Understanding and administering systemd
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.
Requisitos previos
You are logged in as a user with administrator-level permissions.
Procedimiento
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.
Requisitos previos
-
You are logged in as a user with administrator-level permissions.
-
You have a configured
httpd
server running through systemd.
Procedimiento
-
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.
Requisitos previos
-
You are logged in as a user with administrator-level permissions.
Procedimiento
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:
-
The
[Unit]
section provides basic information about the service. Thefoo
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 thefoo
services specifiesnetwork.target
as anAfter=
condition.The resulting
[Unit]
section looks like this:[Unit] Description=My custom service After=network.target
-
The
[Service]
section provides instructions on how to control the service. Thefoo
service uses the following parameters:Type
-
Defines the type of systemd service. In this example, the
foo
service is asimple
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
-
The
[Install]
section provides instructions on how systemd installs the service. Thefoo
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
usesmulti-user.target
, which startsfoo.service
when systemd loadsmulti-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.
Requisitos previos
-
You are logged in as a user with administrator-level permissions.
-
You have a custom SysVinit script to convert to a systemd configuration.
Procedimiento
-
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 themulti-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 theExecStart
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 thereload
subcommand. Set theExecReload
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 theshutdown
subcommand. Set theExecStop
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
andWants
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 tosimple
, except the process must exit before systemd starts follow-up services. -
dbus
- Similar tosimple
, except the daemon acquires a name of the D-Bus bus. -
notify
- Similar tosimple
, except the daemon sends a notification message usingsd_notify
or an equivalent call after starting up. -
idle
- Similar tosimple
, 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 andPIDFile
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
andTimeoutStopSec
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.
-
Mapeo de niveles de ejecución a objetivos
Los objetivos Systemd sirven a un propósito similar que los niveles de ejecución SysVinit pero actúan un poco diferente. Cada objetivo tiene un nombre en lugar de un número y cada uno sirve a un propósito específico. Systemd implementa algunos objetivos heredando todos los servicios de otro objetivo y añadiéndole servicios adicionales. Algunos objetivos systemd imitan los niveles de ejecución comunes de sysvinit, lo que significa que puede cambiar de objetivo con el familiar comando telinit RUNLEVEL
. Los niveles de ejecución que asignaron un propósito específico en las instalaciones básicas de Fedora (0, 1, 3, 5 y 6) tienen un mapeo 1:1 con un objetivo específico systemd.
Sin embargo, este no es el caso de los niveles de ejecución definidos por el usuario 2 y 4. Para hacer uso de estos niveles de ejecución, cree un nuevo objetivo systemd con un nombre como /etc/systemd/system/$YOURTARGET`que toma uno de los niveles de ejecución existentes como base haga un directorio `/etc/systemd/system/$YOURTARGET.wants
y después un enlace simbólico a los servicios adicionales para habilitar este directorio.
Los siguiente es un mapeo de los niveles de ejecución SysVinit a los objetivos systemd.
Nivel de ejecución Sysvinit | Objetivo systemd | Notas |
---|---|---|
0 |
runlevel0.target, poweroff.target |
Para el sistema. |
1, s, single |
runlevel1.target, rescue.target |
Modo usuarios único. |
2, 4 |
runlevel2.target, runlevel4.target, multi-user.target |
Definido por el usuario/Niveles de ejecución específicos del sitio. Predeterminado, idéntico a 3. |
3 |
runlevel3.target, multi-user.target |
Multi-user, non-graphical. Los usuarios pueden acceder normalmente por medio de múltiples consolas o por red. |
5 |
runlevel5.target, graphical.target |
Multiusuario, gráfico. Normalmente tiene todos los servicios del nivel de ejecución 3 más acceso gráfico. |
6 |
runlevel6.target, reboot.target |
Reinicio |
emergency |
emergency.target |
Shell de emergencia |
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 |
---|---|---|
|
|
Used to start a service (not reboot persistent) |
|
|
Used to stop a service (not reboot persistent) |
|
|
Used to stop and then start a service |
|
|
When supported, reloads the config file without interrupting pending operations. |
|
|
Restarts if the service is already running. |
|
|
Tells whether a service is currently running. |
|
|
Used to list the services that can be started or stopped |
|
|
Turn the service on, for start at next boot, or other trigger. |
|
|
Turn the service off for the next reboot, or any other trigger. |
|
|
Used to check whether a service is configured to start or not in the current environment. |
|
|
Print a table of services that lists which runlevels each is configured on or off |
|
|
Print a table of services that will be started when booting into graphical mode |
|
|
Used to list what levels this service is configured on or off |
|
|
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 .
|
Recursos Adicionales
-
Lennart Poettering’s blog with lots of information about systemd. Lennart is the primary systemd developer
Want to help? Learn how to contribute to Fedora Docs ›