## Configuration Directories and Precedence

Configuration files are read from directories in `/etc/`, `/run/`, and `/usr/lib/`, in order of precedence. Each configuration file in these configuration directories shall be named in the style of `filename.conf`. Files in `/etc/` override files with the same name in `/run/` and `/usr/lib/`. Files in `/run/` override files with the same name in `/usr/lib/`.

Packages should install their configuration files in `/usr/lib/`. Files in `/etc/` are reserved for the local administrator, who may use this logic to override the configuration files installed by vendor packages. All configuration files are sorted by their filename in lexicographic order, regardless of which of the directories they reside in. If multiple files specify the same option, the entry in the file with the lexicographically latest name will take precedence. It is recommended to prefix all filenames with a two-digit number and a dash, to simplify the ordering of the files.

If the administrator wants to disable a configuration file supplied by the vendor, the recommended way is to place a symlink to `/dev/null` in the configuration directory in `/etc/`, with the same filename as the vendor configuration file. If the vendor configuration file is included in the initrd image, the image has to be regenerated.

## Configuration Directories and Precedence

The default configuration is defined during compilation, so a configuration file is only needed when it is necessary to deviate from those defaults. By default, the configuration file in `/etc/systemd/` contains commented out entries showing the defaults as a guide to the administrator. This file can be edited to create local overrides.

When packages need to customize the configuration, they can install configuration snippets in `/usr/lib/systemd/*.conf.d/`. Files in `/etc/` are reserved for the local administrator, who may use this logic to override the configuration files installed by vendor packages. The main configuration file is read before any of the configuration directories, and has the lowest precedence; entries in a file in any configuration directory override entries in the single configuration file. Files in the `*.conf.d/` configuration subdirectories are sorted by their filename in lexicographic order, regardless of which of the subdirectories they reside in. When multiple files specify the same option, for options which accept just a single value, the entry in the file with the lexicographically latest name takes precedence. For options which accept a list of values, entries are collected as they occur in files sorted lexicographically. It is recommended to prefix all filenames in those subdirectories with a two-digit number and a dash, to simplify the ordering of the files.

To disable a configuration file supplied by the vendor, the recommended way is to place a symlink to `/dev/null` in the configuration directory in `/etc/`, with the same filename as the vendor configuration file.