GNU Coding Standards
4.9 File Usage
Programs should be prepared to operate when `/usr' and `/etc'
are read-only file systems. Thus, if the program manages log files,
lock files, backup files, score files, or any other files which are
modified for internal purposes, these files should not be stored in
`/usr' or `/etc'.
There are two exceptions. `/etc' is used to store system
configuration information; it is reasonable for a program to modify
files in `/etc' when its job is to update the system configuration.
Also, if the user explicitly asks to modify one file in a directory, it
is reasonable for the program to store other files in the same