Bug#941198: In support of mandatory unit files

[Guillem Jover <guillem@debian.org>]

On Sun, 2019-12-08 at 15:55:45 -0800, Russ Allbery wrote:
> Guillem Jover <guillem@debian.org> writes:
> > But here you do have another option, but I'm not sure it might be
> > described as nicer TBH, :) something like this, or variations on this
> > theme:
> 
> >   [Service]
> >   Type=simple
> >   EnvironmentFile=/etc/default/service-static-vars
> >   EnvironmentFile=-/run/service-dynamic-vars
> >   ExecStartPre=-/bin/sh -c 'echo NAME=$(hostname) >/run/service-dynamic-vars'
> >   ExecStart=/usr/bin/daemon --option ${NAME}
> 
> This is a nice approach, but I don't think it quite preserves the original
> behavior.  As you wrote it above, if someone changed the setting in the
> /etc/default file, that would have no effect and the default would still
> be used.  If you reverse the order of EnvironmentFile, it avoids that
> problem, but now the legacy setting with $(hostname) will be used if it
> hasn't been changed, and that will result in a literal $(hostname) in the
> value.

Right, it was more about showing the concept than a proper
implementation, but it's true that as is, it's not helpful. :)

I guess the following which starts to get a bit into ugly territory
would do instead:

   [Service]
   Type=simple
   EnvironmentFile=-/run/service-dynamic-vars
   ExecStartPre=-/usr/bin/env -i /bin/sh -a -c '. /etc/default/service-static-vars && env -uPWD >/run/service-dynamic-vars'
   ExecStart=/usr/bin/daemon --option ${NAME}

Thanks,
Guillem