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From Graham Leggett <>
Subject Environment confusion
Date Mon, 20 Oct 2008 17:19:58 GMT
Hi all,

I have just been picking apart the way that environment variables are 
handled at config time within httpd, and there seems to be some 
overloading on concepts that has caused some confusion.

There are two environments within httpd, the first is the read only 
system environment that is read using getenv(), the second is the 
server->vars table that is read/write using mod_env and friends.

A recent addition was made (it's on trunk and 2.2) that allows httpd to 
include system variables in config directives, like this:

DocumentRoot ${DOCUMENT_ROOT}

The system environment variable DOCUMENT_ROOT is parsed and placed in 
the config line, so far so good.

What isn't possible however, is this:

SetEnv WEBAPP app1
<Location /${WEBAPP}>
   ServerAdmin ${WEBAPP}
   ... other cool template style stuff ...

In this case, SetEnv sets a variable within mod_env's server->vars, but 
this is ignored by ap_resolve_env, and so doesn't work as the user might 
expect it to.

Zooming in some more on the way mod_env's environment works.

The mod_env environment in server->vars starts off empty, and then is 
populated by explicit allocation (SetEnv), or by copying the value from 
a system environment variable to a mod_env environment variable (PassEnv).

Would it make sense when parsing the config to try and insert mod_env's 
server->vars variables first, and then if not present, fall back to (the 
current behaviour of) looking at the system environment?

This will make some interesting templating options possible, and will 
probably make lives easier for people doing mass hosting.


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