In linux, why are folders for configuration files always named *.d

Question

In linux, why are folders for configuration files always named *.d?

Say

• /etc/init.d
• /etc/grub.d
• /etc/apparmor.d

Answer 1

The .d stands for directory. It is a convention to distinguish the directory-based configuration from a configuration based on a single configuration file. Often you will have both in some capacity, for example /etc/logrotate.conf and /etc/logrotate.d/.

It is also usually the case that all (reasonably-named) files in such a directory are automatically combined into a single configuration. Packages can then install files into a directory like that, and they will be used automatically. Again, /etc/logrotate.d/ is a good example. By constrast, a directory of configuration files that does not end in .d probably just contains random collation of configuration files belonging to the same package, and you cannot infer anything about how they are processed, example /etc/zsh/.