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Sharing Xresources between systems

In amongst the rants in our watercooler channel at work comes this question:

So then, how do custom settings in Xresources work if you share your home directory between machines?

The question arises from a much larger topic concerning keeping your home directory in a git repository. The issue Carl is having is that he needs to use different options on different machines and doesn’t relish the idea of having to pull and rebase branches just to keep files in sync. Of course, he doesn’t have to do that and that is the point of this tip.

Basic conditionals

With great foresight the authors of xrdb decided that our Xresources file should be preprocessed before it is used, the default preprocessor is cpp. On most of our systems at work, and likely most Linux systems in general, cpp is GNU cpp. Don’t worry about the size of the manual though, the interesting features are easy to understand and use. Let’s look at trimmed example from my configuration file:

#ifndef FIXED_FONT
    #define FIXED_FONT xft:Inconsolata:pixelsize=14:antialias=false
#endif
#ifndef BOLD_FIXED_FONT
    #define BOLD_FIXED_FONT FIXED_FONT:bold
#endif

urxvt.font: FIXED_FONT
urxvt.boldFont: BOLD_FIXED_FONT

irssi.font: FIXED_FONT

This small snippet sets a couple of default fonts that we can reference throughout our resource file as FIXED_FONT and BOLD_FIXED_FONT. This way we can easily change the value in one place and it trickles down to all our programs. Or we can define different options at runtime by specifying values to xrdb, such as xrdb -DFIXED_FONT=xft:Monospace:pixelsize=10. And don’t worry, cpp is smart enough to cope with the use = in our definition.

Per system conditionals

On one system I use Inconsolata is not installed, so the configuration above wouldn’t work on it. xrdb gives us a way around the problem though, it defines a set of symbols we can use including one to test the system’s hostname. nebula, the system that doesn’t have Inconsolata, does have terminus and I can use that on there with a couple of small changes.

#ifndef FIXED_FONT
    #ifdef SRVR_nebula
        #define FIXED_FONT xft:Terminus:pixelsize=14:antialias=false
    #else
        #define FIXED_FONT xft:Inconsolata:pixelsize=14:antialias=false
    #endif
#endif

Note

As we’re using X11’s xrdb it is network aware, it defines SRVR_name for the X server name and CLNT_name for the client name. Using these it is easy to configure systems where the server and clients are on different machines.

Conditionals for server options

You can also change configured settings based on the extensions loaded in to the server, for example to disable the screensaver in muxi if the DPMS extension is supported we’d test for EXT_DPMS:

#ifdef EXT_DPMS
    muxi.screensaver: false
#endif

Testing your resource files

When you’re testing your own ~/.Xresources file you can use the -n option with xrdb, it tells xrdb to dump the settings as they would be used instead of updating the resource database. This makes it easy to check if our conditional statements are working correctly without having to open and close applications constantly. You can also define and cancel symbol definitions with the -D and -U options for xrdb, this allows you to test your modifications that rely on symbols that are normally exported by xrdb.

Using a more featureful preprocessor

You can also choose a different preprocessor if cpp isn’t up to your needs by specifying a -cpp option to xrdb. The only caveat is that must accept -D for defines, -U for symbol cancelling and -I for include paths. An example that does fit these restrictions is m4, and it might be a good choice if you wish to do more advanced things in your configuration file such as fancy filtering or the use of loops for defining colour tables.

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