passenv">PassEnv</a></code></li><li><code class="directive"><a href="./mod/mod_rewrite.html#rewriterule">RewriteRule</a></code></li><li><code class="directive"><a href="./mod/mod_env.html#setenv">SetEnv</a></code></li><li><code class="directive"><a href="./mod/mod_setenvif.html#setenvif">SetEnvIf</a></code></li><li><code class="directive"><a href="./mod/mod_setenvif.html#setenvifnocase">SetEnvIfNoCase</a></code></li><li><code class="directive"><a href="./mod/mod_env.html#unsetenv">UnsetEnv</a></code></li></ul></td></tr></table>
<h3><a name="basic-manipulation" id="basic-manipulation">Basic Environment Manipulation</a></h3>
<p>The most basic way to set an environment variable in Apache
is using the unconditional <code class="directive"><a href="./mod/mod_env.html#setenv">SetEnv</a></code> directive. Variables may also be passed from
the environment of the shell which started the server using the
<code class="directive"><a href="./mod/mod_env.html#passenv">PassEnv</a></code> directive.</p>
<h3><a name="conditional" id="conditional">Conditional Per-Request Settings</a></h3>
<p>For additional flexibility, the directives provided by
mod_setenvif allow environment variables to be set on a
per-request basis, conditional on characteristics of particular
requests. For example, a variable could be set only when a
specific browser (User-Agent) is making a request, or only when
a specific Referer [sic] header is found. Even more flexibility
is available through the mod_rewrite's <code class="directive"><a href="./mod/mod_rewrite.html#rewriterule">RewriteRule</a></code> which uses the
<code>[E=...]</code> option to set environment variables.</p>
<h3><a name="unique-identifiers" id="unique-identifiers">Unique Identifiers</a></h3>
<p>Finally, mod_unique_id sets the environment variable
<code>UNIQUE_ID</code> for each request to a value which is
guaranteed to be unique across "all" requests under very
specific conditions.</p>
<h3><a name="standard-cgi" id="standard-cgi">Standard CGI Variables</a></h3>
<p>In addition to all environment variables set within the
Apache configuration and passed from the shell, CGI scripts and
SSI pages are provided with a set of environment variables
containing meta-information about the request as required by
the <a href="http://cgi-spec.golux.com/">CGI
specification</a>.</p>
<h3><a name="caveats" id="caveats">Some Caveats</a></h3>
<ul>
<li>It is not possible to override or change the standard CGI
variables using the environment manipulation directives.</li>
<li>When <code class="program"><a href="./programs/suexec.html">suexec</a></code> is used to launch
CGI scripts, the environment will be cleaned down to a set of
<em>safe</em> variables before CGI scripts are launched. The
list of <em>safe</em> variables is defined at compile-time in
<code>suexec.c</code>.</li>
<li>For portability reasons, the names of environment
variables may contain only letters, numbers, and the
underscore character. In addition, the first character may
not be a number. Characters which do not match this
restriction will be replaced by an underscore when passed to
CGI scripts and SSI pages.</li>
</ul>
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<div class="section">
<h2><a name="using" id="using">Using Environment Variables</a></h2>
<table class="related"><tr><th>Related Modules</th><th>Related Directives</th></tr><tr><td><ul><li><code class="module"><a href="./mod/mod_access.html">mod_access</a></code></li><li><code class="module"><a href="./mod/mod_cgi.html">mod_cgi</a></code></li><li><code class="module"><a href="./mod/mod_ext_filter.html">mod_ext_filter</a></code></li><li><code class="module"><a href="./mod/mod_headers.html">mod_headers</a></code></li><li><code class="module"><a href="./mod/mod_include.html">mod_include</a></code></li><li><code class="module"><a href="./mod/mod_log_config.html">mod_log_config</a></code></li><li><code class="module"><a href="./mod/mod_rewrite.html">mod_rewrite</a></code></li></ul></td><td><ul><li><code class="directive"><a href="./mod/mod_access.html#allow">Allow</a></code></li><li><code class="directive"><a href="./mod/mod_log_config.html#customlog">CustomLog</a></code></li><li><code class="directive"><a href="./mod/mod_access.html#deny">Deny</a></code></li><li><code class="directive"><a href="./mod/mod_ext_filter.html#extfilterdefine">ExtFilterDefine</a></code></li><li><code class="directive"><a href="./mod/mod_headers.html#header">Header</a></code></li><li><code class="directive"><a href="./mod/mod_log_config.html#logformat">LogFormat</a></code></li><li><code class="directive"><a href="./mod/mod_rewrite.html#rewritecond">RewriteCond</a></code></li><li><code class="directive"><a href="./mod/mod_rewrite.html#rewriterule">RewriteRule</a></code></li></ul></td></tr></table>
<h3><a name="cgi-scripts" id="cgi-scripts">CGI Scripts</a></h3>
<p>One of the primary uses of environment variables is to
communicate information to CGI scripts. As discussed above, the
environment passed to CGI scripts includes standard
meta-information about the request in addition to any variables
set within the Apache configuration. For more details, see the
<a href="howto/cgi.html">CGI tutorial</a>.</p>
<h3><a name="ssi-pages" id="ssi-pages">SSI Pages</a></h3>
<p>Server-parsed (SSI) documents processed by mod_include's
<code>INCLUDES</code> filter can print environment variables
using the <code>echo</code> element, and can use environment
variables in flow control elements to makes parts of a page
conditional on characteristics of a request. Apache also
provides SSI pages with the standard CGI environment variables
as discussed above. For more details, see the <a href="howto/ssi.html">SSI tutorial</a>.</p>
<h3><a name="access-control" id="access-control">Access Control</a></h3>
<p>Access to the server can be controlled based on the value of
environment variables using the <code>allow from env=</code>
and <code>deny from env=</code> directives. In combination with
<code class="directive"><a href="./mod/mod_setenvif.html#setenvif">SetEnvIf</a></code>, this
allows for flexible control of access to the server based on
characteristics of the client. For example, you can use these
directives to deny access to a particular browser (User-Agent).
</p>
<h3><a name="logging" id="logging">Conditional Logging</a></h3>
<p>Environment variables can be logged in the access log using
the <code class="directive"><a href="./mod/mod_log_config.html#logformat">LogFormat</a></code>
option <code>%e</code>. In addition, the decision on whether
or not to log requests can be made based on the status of
environment variables using the conditional form of the
<code class="directive"><a href="./mod/mod_log_config.html#customlog">CustomLog</a></code>
directive. In combination with <code class="directive"><a href="./mod/mod_setenvif.html#setenvif">SetEnvIf</a></code> this allows for flexible control of which
requests are logged. For example, you can choose not to log
requests for filenames ending in <code>gif</code>, or you can
choose to only log requests from clients which are outside your
subnet.</p>
<h3><a name="response-headers" id="response-headers">Conditional Response Headers</a></h3>
<p>The <code class="directive"><a href="./mod/mod_headers.html#header">Header</a></code>
directive can use the presence or
absence of an environment variable to determine whether or not
a certain HTTP header will be placed in the response to the
client. This allows, for example, a certain response header to
be sent only if a corresponding header is received in the
request from the client.</p>
<h3><a name="external-filter" id="external-filter">External Filter Activation</a></h3>
<p>External filters configured by <code class="module"><a href="./mod/mod_ext_filter.html">mod_ext_filter</a></code>
using the <code class="directive"><a href="./mod/mod_ext_filter.html#extfilterdefine">ExtFilterDefine</a></code> directive can
by activated conditional on an environment variable using the
<code>disableenv=</code> and <code>enableenv=</code> options.</p>
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