Module mod_define

Variable Definition For Arbitrary Directives

This module is contained in the mod_define.c file. It provides the definition variables for arbitrary directives, i.e. variables which can be expanded on any(!) directive line. It needs Extended API (EAPI). It is not compiled into the server by default. To use mod_define you have to enable the following line in the server build Configuration file:

    AddModule  modules/extra/mod_define.o


Syntax: Define variable value
Default: none
Context: server config, virtual host, directory, .htaccess
Override: none
Status: Extension
Module: mod_define.c
Compatibility: Apache+EAPI

The Define directive defines a variable which later can be expanded with the unsafe but short construct ``$variable'' or the safe but longer construct ``${variable}'' on any configuration line. Do not intermix this with the third-party module mod_macro. The mod_define module doesn't provide a general macro mechanism, although one can consider variable substitutions as a special form of macros. Because the value of to which ``$variable'' expands has to fit into one line. When you need macros which can span more lines, you've to use mod_macro. OTOH mod_macro cannot be used to expand a variable/macro on an arbitrary directive line. So, the typical use case of mod_define is to make strings variable (and this way easily changeable at one location) and not to bundle things together (as it's the typical use case for macros).

The syntax of the expansion construct ( ``${variable}'') follows the Perl and Shell syntax, but can be changed via the Define directive, too. Four internal variables can be used for this. The default is:

Define mod_define::escape "\\"
Define mod_define::dollar "$"
Define mod_define::open   "{"
Define mod_define::close  "}"

When you need to escape some of the expansion constructs you place the mod_define::escape character in front of it. The default is the backslash as in Perl or the Shell.


Define master     "Joe Average <joe@average.dom>"
Define docroot    /usr/local/apache/htdocs
Define hostname   foo
Define domainname bar.dom
Define portnumber 80
<VirtualHost $hostname.$domainname:$portnumber>
SetEnv       SERVER_MASTER "$master"
ServerName   $hostname.$domainname
ServerAlias  $hostname
Port         $portnumber
DocumentRoot $docroot
<Directory $docroot>

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