[lxc-devel] Variable expansions in default config.

[Serge Hallyn]

Quoting Michael H. Warfield (mhw at WittsEnd.com):
> Hey all,
> 
> I'm in the middle of some cleanup and fix-ups in the Fedora and CentOS
> templates including hardening the root password and some static MAC
> address code and generally reconciling the two templates into some
> semblance of coherency.
> 
> I noticed this in the comments emitted from the CentOS template into the
> container config:
> 
> #lxc.network.veth.pair = v-$name-e0
> 
> Hmmm...  That's interesting.  I wonder if that would work in the default
> config?  That could be real handy (subject to name collisions
> understood.)  Nope.  It's a literal copy from the default config over to
> the initial starting point container config.  Sigh...
> 
> That's an idea, though, if variables could be expanded.  That's easy
> enough to deal with in the templates.  Since I'm already recopying that
> initialized config to add some hwaddr addresses (in my upcoming
> patches), I can add this to those templates and do the expansions
> (reading from the initial default config and copying back to the target
> config)...
> 
>     # This should catch variable expansions from the default config...
>     if expr "${LINE}" : '.*\$' > /dev/null 2>&1
>     then
>             LINE=$(eval "echo \"${LINE}\"")
>     fi
> 
> I've got that working and will submit it along with my other patches
> shortly...
> 
> It's a thought, though there may be other ways we could do this.  It
> could be useful.  I can use it.  Does no harm if nobody puts environment
> variables in the default config.  Not sure what the potential for random
> acts of terrorism is if someone stuffs something STUPID in there...
> Simple expansions like above are prone to abuse in unexpected ways...
> Other shell meta characters could be a PITA...
> 
> Thoughts?

Sounds reasonable.