Linux distributions are moving away from their old System V-style startup processes to a newer mechanism, the systemd daemon, and its associated systemctl command-line tool. Services managed by systemd/systemctl require, at a minimum, a configuration file that defines startup and shutdown processes, a type definition that controls how those processes will be handled by the OS, and whatever executables are needed to start or stop the service processes.
On most new distributions that use the system, we would manage processes using the systemctl command. The Linux developers have also left support for the service command; if we try to terminate a process using the service command, then we will see that it is actually going to redirect our request to the systemctl command.
Configuration Files
- /usr/lib/systemd/system/: It contains system default unit files.
- /etc/systemd/system: It contains system-specific parameters to modify the default behavior of systemd.
- /run/systemd/system/: It contains the runtime configuration of unit files.
systemctl Command Examples
1. Viewing states of service with systemctl:
# systemctl status sshd.service
| Status | Description |
|---|---|
| Loaded | Unit configuration file is processed successfully |
| Active (running) | Running with one or more active processes |
| Active (exited) | Successfully completed a one-time configuration |
| Active (waiting) | Running and waiting for an event to take place |
| Inactive | Not running currently |
| Enabled | Will get started at boot time |
| Disabled | Will not get started at boot time |
| Static | Cannot be enabled directly, but may be started by another enabled unit automatically |
2. Displays the state of all the different units that are active and loaded on startup:
# systemctl
3. Displays the state of only service units that are active:
# systemctl --type=service
or
# systemctl list-units --type=service
4. Displays the state of all service units loaded, whether active or inactive:
# systemctl --type=service –all
or
# systemctl list-units --type=service --all
5. Displays all services that failed:
# systemctl --failed --type=service
6. Displays whether the particular service is currently active or not:
# systemctl is-active sshd
7. Displays whether the particular service in enabled to start at boot time or not:
# systemctl is-enabled sshd
8. Displays the enabled, disabled, or static settings of all units of the specified type:
# systemctl list-unit-files --type=service
9. Displays detailed status information about the specified service:
# systemctl status sshd.service -l