Managing systemd services (2024)

Publication Date: 08Aug2024

WHAT?

systemd is used to manage system settings and services. systemd organizes tasks into components called units and groups of units into targets.

WHY?

Learn how to manage your system services by using systemd to start,stop, enable and disable a service, send termination signals, use the systemctl command and manage systemd targets. The article also includes troubleshooting and best practices.

EFFORT

20 minutes of reading time.

REQUIREMENTS

Basic understanding of Linux commands
Basic understanding of Linux processes, daemons and control groups

Revision History: Managing systemd Services

1 What is `systemd`? #

systemd is a system and service manager for Linux operating systems. It is the default initialization system for major Linux distributions. systemd is not directly initiated by the user, but installed through the /sbin/init and started during the early boot. systemd acts as the init system that brings up and maintains user space services when run as the first process on boot (PID 1). PID 1 is known as init and is the first Linux user-mode process created. It runs until the system shutdown.

systemd owns PID 1, and is started directly by the kernel. All other processes are started directly by systemd or one of its child processes. systemd mounts the host's file system and manages temporary files. It is backward compatible with the SysV init scripts. SysV is an initialization system that predates systemd.

In systemd, a unit is a resource that the system knows how to operate on and manage. This is the primary object that the systemd tools use. These resources are defined with configuration files called unit files.

systemctl is the central management tool for controlling the init system. It is used to examine and control the state of the systemd system and service manager.

Targets in systemd are groups of related units that act as synchronization points during a system boot. Target unit files have a .target file extension. Target units group together various systemd units through a chain of dependencies.

For troubleshooting, you can use journalctl, which is used to query and display log messages from the systemd journal.

For more information on systemd, you can refer to https://systemd.io and man 1 systemd.

2 Starting and stopping `systemd` services #

You can start and stop systemd services by using units which are resources. The target units are service units, which have a .service suffix. For service management tasks, you do not have to add the .service because systemd knows that you want to execute a service.

To start and stop systemd services, use the following commands:

To start a systemd service:
```
> sudo systemctl start SERVICE
```
To check the status of a systemd service:
```
> sudo systemctl status SERVICE
```
To stop a currently running systemd service:
```
> sudo systemctl stop SERVICE
```

You can also use the following systemctl commands:

Stops the service and then starts it. If the service is not running, it is started.
```
> sudo systemctl restart SERVICE
```
Reloads the systemd service's configuration file without interrupting operations.
```
>  systemctl reload SERVICE
```
Reloads the service if it supports reloading, otherwise it restarts the service. If the service is not running, it is started.
```
> sudo systemctl reload-or-restart SERVICE
```

3 Enabling and disabling `systemd` services #

You can enable and disable systemd services by using units which are resources.

4 Sending termination signals to `systemd` services #

Each service in systemd is categorized into a control group which makes it easy to identify all parent and child processes of a service. This enables you to send a signal to each of these processes. You can send a signal to a systemd service or to the individual processes that belong to a service.

Warning: Do not cancel the D-Bus service in a running system

The D-Bus service is the message bus for communication between systemd clients and the systemd manager that is running as PID 1. D-Bus is a stand-alone daemon and an integral part of the init infrastructure.

Terminating or restarting the D-Bus service is neither recommended nor supported. This breaks the systemd client and server communication and makes most of the systemd functions unusable.

The systemd-cgls command displays all processes that belong to a systemd service. It recursively shows the contents of the specific Linux control group hierarchy in a tree.

Lists all control groups and their processes:
```
> systemd-cgls
```
Displays all processes that belong to a particular service, for example: the libvirtd.service service:
```
> systemd-cgls -u libvirtd.service
```
Sends a termination signal to the service:
```
> sudo systemctl kill SERVICE
```
By default, the kill command sends the signal to all processes of the specified control group. You can restrict it to the control or the main process. The latter is, for example, useful to force a service to reload its configuration by sending SIGHUP:
```
> sudo systemctl kill -s SIGHUP --kill-who=main SERVICE
```
Sends a different signal to a service with the -s option, such as SIGKILL (9):
```
> sudo systemctl kill -s 9 SERVICE
```

5 Managing `systemd` targets with the `systemctl` command #

systemd targets are special unit files that describe a system or synchronization state. They are used to group units together in order or bring the system to a certain state, just like other init systems use runlevels.You can use the systemctl command to manage systemd targets.

Getting and setting the default target

systemd has a default target that is used when booting the system. To find the default target for your system:

> sudo systemctl get-default multi-user.target

You can also change the default target:

> sudo systemctl set-default graphical.target

Listing the available targets

To get the available targets on your system:

> sudo systemctl list-unit-files --type=target STATE VENDOR PRESET basic.target static - blockdev@.target static - bluetooth.target static - boot-complete.target static - cryptsetup-pre.target static - cryptsetup.target static - ctrl-alt-del.target alias - cvs.target static - default.target alias - emergency.target static - exit.target disabled disabled final.target static - first-boot-complete.target static - getty-pre.target static - getty.target static - .....

