Understanding and administering systemd

Christopher Engelhard, Kamil Páral, Caleb McKee Версія unknown Last review: 2020-08-05

Learn the basic principles of the systemd init system: how to configure it and use it to administer the system.

Understanding systemd

Systemd is a system and service manager for Linux, compatible with SysV and LSB init scripts. Systemd provides:

Aggressive parallelization capabilities
Uses socket and D-Bus activation for starting services
Offers on-demand starting of daemons, keeps track of processes using Linux cgroups
Supports snapshotting and restoring of the system state
Maintains mount and automount points
Implements an elaborate transactional dependency-based service control logic.

The systemctl command is the primary tool to manage systemd. It combines the functionality of SysVinit’s service and chkconfig commands into a single tool you can use to enable and disable services permanently or only for the current session.

Systemd manages so-called units, which are representations of system resources and services. This following list shows the unit types that systemd can manage:

service: A service on the system, including instructions for starting, restarting, and stopping the service.
socket: A network socket associated with a service.
device: A device specifically managed with systemd.
mount: A mountpoint managed with systemd.
automount: A mountpoint automatically mounted on boot.
swap: Swap space on the system.
target: A synchronization point for other units. Usually used to start enabled services on boot.
path: A path for path-based activation. For example, you can start services based on the state of a certain path, such as whether it exists or not.
timer: A timer to schedule activation of another unit.
snapshot: A snapshot of the current systemd state. Usually used to rollback after making temporary changes to systemd.
slice: Restriction of resources through Linux Control Group nodes (cgroups).
scope: Information from systemd bus interfaces. Usually used to manage external system processes.

Starting, stopping, and querying systemd services

You can perform various management tasks to control systemd services using the systemctl command. The following is a set of example commands to demonstrate how to use systemctl to manage systemd services.

Prerequisites

You are logged in as a user with administrator-level permissions.

Procedure

The following commands control the foo service:

Activate a service immediately:
```
# systemctl start foo
```
Deactivate a service immediately:
```
# systemctl stop foo
```
Restart a service:
```
# systemctl restart foo
```
Show the status of a service including, whether it is running or not:
```
# systemctl status foo
```
Enable a service to be started on boot:
```
# systemctl enable foo
```
Disable a service to not start during boot:
```
# systemctl disable foo
```
Prevent a service from starting dynamically or even manually unless unmasked:
```
# systemctl mask foo
```
Check if a service is enabled or not:
```
# systemctl is-enabled foo
```

Related Information

Run man systemctl for more details.

Modifying existing systemd services

This example shows how to modify an existing service. Service modification are stored within /etc/systemd/system, in a single file or in a subdirectory named after the service. For example, this procedure modifies the httpd service.

Prerequisites

You are logged in as a user with administrator-level permissions.
You have a configured httpd server running through systemd.

Procedure

Systemd services can be modified using the systemctl edit command.
```
# systemctl edit httpd.service
```
This creates an override file /etc/systemd/system/httpd.service.d/override.conf and opens it in your text editor. Anything you put into this file will be added to the existing service file.
Add your custom configuration. For example:
```
[Service]
Restart=always
RestartSec=30
```
To replace an option that can be set multiple times, it must cleared first, otherwise the override file will add the option a second time.
```
[Service]
ExecStart=
ExecStart=<new command>
```
Save the file. Systemd automatically loads the new service configuration.
Restart the httpd service:
```
# systemctl restart httpd
```

To completely replace (instead of just add to/modify) an existing service file, use systemctl edit --full, e.g. systemctl edit --full httpd.service. This will create /etc/systemctl/system/httpd.service, which will be used instead of the existing service file.

Related Information

See Common service parameters for more information about the parameters used in this procedure.

Creating new systemd services

This example shows how to create a unit file for a custom service. Custom unit files are located in /etc/systemd/system/ and have a .service extension. For example, a custom foo service uses /etc/systemd/system/foo.service unit file.

Prerequisites

You are logged in as a user with administrator-level permissions.

Procedure

This procedure creates a basic configuration file to control the foo service.

Create and edit the new configuration file:
```
# nano /etc/systemd/system/foo.service
```
The next few steps describe each section its parameters to add to the file:
1. The [Unit] section provides basic information about the service. The foo service uses the following parameters:
  Description
  
  A string describing the unit. Systemd displays this description next to the unit name in the user interface.
  
  After
  
  Defines a relationship with a second unit. If you activate the unit, systemd activates it only after the second one. For example, the foo service might require network connectivity, which means the foo services specifies network.target as an After= condition.
  
  The resulting [Unit] section looks like this:
  
  [Unit] Description=My custom service After=network.target
2. The [Service] section provides instructions on how to control the service. The foo service uses the following parameters:
  Type
  
  Defines the type of systemd service. In this example, the foo service is a simple service, which starts the service without any special consideration.
  
