Environment Variables in Apache

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There are two kinds of environment variables that affect the Apache HTTP Server.

First, there are the environment variables controlled by the underlying operating system. These are set before the server starts. They can be used in expansions in configuration files, and can optionally be passed to CGI scripts and SSI using the PassEnv directive.

Second, the Apache HTTP Server provides a mechanism for storing information in named variables that are also called environment variables. 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.

Although these variables are referred to as environment variables, 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.

Setting Environment Variables
Using Environment Variables
Special Purpose Environment Variables
Examples

Setting Environment Variables

Related Modules	Related Directives
`mod_cache` `mod_env` `mod_rewrite` `mod_setenvif` `mod_unique_id`	`BrowserMatch` `BrowserMatchNoCase` `PassEnv` `RewriteRule` `SetEnv` `SetEnvIf` `SetEnvIfNoCase` `UnsetEnv`

Basic Environment Manipulation

The most basic way to set an environment variable in Apache is using the unconditional SetEnv directive. Variables may also be passed from the environment of the shell which started the server using the PassEnv directive.

Conditional Per-Request Settings

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 RewriteRule which uses the [E=...] option to set environment variables.

Unique Identifiers

Finally, mod_unique_id sets the environment variable UNIQUE_ID for each request to a value which is guaranteed to be unique across "all" requests under very specific conditions.

Standard CGI Variables

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 CGI specification.

Some Caveats

It is not possible to override or change the standard CGI variables using the environment manipulation directives.
When suexec is used to launch CGI scripts, the environment will be cleaned down to a set of safe variables before CGI scripts are launched. The list of safe variables is defined at compile-time in suexec.c.
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.
A special case are HTTP headers which are passed to CGI scripts and the like via environment variables (see below). They are converted to uppercase and only dashes are replaced with underscores; if the header contains any other (invalid) character, the whole header is silently dropped. See below for a workaround.
The SetEnv directive runs late during request processing meaning that directives such as SetEnvIf and RewriteCond will not see the variables set with it.
When the server looks up a path via an internal subrequest such as looking for a DirectoryIndex or generating a directory listing with mod_autoindex, per-request environment variables are not inherited in the subrequest. Additionally, SetEnvIf directives are not separately evaluated in the subrequest due to the API phases mod_setenvif takes action in.

Using Environment Variables

Related Modules	Related Directives
`mod_authz_host` `mod_cgi` `mod_ext_filter` `mod_headers` `mod_include` `mod_log_config` `mod_rewrite`	`Require` `CustomLog` `Allow` `Deny` `ExtFilterDefine` `Header` `LogFormat` `RewriteCond` `RewriteRule`

CGI Scripts

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 CGI tutorial.

SSI Pages

Server-parsed (SSI) documents processed by mod_include's INCLUDES filter can print environment variables using the echo 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 SSI tutorial.

Access Control

Access to the server can be controlled based on environment variables using the Require env and Require not env directives. In combination with SetEnvIf, 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).

Conditional Logging

Environment variables can be logged in the access log using the LogFormat option %e. 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 CustomLog directive. In combination with SetEnvIf this allows for flexible control of which requests are logged. For example, you can choose not to log requests for filenames ending in gif, or you can choose to only log requests from clients which are outside your subnet.

Conditional Response Headers

The Header 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.

External Filter Activation

External filters configured by mod_ext_filter using the ExtFilterDefine directive can by activated conditional on an environment variable using the disableenv= and enableenv= options.

URL Rewriting

The %{ENV:variable} form of TestString in the RewriteCond allows mod_rewrite's rewrite engine to make decisions conditional on environment variables. Note that the variables accessible in mod_rewrite without the ENV: prefix are not actually environment variables. Rather, they are variables special to mod_rewrite which cannot be accessed from other modules.

Environment Variables in Apache

See also