Almost all of XTerm's configuration is described in terms of X resources (see archlinux.org and stuff.mit.edu). For desktop users, X resources are typically stored in the ~/.Xresources dotfile. (For additional details, see the section ~/.Xresources for advanced use cases further below.)
The X resources that you can configure for XTerm are listed in the XTerm manpage, which is available as $ man xterm or online.
To configure XTerm on a system, simply download or create and edit your ~/.Xresources dotfile, and then run the xrdb command to load the X resources from your ~/.Xresources dotfile. See the next sections for step-by-step instructions.
Before doing anything regarding the ~/.Xresources dotfile, first check if you already have it in the system's user home directory. Because ~/.Xdefaults and ~/.Xdefaults-* are other dotfiles where X resources might be stored, you can check for their presence too. Read the output of the command to find out:
$ ls -a ~/.Xresources ~/.Xdefaults ~/.Xdefaults-*
TIP The easiest way to start with a ~/.Xresources dotfile is to download and use the sample file (see instructions inside that file). Alternatively, you can run $ touch ~/.Xresources to create a blank one where you can add XTerm configuration lines.
Run $ xrdb -query to view the loaded X resources.
TIP This command might be useful after editing the ~/.Xresources dotfile.
Run $ xrdb -merge ~/.Xresources.
1. Edit your ~/.Xresources dotfile, adding or removing XTerm parameters or changing their values.
2. Run $ xrdb -merge ~/.Xresources to load the updated X resources. (If you get any error messages in the output, edit the ~/.Xresources file to resolve the errors.)
3. Open a new XTerm window by running $ xterm & to visually verify the changes in the window.
~/.XresourcesYou can reference another file in the ~/.Xresources dotfile with an absolute path, for example /home/<user>/..., not using the tilde ~. This means you can load XTerm's X resources from another file by adding the following statement on a line in ~/.Xresources:
#include "/<absolute_path_to_file>/<file_with_x_resources_for_xterm>"
.Xresources and .Xdefaults.Xresources are loaded into the X server, so are linked to a particular display environment."For most desktop environments, ~/.Xresources should be loaded at login. Note that if you make changes, you will need to run xrdb to pick them up. In my case, typically I care more about the server than the client for things like font size: for example, when using my laptop the resources are configured to suit a 14" screen at a particular resolution, and those same settings are used when I run xvile either locally, or remotely on my workstation over ssh. When sitting at my workstation which has a somewhat larger screen, I want different settings configured in the X server. By using .Xresources rather than .Xdefaults the same invocation of xvile behaves appropriately depending on where it is being displayed, rather than where was invoked from." Source ".Xresources works using ssh -x. I thought that my window manager would automatically load .Xresources, but it doesn't do so. Modifying my .xinitrc fixes the problem." Source
.Xdefaults are set based on the client."On POSIX-based systems, the user's environment resource file name is specified by the value of the XENVIRONMENT Environment Variable. If this environment variable does not exist, the user's home directory is searched for a file named .Xdefaults-<host>, where <host> is the host name of the machine on which the application is running." Source
Another in-depth alternative is to set XAPPLRESDIR, which lets one set up a directory where X will search for an .Xresources dotfile:
application-specific files
Directories named by the environment variable XUSERFILESEARCH‐
PATH or the environment variable XAPPLRESDIR (which names a
single directory and should end with a '/' on POSIX systems),
plus directories in a standard place (usually under
/usr/share/X11/, but this can be overridden with the XFILE‐
SEARCHPATH environment variable) are searched for for applica‐
tion-specific resources. For example, application default
resources are usually kept in /usr/share/X11/app-defaults/.
See the X Toolkit Intrinsics - C Language Interface manual for
details.
Source
X Toolkit Intrinsics - C Language Interface discusses this as well.
Run $ xrdb -help or see the xrdb manpage for more xrdb options.
WARNING If you decide to use $ xrdb -remove, be aware that it clears (!) all previously loaded X resources, including the X resources that were loaded at system startup.