User Tools

Site Tools


systemd

Understanding and administering Systemd

Learn the basic principles of Systemd: how to configure it and use to administer the system. Understanding Systemd

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

  • Uses socket and D-Bus activation for starting services
  • Aggressive parallelization capabilities
  • 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 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

  Restrivtion 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 if it is running or not:
  # systemctl status foo
  Enable a service to be started on bootup:
  # systemctl enable foo
  Disable a service to not start during bootup:
  # systemctl disable foo
  Prevent a service from starting dynamically or even manually unless unmasked:
  # systemctl mask foo
  Check if a service is already enabled or not:
  # systemctl is-enabled foo
  
  Run man systemctl for more details.

Modifying existing Systemd services

This example shows how to modify an existing service. The files for service modification are stored in a directory within /etc/systemd/system. This directory is 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

  Create a directory for the service modification in the following format: [SERVICE NAME].service.d. For example, the directory for the httpd.service modification is httpd.service.d:
  # mkdir /etc/systemd/system/httpd.service.d/
  Create a configuration file within this directory:
  # vi /etc/systemd/system/httpd.service.d/custom.conf
  Add your custom configuration. For example:
  [Service]
  Restart=always
  RestartSec=30
  Save the file.
  Restart the httpd service:
  # systemctl restart httpd
  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:
  # vi /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. 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.
      Requires
          Defines unit to use as a dependency for the service. If you activate the unit, Systemd activates the units listed in Requires as well. For example, the foo service might require network connectivity, which means the foo services requires network.target as a dependency.
          The resulting [Unit] section looks like this:
          [Unit]
          Description=My custom service
          Requires=network.target
      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
      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
  Requires=network.target
  [Service]
  Type=simple
  ExecStart=/usr/bin/sleep infinity
  [Install]
  WantedBy=multi-user.target
  Save the file.
  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.
  See Common service parameters for more information about the parameters used in this procedure.

Converting SysVinit services

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
  Requires=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 exits 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 motification 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 executed 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 different. 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.

systemd.txt · Last modified: 2018/03/01 19:29 by avouchor

Logo by Jakub T. Jankiewicz; license CC-BY-SA

Powered by DokuWiki