Configuration

Most programs follow the same paradigm for configuration. The effective configuration for a program is assembled by layering parameters from the following sources in order:

Default parameters

Stored within the program

System-wide parameters

Stored in a shared directory

User parameters

Stored in the user's directory

Invocation parameters

Specified at invocation

For example, Xdefaults are typically compiled into the program and augmented by specifications in /usr/lib/X11/app-defaults/, $HOME/.Xdefaults, and the command line. The default color of the xclock background is white; a line in /usr/lib/X11/app-defaults/XClock like
XClock*background: orange
shows no sense of aesthetics and could be overridden with a line in $HOME/.Xdefaults:
XClock*background: cornsilk.
You might use the following to remind yourself to go home at 9pm:
$echo "xclock -display :0 -geometry 500x500 -bg red" | at 9pm