systemctl

Understanding Systemd Services

Before diving into systemctl commands, it’s important to grasp the concept of systemd services. These are essentially processes or daemons that run in the background, providing essential system functionality. Examples include network management (networkd), logging (journalctl), and the display manager (gdm3). Each service is defined by a unit file, typically located in /etc/systemd/system/. These files specify how the service should be started, stopped, and managed.

Common systemctl Commands

systemctl offers a rich set of commands to interact with systemd services. Here are some of the most frequently used:

1. Starting and Stopping Services

To start a service, use the start command:

sudo systemctl start sshd.service

This command starts the SSH daemon. The .service suffix is essential. To stop a running service:

sudo systemctl stop sshd.service

2. Restarting and Reloading Services

Restarting a service terminates and restarts it:

sudo systemctl restart sshd.service

Reloading a service, on the other hand, reloads its configuration without restarting the process itself (useful for configuration changes):

sudo systemctl reload sshd.service

3. Checking Service Status

To check the status of a service, use the status command:

sudo systemctl status sshd.service

This provides detailed information about the service’s current state, including whether it’s running, active, failed, or inactive. The output usually includes logs and other relevant details.

4. Enabling and Disabling Services

Enabling a service ensures it starts automatically during system boot:

sudo systemctl enable sshd.service

Disabling a service prevents it from starting at boot:

sudo systemctl disable sshd.service

You can check the enabled status of a service with:

sudo systemctl is-enabled sshd.service

This will output enabled or disabled.

5. Listing Services

To list all active services:

systemctl list-units --type=service

You can filter this list. For example to list only running services:

systemctl list-units --type=service --state=running

To list all available service units:

systemctl list-unit-files --type=service

6. Masking a Service

Sometimes, you want to prevent a service from ever being started, even manually. This is done by masking:

sudo systemctl mask sshd.service

Unmasking is done similarly:

sudo systemctl unmask sshd.service

7. Working with Other Unit Types

While we focused on .service units, systemctl manages other unit types, including:

• .socket: Network sockets
• .target: Groups of units (e.g., graphical.target for the graphical desktop)
• .mount: Mount points
• .automount: Automounted file systems
• .swap: Swap devices
• .device: Hardware devices

The commands remain the same, simply replace .service with the appropriate suffix. For instance:

sudo systemctl start getty.target

8. Using --user for User Services

systemctl also manages user services. These are services that run under a specific user’s context. Use the --user option for these:

systemctl --user start my-user-service.service

Remember to create your user service files appropriately.

These examples provide a solid foundation for working with systemctl. Mastering these commands is essential for any Linux administrator. Remember to always use sudo when necessary to execute commands with root privileges. Further exploration of the man systemctl page is highly recommended for advanced usage.