6 Troubleshooting `systemd` management #

If you are experiencing problems with systemd, try the following troubleshooting tips to identify and resolve issues:

Check the runtime status of a service

To find out the current runtime status of a service:

> sudo systemctl status SERVICE

Display all processes that belong to a particular service, for example: the libvirtd.service service:

> systemd-cgls -u libvirtd.service

Check the logs for your service with the journalctl -u SERVICE command

If you experience any issue with a systemd service, check the service's log. For example:

> sudo journalctl -u my-custom-service.service

The command displays logs for the specified service, including any error messages, warnings or other relevant information. You can use these logs to identify and fix issues with the service.

7 Best practices for `systemd` management #

To manage systemd services, try certain best practices that are equipped to handle different situations:

Check the runtime status of a service

To find out the current runtime status of a service:

> sudo systemctl status SERVICE

Use absolute path in your systemd unit file

Use an absolute path for executable files and required files, such as configuration files or scripts in your systemd unit file. systemd does not rely on the user's environment variables like $PATH to locate files. Use the ExecSearchPath directive in the [SERVICE] section.

[Service] ExecSearchPath=ABSOLUTE_PATH

Use the ExecReload directive

Use the ExecReload directive in the [SERVICE] section when you want to define a specific command that should be executed when you reload a service with the systemctl reload command. This is useful for services that can dynamically reload their configuration without a restart.

[Service]ExecStart=PATH_TO_EXECUTABLEExecReload=PATH_TO_RELOAD_SCRIPT

Use the RestartSec directive

Use the RestartSec directive in the [SERVICE] section when you want to define a delay (in seconds) before the service is restarted after a failure. This is useful for services that require a specified time to release resources or prevent rapid restart loops that can cause high system load.

[Service]ExecStart=PATH_TO_EXECUTABLERestart=on-failureRestartSec=5

Disable emergency mode on a remote machine

You can disable emergency mode on a remote machine, for example, a virtual machine hosted on Google Cloud. If this mode is enabled, the machine is blocked from connecting to the network. For example:

> sudo systemctl mask emergency.service

> sudo systemctl mask emergency.target

8 `systemctl` commands overview #

The systemctl command is used to examine and control the state of systemd and service manager.

You can use the following common systemctl commands and refer to the man systemctl page.

8.1 Viewing `systemd` information #

To view information about systemd components, you can use the following commands:

systemctl list-units: Lists the systemd units. You can use the optional arguments: --state=running to show the active units and --type=service to show the exited and active units.
systemctl list-unit-files: Lists the systemd units and the status, such as static, generated, disabled, alias, masked, and enabled.
systemctl list-dependencies: Lists the dependency tree.
systemctl list-dependencies UNIT_FILE: Lists the dependencies of a unit file.

8.2 Managing `systemd` services #

The systemctl command enables you to perform the following tasks with services.

systemctl status SERVICE

Checks the status of the specific service.

systemctl show SERVICE

Displays the service information.

systemctl start SERVICE

Instead of manually starting the service, use the start command. When a change is made to the configuration file, the related service must be started again.

systemctl stop SERVICE

Stops a specific running service.

systemctl restart SERVICE

Instead of manually restarting the service, use the restart command. When a change is made to the configuration file, the related service must be restarted again.

systemctl enable SERVICE

Enables the service on boot.

systemctl disable SERVICE

Disables the service on boot.

systemctl reload-or-restart SERVICE

Reload the service if it supports reloading, otherwise it restarts the service. If the service is not running, it is restarted.

systemctl mask SERVICE

When a service is masked, this means the unit file is symlinked to /dev/null. A symlink for a masked service is created from /etc/systemd/system to point to /dev/null. This makes it impossible to load the service even if another enabled service requires it. It must be stopped manually, or it continues to run in the background. You can use --runtime option to only mask temporarily until the next reboot of the system.

Created symlink /etc/systemd/system/FOSSLinux.service → /dev/null.

systemctl unmask SERVICE

Unmasks the service. It is effective when the system is started or restarted manually.

8.3 Managing system states #

The systemctl command enables you to perform power management processes on your system, like restarting, shutting down and so on, as described below.

systemctl reboot: Reboots the system reboot.target.
systemctl poweroff: Powers off the system poweroff.target.
systemctl emergency: Goes into the emergency mode emergency.target.
systemctl default: Goes back to default target multi-user.target.

9 More information #

Man 1 systemd
Man 1 journalctl
Man 1 systemd-cgls
Man 1 systemctl
Man 5 systemd.service
Man 5 systemd.kill
Man 7 systemd.boot
Introduction to systemd basics
System and Service Manager

10 Legal Notice #

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or (at your option) version 1.3; with the Invariant Section being this copyright notice and license. A copy of the license version 1.2 is included in the section entitled “GNU Free Documentation License”.

For SUSE trademarks, see https://www.suse.com/company/legal/. All other third-party trademarks are the property of their respective owners. Trademark symbols (®, ™ etc.) denote trademarks of SUSE and its affiliates. Asterisks (*) denote third-party trademarks.

All information found in this book has been compiled with utmost attention to detail. However, this does not guarantee complete accuracy. Neither SUSE LLC, its affiliates, the authors, nor the translators shall be held liable for possible errors or the consequences thereof.

Managing systemd services (2024)

1 What is systemd? #

2 Starting and stopping systemd services #

3 Enabling and disabling systemd services #

4 Sending termination signals to systemd services #

5 Managing systemd targets with the systemctl command #

6 Troubleshooting systemd management #

7 Best practices for systemd management #

8 systemctl commands overview #

8.1 Viewing systemd information #

8.2 Managing systemd services #