Master Systemd: Enable, Start, Stop, and Configure Linux Services

1. Enable service at boot

For software that supports systemd, installation adds a unit file to /usr/lib/systemd/system. To enable the service to start on boot, run sudo systemctl enable httpd.service. This creates a symbolic link in /etc/systemd/system that points to the original unit file.

2. Start a service

Enabling a service does not start it immediately; to run it now, execute sudo systemctl start httpd.service. If the start fails, check the service status with sudo systemctl status httpd.service. The status output includes fields such as Loaded (location and enablement), Active (running state), Main PID, Status (application‑provided state), and CGroup (process tree).

3. Stop a service

To stop a running service, use sudo systemctl stop httpd.service. If the service does not respond, you can force termination with sudo systemctl kill httpd.service. Restarting is done via sudo systemctl restart httpd.service.

4. Read a unit file

Unit files reside in /usr/lib/systemd/system or /etc/systemd/system. Use systemctl cat sshd.service to view the file. A typical unit file is divided into sections: [Unit], [Service], and [Install], each containing key‑value pairs.

5. [Unit] section – ordering and dependencies

The Description and Documentation fields provide a brief description and docs. After and Before define start order, while Wants and Requires express weak and strong dependencies respectively. Weak dependencies allow the dependent service to start even if the required service fails, whereas strong dependencies cause the dependent service to stop if the required one fails.

6. [Service] section – start behavior

6.1 Exec commands : ExecStart defines the command to launch the service; optional EnvironmentFile can supply key=value pairs. Additional commands include ExecReload, ExecStop, ExecStartPre, ExecStartPost, ExecStopPost. A leading hyphen (e.g., EnvironmentFile=-/etc/sysconfig/sshd) suppresses errors if the file is missing.

6.2 Startup type : The Type field can be simple (default), forking, oneshot, dbus, notify, or idle, each affecting how systemd treats the process lifecycle.

6.3 Restart behavior : KillMode controls which processes are terminated (options: control-group, process, mixed, none). Restart defines when to restart (e.g., on-failure, always, etc.), and RestartSec sets the delay before a restart.

7. [Install] section – enabling the unit

The WantedBy (or RequiredBy) field specifies the target that pulls the unit in. Enabling a service creates a symlink under /etc/systemd/system/<target>.wants/, linking the unit to the chosen target (commonly multi-user.target).

8. Targets

Targets group units. The default boot target is obtained with systemctl get-default (usually multi-user.target). Targets have no Exec commands; they only declare ordering ( After), requirements ( Requires), conflicts, and whether they can be isolated ( AllowIsolate=yes). Use systemctl list-dependencies to view a target’s units and systemctl isolate to switch to a different target.

9. Reloading after changes

After editing unit files, reload the daemon with sudo systemctl daemon-reload and restart the affected service, e.g., sudo systemctl restart foobar.