  ExecStart
  
  The command to run to start the service. This includes the full path to the command and arguments to modify the service.
  
  The resulting [Service] section looks like this:
  
  [Service] Type=simple ExecStart=/usr/bin/sleep infinity
3. The [Install] section provides instructions on how systemd installs the service. The foo service uses the following parameters:
  
  WantedBy
  
  Defines which service triggers the custom service if enabled with systemctl enable. This is mostly used for starting the custom service on boot. In this example, foo.service uses multi-user.target, which starts foo.service when systemd loads multi-user.target on boot.

The full foo.service file contains the following contents:

[Unit]
Description=My custom service
After=network.target

[Service]
Type=simple
ExecStart=/usr/bin/sleep infinity

[Install]
WantedBy=multi-user.target

Save the file.

To make systemd aware of the new service, reload its service files
```
# systemctl daemon-reload
```
Start the custom foo service:
```
# systemctl start foo
```

Check the status of the service to ensure the service is running:

$ systemctl status foo
● foo.service - My custom service
   Loaded: loaded (/etc/systemd/system/foo.service; static; vendor preset: disabled)
   Active: active (running) since Thu 2017-12-14 14:09:12 AEST; 6s ago
 Main PID: 31837 (sleep)
    Tasks: 1 (limit: 4915)
   CGroup: /system.slice/foo.service
           └─31837 /usr/bin/sleep infinity

Dec 14 14:09:12 dansmachine systemd[1]: Started My custom service.

Related Information

See Common service parameters for more information about the parameters used in this procedure.

Converting SysVinit services to systemd

Older versions of Fedora use SysVinit scripts to manage services. This section provides some guidelines on how to convert a SysVinit script to a systemd equivalent.

Prerequisites

You are logged in as a user with administrator-level permissions.
You have a custom SysVinit script to convert to a systemd configuration.

Procedure

Identify the runlevels in your SysVinit script. This is usually defined with chkconfig directive in the commented section at the beginning of the script. For example, the following indicates the service is using runlevels 3, 4, and 5:
```
# chkconfig: 235 20 80
```
systemd uses targets instead of runlevels. Use the table in [converting-sysvinit-services] to map the runlevels to targets. In this example, runlevels 2, 3, and 5 are all multi-user runlevels, so the systemd service can use the following:
```
[Install]
WantedBy=multi-user.target
```
If you enable the custom systemd service to start at boot (systemctl enable foo.service), systemd loads the service when loading the multi-user.target at boot time.
Identify the dependent services and targets. For example, if the custom service requires network connectivity, specify the network.target as a dependency:
```
[Unit]
Description=My custom service
After=network.target
```
Identify the command used to start the service in the SysVinit script and convert this to the systemd equivalent. For example, the script might contain a start function in the following format:
```
start() {
  echo "Starting My Custom Service..."
  /usr/bin/myservice -D
}
```
In this example, the /usr/bin/myservice command is the custom service command set to daemonize with the -D option. Set the ExecStart parameter to use this command:
```
[Service]
ExecStart=/usr/bin/myservice -D
```
Check the SysVinit script to see if the service uses a special command to restart the service. For example, the script might contain a reboot function that reloads the service:
```
reboot() {
  echo "Reloading My Custom Service..."
  /usr/bin/myservice reload
}
```
In this example, the /usr/bin/myservice command is the custom service command and reloads the service using the reload subcommand. Set the ExecReload parameter to use this command:
```
[Service]
ExecReload=/usr/bin/myservice reload
```
Alternatively, you can omit ExecReload and use the default behavior, which kills the service and starts it again.
Check the SysVinit script to see if the service uses a special command to stop the service. For example, the script might contain a stop function that reloads the service:
```
reboot() {
  echo "Stopping My Custom Service..."
  /usr/bin/myservice shutdown
}
```
In this example, the /usr/bin/myservice command is the custom service command and stop the service gracefully using the shutdown subcommand. Set the ExecStop parameter to use this command:
```
[Service]
ExecStop=/usr/bin/myservice shutdown
```
Alternatively, you can omit ExecStop and use the default behavior, which kills the service.
Review the SysVinit script and identify any additional parameters or functions. Use systemd parameters to replicate any identified SysVinit functions that might be relevant to your service.

Related Information

See Common service parameters for more information about the parameters used in this procedure.

Common service parameters

Unit Parameters

This section contains parameters you can use in the [Unit] section of a service. These parameters are common to other systemd units.

This list is a summarized version. For a full list of these parameters and their descriptions, run man systemd.unit.

