第 1 章 Working with systemd unit files

A unit file contains configuration directives that describe the unit and define its behavior. Several systemctl commands work with unit files in the background. To make finer adjustments, you can edit or create unit files manually.

You can find three main directories where unit files are stored on the system, the /etc/systemd/system/ directory is reserved for unit files created or customized by the system administrator.

Unit file names take the following form:

<unit_name>.<type_extension>

Here, unit_name stands for the name of the unit and type_extension identifies the unit type.

For example, you can find an sshd.service as well as an sshd.socket unit present on your system.

Unit files can be supplemented with a directory for additional configuration files. For example, to add custom configuration options to sshd.service, create the sshd.service.d/custom.conf file and insert additional directives there. For more information on configuration directories, see Modifying existing unit files.

The systemd system and service manager can also create the sshd.service.wants/ and sshd.service.requires/ di. These directories contain symbolic links to unit files that are dependencies of the sshd service. systemd creates the symbolic links automatically either during installation according to [Install] unit file options or at runtime based on [Unit] options. You can also create these directories and symbolic links manually.

Also, the sshd.service.wants/ and sshd.service.requires/ directories can be created. These directories contain symbolic links to unit files that are dependencies of the sshd service. The symbolic links are automatically created either during installation according to [Install] unit file options or at runtime based on [Unit] options. It is also possible to create these directories and symbolic links manually. For more details on [Install] and [Unit] options, see the tables below.

Many unit file options can be set by using the unit specifiers - wildcard strings that are dynamically replaced with unit parameters when the unit file is loaded. This enables creation of generic unit files that serve as templates for generating instantiated units. See Working with instantiated units.