Managing systemd services and units using systemctl commands

The default system and service manager for most Linux distributions now is systemd. It is designed to be backward compatible with SysV init scripts and provides several features such as parallel startup of system services at boot time, on-demand activation of daemons, or dependency-based service control logic. In Red Hat Enterprise Linux 7, systemd replaces Upstart as the default init system.

• Service Units
• Starting and Stopping Services
• Restarting and Reloading services
• Enabling and Disabling Services
• Viewing service status
• Viewing System State
• List all installed services

Along with systemd, a new concept called systemd units was introduced. These units are represented by unit configuration files located in one of the directories listed under

And encapsulate information about system services, listening sockets, and other objects relevant to the init system.

The systemd runs as the first process after the kernel boot and is responsible for bringing the Linux host up. It is responsible for starting and managing the services, mounting filesystems, managing hardware, generating the login prompt, and much more. A key benefit over SysV is that systemd starts as many services as possible in parallel, thus speeding up the startup process, bringing up the login screen faster.

Service Units

The target units are service units for service management, which have unit files with a suffix of .service.

Managing systemd services

The command to manage systemd units is systemctl

Starting and Stopping Services

To start a systemd service, use the systemctl start command:

It is not mandatory to include .service suffix. For example, to start the apache server on Ubuntu:

To stop a running service:

So, to stop the apache server on Ubuntu:

Restarting and Reloading services

To restart a running service, use the restart command:

And where the only reloading configuration file is required, use reload

Enabling and Disabling Services

If you want a service to start automatically at system boot, use enable command:

To disable a service from starting at system boot, use disable

Disabling does not stop a running service but will only come into effect once the system is rebooted.

Viewing service status

To view information about a service, use the status command.

This displays the service status and the first few lines of the log file. So, while the service is running is, the output of

output

Let us now see the command to check if a service is active:

So, while the apache2 service is running, the output of systemctl is-active command is :

Output

To check if a service is enabled:

Viewing System State

All commands you have seen till now are used to manage a single service. When you want an overview of the system state, use the following set of commands:

To see all unit types

Output

To list all installed units, use list-unit-files.

Output

Note: The above output is only partial for demonstration purposes only. The actual list is long and may vary.

The output has only two columns Unit File and State. The state will usually be enabled, disabled, static, or masked.

Static: This means the unit cannot be enabled, performs a one-off action, or is a dependency of another unit and cannot be run by itself.

Masked: A unit listed as masked means it is entirely unstartable, as it is linked to /dev/null. This is called masking the unit. This prevents the service from being started, manually or automatically.

List all installed services.

The systemctl list-unit-files command with -t or –type service filter shows the state of installed services only.

Output

To see all active service units, use list-units with -t service filter.

Output

LOAD = Reflects whether the unit definition was loaded correctly.
ACTIVE = The high-level unit activation state, i.e., generalization of SUB.
SUB = The low-level unit activation state, values depend on unit type.

In the above article, we looked at systemd units in more detail and used the systemctl commands to explore and manage units. It also showed how to stop, start, enable and disable units. If you have found this article helpful, please show us some love by like the post.