Starting and Stopping Services
Services need to be stopped or started manually for a number of reasons: perhaps the service needs to be updated; the conﬁguration ﬁle may need to be changed; or a service may need to be uninstalled, or an administrator may manually start an infrequently used service.
To start a service, ﬁrst verify that it is not running with systemctl status. Then, use the systemctl start command as the root user (using sudo if necessary). The example below shows how to start the sshd.service service:
[root@host ~]# systemctl start sshd.service
The systemd service looks for .service ﬁles for service management in commands in the absence of the service type with the service name. Thus the above command can be executed as:
[root@host ~]# systemctl start sshd
To stop a currently running service, use the stop argument with the systemctl command. The example below shows how to stop the sshd.service service:
[root@host ~]# systemctl stop sshd.service
Restarting and Reloading Services
During a restart of a running service, the service is stopped and then started. On the restart of service, the process ID changes and a new process ID gets associated during the startup. To restart a running service, use the restart argument with the systemctl command. The example below shows how to restart the sshd.service service:
[root@host ~]# systemctl restart sshd.service
Some services have the ability to reload their conﬁguration ﬁles without requiring a restart. This process is called a service reload. Reloading a service does not change the process ID associated with various service processes. To reload a running service, use the reload argument with the systemctl command. The example below shows how to reload the sshd.service service after conﬁguration changes:
[root@host ~]# systemctl reload sshd.service
In case you are not sure whether the service has the functionality to reload the conﬁguration ﬁle changes, use the reload-or-restart argument with the systemctl command. The command reloads the conﬁguration changes if the reloading functionality is available. Otherwise the command restarts the service to implements the new conﬁguration changes:
[root@host ~]# systemctl reload-or-restart sshd.service
Listing Unit Dependencies
Some services require that other services be running ﬁrst, creating dependencies on the other services. Other services are not started at boot time but rather only on demand. In both cases, systemd and systemctl start services as needed whether to resolve the dependency or to start an infrequently used service. For example, if the CUPS print service is not running and a ﬁle is placed into the print spool directory, then the system will start CUPS-related daemons or commands to satisfy the print request.
[root@host ~]# systemctl stop cups.service Warning: Stopping cups, but it can still be activated by: cups.path cups.socket
To completely stop printing services on a system, stop all three units. Disabling the service disables the dependencies. The ‘systemctl list-dependencies UNIT‘ command displays a hierarchy mapping of dependencies to start the service unit. To list reverse dependencies (units that depend on the speciﬁed unit), use the –reverse option with the command.
[root@host ~]# systemctl list-dependencies sshd.service sshd.service ● ├─system.slice ● ├─sshd-keygen.target ● │ ├─email@example.com ● │ ├─firstname.lastname@example.org ● │ └─email@example.com ● └─sysinit.target ...output omitted...
Masking and Unmasking Services
At times, a system may have different services installed that are conﬂicting with each other. For example, there are multiple methods to manage mail servers (postfix and sendmail, for example). Masking a service prevents an administrator from accidentally starting a service that conﬂicts with others. Masking creates a link in the conﬁguration directories to the /dev/null ﬁle which prevents the service from starting.
[root@host ~]# systemctl mask sendmail.service Created symlink /etc/systemd/system/sendmail.service → /dev/null.
[root@host ~]# systemctl list-unit-files --type=service UNIT FILE STATE sendmail.service masked ...output omitted...
Attempting to start a masked service unit fails with the following output:
[root@host ~]# systemctl start sendmail.service Failed to start sendmail.service: Unit sendmail.service is masked
Use the systemctl unmask command to unmask the service unit.
[root@host ~]# systemctl unmask sendmail Removed /etc/systemd/system/sendmail.service.
Enabling Services to Start or Stop at Boot
Starting a service on a running system does not guarantee that the service automatically starts when the system reboots. Similarly, stopping a service on a running system does not keep it from starting again when the system reboots. Creating links in the systemd conﬁguration directories enables the service to start at boot. The systemctl commands create and remove these links.
To start a service at boot, use the systemctl enable command.
[root@root ~]# systemctl enable sshd.service Created symlink /etc/systemd/system/multi-user.target.wants/sshd.service → /usr/ lib/systemd/system/sshd.service.
The above command creates a symbolic link from the service unit ﬁle, usually in the /usr/lib/systemd/system directory, to the location on disk where systemd looks for ﬁles, which is in the /etc/systemd/system/TARGETNAME.target.wants directory. Enabling a service does not start the service in the current session. To start the service and enable it to start automatically during boot, execute both the systemctl start and systemctl enable commands.
To disable the service from starting automatically, use the following command, which removes the symbolic link created while enabling a service. Note that disabling a service does not stop the service.
[root@host ~]# systemctl disable sshd.service Removed /etc/systemd/system/multi-user.target.wants/sshd.service.
To verify whether the service is enabled or disable, use the systemctl is-enabled command.
Summary of systemctl Commands
Services can be started and stopped on a running system and enabled or disabled for an automatic start at boot time.
Useful Service Management Commands:
|View detailed information about a unit state.||systemctl status UNIT|
|Stop a service on a running system.||systemctl stop UNIT|
|Start a service on a running system.||systemctl start UNIT|
|Restart a service on a running system.||systemctl restart UNIT|
|Reload the configuration file of a running service.||systemctl reload UNIT|
|Completely disable a service from being started, both manually and at boot.||systemctl mask UNIT|
|Make a masked service available.||systemctl unmask UNIT|
|Configure a service to start at boot time.||systemctl enable UNIT|
|Disable a service from starting at boot time.||systemctl disable UNIT|
|List units required and wanted by the specified unit.||systemctl list-dependencies UNIT|
How to Control(start/stop/mask/unmask) Services Using Systemd