Description: A free-form string describing the service.
Documentation: A space-separated list of URIs referencing documentation for this service or its configuration. Accepted are only URIs of the following types: http://, https://, file:, info:, man:.
Requires: Configures requirement dependencies on other services. If this service gets activated, the units listed here are activated too. If one of the dependent services fails to activate, systemd does not start this service. This option may be specified more than once or you can specify multiple space-separated units.
Wants: Similar to Requires, except failed units do not have any effect on the service.
BindsTo: Similar to Requires, except stopping the dependent units also stops the service.
PartOf: Similar to Requires, except the stopping and restarting dependent units also stop and restart the service.
Conflicts: A space-separated list of unit names that, if running, cause the service not to run.
Before, After: A space-separated list of unit names that configures the ordering of dependencies between services.
OnFailure: A space-separated list of unit names that are activated when this service enters a failed state.

Install Parameters

This section contains parameters you can use in the [Install] section of a service. These parameters are common to other systemd units.

This list is a summarized version. For a full list of these parameters and their descriptions, run man systemd.unit.

Alias: A space-separated list of additional names this service shall be installed under. The names listed here must have the same suffix (i.e. type) as the service filename.
RequiredBy, WantedBy: Defines the service as dependent of another service. This usually define the target to trigger an enabled service to run. These options are analogous to the Requires and Wants in the [Units] section.
Also: Additional units to install or uninstall when this service is installed or uninstalled.

Service Parameters

This section contains parameters you can use in the [Service] section of a service unit. These parameters are specific only to systemd service units.

This list is a summarized version. For a full list of these parameters and their descriptions, run man systemd.unit.

Type

Configures the process start-up type for this service service:

simple - The service starts as the main process. This is the default.
forking - The service calls forked processes and run as part of the main daemon.
oneshot - Similar to simple, except the process must exit before systemd starts follow-up services.
dbus - Similar to simple, except the daemon acquires a name of the D-Bus bus.
notify - Similar to simple, except the daemon sends a notification message using sd_notify or an equivalent call after starting up.
idle - Similar to simple, except the execution of the service is delayed until all active jobs are dispatched.

RemainAfterExit

A boolean value that specifies whether the service shall be considered active even if all its processes exited. Defaults to no.

GuessMainPID

A boolean value that specifies whether systemd should guess the main PID of a service if it cannot be determined reliably. This option is ignored unless Type=forking is set and PIDFile is not set. Defaults to yes.

PIDFile

An absolute filename pointing to the PID file of this daemon. Use of this option is recommended for services where Type=forking. Systemd reads the PID of the main process of the daemon after start-up of the service. Systemd does not write to the file configured here, although it removes the file after the service has shut down.

BusName

A D-Bus bus name to reach this service. This option is mandatory for services where Type=dbus.

ExecStart

The commands and arguments executed when the service starts.

ExecStartPre, ExecStartPost

Additional commands that are executed before or after the command in ExecStart.

ExecReload

The commands and arguments to execute when the service reloads.

ExecStop

The commands and arguments to execute when the service stops.

ExecStopPost

Additional commands to execute after the service stops.

RestartSec

The time in seconds to sleep before restarting a service.

TimeoutStartSec

The time in seconds to wait for the service to start.

TimeoutStopSec

The time in seconds to wait for the service to stop.

TimeoutSec

A shorthand for configuring both TimeoutStartSec and TimeoutStopSec simultaneously.

RuntimeMaxSec

A maximum time in seconds for the service to run. Pass infinity (the default) to configure no runtime limit.

Restart

Configures whether to restart the service when the service’s process exits, is killed, or reaches a timeout:

no - The service will not be restarted. This is the default.
on-success - Restart only when the service process exits cleanly (exit code 0).
on-failure - Restart only when the service process does not exit cleanly (node-zero exit code).
on-abnormal - Restart if the process terminates with a signal or when a timeout occurs.
on-abort - Restart if the process exits due to an uncaught signal not specified as a clean exit status.
always - Always restart.

Mapping runlevels to targets

Systemd targets serve a similar purpose to SysVinit runlevels but act a little differently. Each target has a name instead of a number and each serves a specific purpose. Systemd implements some targets by inheriting all of the services of another target and adding additional services to it. Some systemd targets mimic the common sysvinit runlevels, which means you can switch targets with the familiar telinit RUNLEVEL command. The runlevels assigned a specific purpose on vanilla Fedora installs (0, 1, 3, 5, and 6) have a 1:1 mapping with a specific systemd target.

However, this is not the case for user-defined runlevels 2 and 4. To make use of those runlevels, create a new named systemd target such as /etc/systemd/system/$YOURTARGET that takes one of the existing runlevels as a base, make a directory /etc/systemd/system/$YOURTARGET.wants, and then symlink the additional services to enable into that directory.

The following is a mapping of SysVinit runlevels to systemd targets.

