Suppose you're making configuration changes to a Linux server. Perhaps you just fired up Vim and made edits to the
/etc/ssh/sshd_config file, and it's time to test your new settings. Now what?
Services such as SSH pull their settings from configuration files during the startup process. To let the service know about changes to the file, you need to restart the service so that it rereads the file. You can use the
systemctl command to manage services and control when they start.
Restart a service
After editing the
/etc/ssh/sshd_config file, use the
systemctl restart command to make the service pick up the new settings:
$ sudo systemctl restart sshd
You can verify the service is running by using the
$ sudo systemctl status sshd
Stop and start a service
Perhaps while troubleshooting you need to stop a service to determine whether it is the culprit or interfering with some other process. Use the
stop subcommand for this:
$ sudo systemctl stop sshd
Once you determine if this service is associated with the issue, you can restart it:
$ sudo systemctl start sshd
restart subcommand is useful for refreshing a service's configuration, the
start features give you more granular control.
Control whether the service starts with the system
One consideration with using
start is that the two commands apply only to the current runtime. The next time you boot the system, the service will either start or not start, depending on its default settings. You can use the
disable subcommands to manage those defaults.
disable the service, it doesn't start the next time the system boots. You might use this setting as part of your security hardening process or for troubleshooting:
$ sudo systemctl disable sshd
Reboot the system with
sudo systemctl reboot, and the service won't automatically start.
You may determine that you need the service to start automatically. In that case, use the
$ sudo systemctl enable sshd
enable subcommand doesn't start a service, it only marks it to start automatically at boot. To enable and start a service at the same time, use the
$ sudo systemctl enable --now sshd
[ Free download: Advanced Linux commands cheat sheet. ]
Mask a service
You can manually start a disabled service with the
systemctl start command after the system boots. To prevent this, use the
mask subcommand. Masking the service links its configuration to
/dev/null. A user or process will not be able to start this service at all (whereas with a disabled service, a user or process can still start it). Use the
unmask subcommand to reverse the setting:
$ sudo systemctl mask sshd
Display all subcommands
Bash's built-in tab-completion feature is one of my favorite tricks for
systemctl (and other commands). When working with commands that support subcommands, this feature saves you a lot of time. Simply type
systemctl and add a space, then tap the Tab key twice. Bash displays all available subcommands.
Do you think you're ready to use
systemctl to manage your services? Fire up a lab virtual machine and choose a service to work with. Don't do this on a production system! Make sure you can accomplish the following tasks:
- Check the status of your service. Is it started? Enabled?
- Stop the service and recheck its status.
- Disable the service, reboot the system, and confirm the service did not start.
- Enable the service, reboot the system, and confirm the service did start.
- Stop the service and use the
masksubcommand to prevent it from launching. Use the
systemctl startcommand to attempt to start it. Were you successful?
- Use Bash's tab-completion feature to display all available subcommands for
Many management tasks involve the
systemctl command, but the ones covered above represent the majority of them. Service management is critical, especially when editing configuration files and hardening a system. Plan to be confident, competent, and quick at using
systemctl and its common subcommands.