Apache lets one do this:

apachectl start -D VariableName

and in the config

<ifDefine VariableName> Do Stuff if we specified the variable </ifDefine> <ifDefine !VariableName> or Don't Do Stuff if we didn't specify it </ifDefine>

nginx doesn't seem to have that facility (at least in the docs it doesnt). I was hoping it could be added if its not in there already

why is it useful? i use it on apache to switch from uptime -> downtime when there's only 1-2 lines different in the config files. big timesaver. also, when you're hosting mutiple sites, you might want to include a different file depending on a flag being set or not