X resources are key-value pairs used for program configuration mediated by the X Window
System. They are manipulated by xrdb(1) (used in my xinitrc).
Newer programs usually don't use this configuration mechanism.
Values are assigned to keys such as XTerm.termName (capitalization matters: XTerm
and xterm will work, but not Xterm). Generally, the keys consist of a bunch of
alphanumeric strings separated by dots (.) where the first string is a class or
instance name of an application:
The first element in a resource specification is either a "class" or an "instance". A class consists of ALL the invocations of a particular program. An instance is a process which is given a particular name.
—"How do I use X Resources?" at kb.mit.edu
A fully specified resource may look like this:
application.widget.widget.attribute: value
Everything to the left of the colon is the key. The space between the colon and the value seems to be optional.
Class names typically start with a capital letter. For example, XTerm is the default
resource class for xterms. The default instance name for xterms is xterm (the name of
the executable: renaming changes it). While xterm allows overriding both the class name
and the instance name (with the -class and -name command-line options), I don't think
this is at all commonly done (for xterm or any program).
Whether a class name or instance name is used does not seem to matter much. X resource snippets on the web mostly use one or the other randomly without providing any reasoning. Class names may be intended to be more general and (mainly for consistency) I am always using them here.
The last string of a key is the resource name or attribute. In between can be any
number of strings referred to as components or widgets, whose names and relations
are (probably) defined by the program to be configured. For example, two widget names
used by xterm are vt100 and tek4014 (class names: VT100 and Tek4014).
Lastly, the keys are actually patterns (sometimes called resource patterns) as two wildcard characters can appear anywhere but at the end:
* matches any number of components, including the class or instance name, so this
is valid: *background: black.
? matches a single component. The dots still have to be specified. It seems to be
less commonly used than *.
Further informative links:
- X resources ArchWiki page
- Inessential X Resources for Techno-Dweebs
- Gilles' and Thomas Dickey's answers to this question on unix.stackexchange.com