Chapter 2. Managing system services with systemctl

Prerequisites

• You have administrative privileges.

Procedure

• List all currently loaded service units:

  $ systemctl list-units --type service

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  UNIT                     LOAD   ACTIVE SUB     DESCRIPTION
  abrt-ccpp.service        loaded active exited  Install ABRT coredump hook
  abrt-oops.service        loaded active running ABRT kernel log watcher
  abrtd.service            loaded active running ABRT Automated Bug Reporting Tool
  ...
  systemd-vconsole-setup.service loaded active exited  Setup Virtual Console
  tog-pegasus.service            loaded active running OpenPegasus CIM Server
  
  LOAD   = Reflects whether the unit definition was properly loaded.
  ACTIVE = The high-level unit activation state, or a generalization of SUB.
  SUB    = The low-level unit activation state, values depend on unit type.
  
  46 loaded units listed. Pass --all to see loaded but inactive units, too.
  To show all installed unit files use 'systemctl list-unit-files'

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  By default, the systemctl list-units command displays only active units. For each service unit file, the command provides an overview of the following parameters:

  UNIT

  The full name of the service unit

  LOAD

  The load state of the configuration file

  ACTIVE or SUB

  The current high-level and low-level unit file activation state

  DESCRIPTION

  A short description of the unit’s purpose and functionality

• List all loaded units regardless of their state:

  $ systemctl list-units --type service --all

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• List the status, for example enabled or disabled, of all available service units:

  $ systemctl list-unit-files --type service

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  UNIT FILE                               STATE
  abrt-ccpp.service                       enabled
  abrt-oops.service                       enabled
  abrtd.service                           enabled
  ...
  wpa_supplicant.service                  disabled
  ypbind.service                          disabled
  
  208 unit files listed.

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  For each service unit, this command displays:

  • UNIT FILE: The full name of the service unit
  • STATE: The information whether the service unit is enabled or disabled to start automatically during boot

• Display detailed information about a service unit that corresponds to a system service:

  $ systemctl status <name>.service

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  Replace <name> with the name of the service unit you want to inspect, for example, gdm.

  This command displays the following information:

  • The name of the selected service unit followed by a short description
  • The execution of the service unit, if the unit is executed by the root user
  • The most recent log entries
  • Detailed information about the service unit

• Verify that a particular service unit is running:

  $ systemctl is-active <name>.service

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• Decide whether a particular service unit is enabled to start during boot time:

  $ systemctl is-enabled <name>.service

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  Both systemctl is-active and systemctl is-enabled commands return an exit status of 0 if the specified service unit is running or enabled.

• Check which services systemd scheduled to start before the specified service unit:

  # systemctl list-dependencies --after <name>.service

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  For example, to view the list of services ordered to start before gdm, enter:

  # systemctl list-dependencies --after gdm.service

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  `gdm.service
  ├─dbus.socket
  ├─getty@tty1.service
  ├─livesys.service
  ├─plymouth-quit.service
  ├─system.slice
  ├─systemd-journald.socket
  ├─systemd-user-sessions.service
  └─basic.target
  ...

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• Check what services systemd orders to start after the specified service unit:

  # systemctl list-dependencies --before <name>.service

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  For example, to view the list of services systemd orders to start after gdm, enter:

  # systemctl list-dependencies --before gdm.service

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  gdm.service
  ├─dracut-shutdown.service
  ├─graphical.target
  │ ├─systemd-readahead-done.service
  │ ├─systemd-readahead-done.timer
  │ └─systemd-update-utmp-runlevel.service
  └─shutdown.target
    ├─systemd-reboot.service
    └─final.target
      └─systemd-reboot.service
  ...

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Prerequisites

• You have administrative privileges.

Procedure

• To start or stop a system service:

  # systemctl <action> <systemd_unit>

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  Replace <action> with the action you want to perform to enable or disable the service, for example, start or stop, and replace <systemd_unit> with the service unit name, for example, httpd.

  Note

  Starting a particular service might require starting one or more other services (positive dependency) or stopping one or more services (negative dependency). In systemd, positive and negative dependencies between services exist.

  When you try to start a new service, systemd resolves all dependencies automatically, without explicit notification to the user. This means that if you are already running a service, and you try to start another service with a negative dependency, the first service is automatically stopped.

  For example, if you are running the httpd service (Apache web server) and then try to start the NGINX service, systemd will automatically stop httpd first. This is because both services are web servers that cannot run simultaneously on the same port (usually port 80 or 443).

Prerequisites

• You have administrative privileges.

Procedure

• Restart a system service:

  # systemctl restart <name>.service

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  Replace <name> with the name of the service unit you want to restart (for example, httpd).

• Restart a service unit only if the corresponding service is already running:

  # systemctl try-restart <name>.service

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• Reload the configuration without interrupting service execution:

  # systemctl reload <name>.service

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  Certain services do not support reload operations. If you use systemctl reload-or-restart or systemctl reload-or-try-restart, systemd automatically restarts the service if a reload is not possible. Note that a restart of a service might interrupt connections to this service.

  For details, see systemctl(1) man page on your system.

Prerequisites

• You have administrative privileges.

Procedure

• Check if the unit is masked:

  # systemctl status <systemd_unit>

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• If the unit is masked, unmask it first:

  # systemctl unmask <systemd_unit>

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• Enable a service to start at boot time:

  # systemctl enable --now <systemd_unit>

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  Replace <systemd_unit> with the name of the service unit you want to enable, for example, httpd.

Prerequisites

• You have administrative privileges.

Procedure

• Disable a service to start at boot:

  # systemctl disable <name>.service

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  Replace <name> with the name of the service unit you want to disable (for example, bluetooth). Optionally, pass the --now command to also stop the service if it is currently running.

• To prevent the unit from being accidentally started by an administrator or as a dependency of other units, mask the service:

  # systemctl mask <name>.service

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  For details, see the systemctl(1) man page on your system.