Таблиця 1. Runlevel to target mapping
Sysvinit Runlevel	systemd Target	Notes
0	runlevel0.target, poweroff.target	Halt the system.
1, s, single	runlevel1.target, rescue.target	Single user mode.
2, 4	runlevel2.target, runlevel4.target, multi-user.target	User-defined/Site-specific runlevels. By default, identical to 3.
3	runlevel3.target, multi-user.target	Multi-user, non-graphical. Users can usually login via multiple consoles or via the network.
5	runlevel5.target, graphical.target	Multi-user, graphical. Usually has all the services of runlevel 3 plus a graphical login.
6	runlevel6.target, reboot.target	Reboot
emergency	emergency.target	Emergency shell

Mapping service commands

The following table demonstrates the systemd equivalent of SysVinit commands.

All recent versions of systemctl assume the .service suffix if left off the service name. For example, systemctl start frobozz.service is the same as systemctl start frobozz.

Sysvinit Command systemd Command Notes

Sysvinit Command	systemd Command	Notes
`service frobozz start`	`systemctl start frobozz`	Used to start a service (not reboot persistent)
`service frobozz stop`	`systemctl stop frobozz`	Used to stop a service (not reboot persistent)
`service frobozz restart`	`systemctl restart frobozz`	Used to stop and then start a service
`service frobozz reload`	`systemctl reload frobozz`	When supported, reloads the config file without interrupting pending operations.
`service frobozz condrestart`	`systemctl condrestart frobozz`	Restarts if the service is already running.
`service frobozz status`	`systemctl status frobozz`	Tells whether a service is currently running.
`ls /etc/rc.d/init.d/`	`systemctl` or `systemctl list-unit-files --type=service` or `ls /lib/systemd/system/.service /etc/systemd/system/.service`	Used to list the services that can be started or stopped Used to list all the services and other units
`chkconfig frobozz on`	`systemctl enable frobozz`	Turn the service on, for start at next boot, or other trigger.
`chkconfig frobozz off`	`systemctl disable frobozz`	Turn the service off for the next reboot, or any other trigger.
`chkconfig frobozz`	`systemctl is-enabled frobozz`	Used to check whether a service is configured to start or not in the current environment.
`chkconfig --list`	`systemctl list-unit-files --type=service` or `ls /etc/systemd/system/*.wants/`	Print a table of services that lists which runlevels each is configured on or off
`chkconfig --list \| grep 5:on`	`systemctl list-dependencies graphical.target`	Print a table of services that will be started when booting into graphical mode
`chkconfig frobozz --list`	`ls /etc/systemd/system/*.wants/frobozz.service`	Used to list what levels this service is configured on or off
`chkconfig frobozz --add`	`systemctl daemon-reload`	Used when you create a new service file or modify any configuration

service frobozz start

systemctl start frobozz

Used to start a service (not reboot persistent)

service frobozz stop

systemctl stop frobozz

Used to stop a service (not reboot persistent)

service frobozz restart

systemctl restart frobozz

Used to stop and then start a service

service frobozz reload

systemctl reload frobozz

When supported, reloads the config file without interrupting pending operations.

service frobozz condrestart

systemctl condrestart frobozz

Restarts if the service is already running.

service frobozz status

systemctl status frobozz

Tells whether a service is currently running.

ls /etc/rc.d/init.d/

systemctl or systemctl list-unit-files --type=service or
ls /lib/systemd/system/*.service /etc/systemd/system/*.service

Used to list the services that can be started or stopped
Used to list all the services and other units

chkconfig frobozz on

systemctl enable frobozz

Turn the service on, for start at next boot, or other trigger.

chkconfig frobozz off

systemctl disable frobozz

Turn the service off for the next reboot, or any other trigger.

chkconfig frobozz

systemctl is-enabled frobozz

Used to check whether a service is configured to start or not in the current environment.

chkconfig --list

systemctl list-unit-files --type=service or ls /etc/systemd/system/*.wants/

Print a table of services that lists which runlevels each is configured on or off

chkconfig --list | grep 5:on

systemctl list-dependencies graphical.target

Print a table of services that will be started when booting into graphical mode

chkconfig frobozz --list

ls /etc/systemd/system/*.wants/frobozz.service

Used to list what levels this service is configured on or off

chkconfig frobozz --add

systemctl daemon-reload

Used when you create a new service file or modify any configuration

All /sbin/service and /sbin/chkconfig commands listed in the table continue to work on systemd-based systems and are translated to native equivalents as necessary. The only exception is chkconfig --list.

Additional resources

Project homepage
Lennart Poettering’s blog with lots of information about systemd. Lennart is the primary systemd developer
freedesktop.org’s systemd FAQ
freedesktop.org’s systemd Tips & Tricks
Interview with the developer

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