Chapter 14. Managing systemd

• List all currently loaded service units:

  $ systemctl list-units --type service
  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'

  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, by using the following command with the --all or -a command line option:

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

• List the status (enabled or disabled) of all available service units:

  $ systemctl list-unit-files --type service
  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.

  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

  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
  • One or more fields described in Available service unit information
  • The execution of the service unit: if the unit is executed by the root user

  • The most recent log entries

    Table 14.2. Available service unit information

    Field	Description
    Loaded	Information whether the service unit has been loaded, the absolute path to the unit file, and a note whether the unit is enabled to start during boot.
    Active	Information whether the service unit is running followed by a time stamp.
    Main PID	The process ID and the name of the corresponding system service.
    Status	Additional information about the corresponding system service.
    Process	Additional information about related processes.
    CGroup	Additional information about related control groups (cgroups).

  Example 14.1. Displaying service status

  The service unit for the GNOME Display Manager is named gdm.service. To determine the current status of this service unit, type the following at a shell prompt:

  # systemctl status gdm.service
  gdm.service - GNOME Display Manager
     Loaded: loaded (/usr/lib/systemd/system/gdm.service; enabled)
     Active: active (running) since Thu 2013-10-17 17:31:23 CEST; 5min ago
   Main PID: 1029 (gdm)
     CGroup: /system.slice/gdm.service
             ├─1029 /usr/sbin/gdm
             └─1047 /usr/bin/Xorg :0 -background none -verbose -auth /r...
  
  Oct 17 17:31:23 localhost systemd[1]: Started GNOME Display Manager.

• Verify that a particular service unit is running:

  $ systemctl is-active <name>.service

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

  $ systemctl is-enabled <name>.service

  Note

  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 what services systemd orders to start before the specified service unit

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

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

  # systemctl list-dependencies --after gdm.service
  gdm.service
  ├─dbus.socket
  ├─getty@tty1.service
  ├─livesys.service
  ├─plymouth-quit.service
  ├─system.slice
  ├─systemd-journald.socket
  ├─systemd-user-sessions.service
  └─basic.target
  [output truncated]

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

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

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

  # systemctl list-dependencies --before gdm.service
  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

Prerequisites

• Root access

Procedure

• Start a system service in the current session:

  # systemctl start <name>.service

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

  Note

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

  When you attempt 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 attempt to start another service with a negative dependency, the first service is automatically stopped.

  For example, if you are running the postfix service, and you attempt to start the sendmail service, systemd first automatically stops postfix, because these two services are conflicting and cannot run on the same port.

Prerequisites

• Root access

Procedure

• Stop a system service:

  # systemctl stop <name>.service

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

• Stop the selected service unit in the current session and immediately start it again.
• Restart a service unit only if the corresponding service is already running.
• Reload configuration of a system service without interrupting its execution.

Prerequisites

• Root access

Procedure

• Restart a system service:

  # systemctl restart <name>.service

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

  Note

  If the selected service unit is not running, this command starts it too.

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

  # systemctl try-restart <name>.service

• Optional: Reload the configuration without interrupting service execution:

  # systemctl reload <name>.service

  Note

  System services that do not support this feature, ignore this command. To restart such services, use the reload-or-restart and reload-or-try-restart commands instead.

Prerequisites

• Root access

• The service you want to enable must not be masked. If you have a masked service, unmask it first:

  # systemctl unmask <name>.service

Procedure

• Enable a service to start at boot:

  # systemctl enable <name>.service

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

• Optional: You can also enable and start a service by using a single command:

  # systemctl enable --now <name>.service

Prerequisites

• Root access

Procedure

• Disable a service to start at boot:

  # systemctl disable <name>.service

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

• Optional: If you want to make a service permanently unusable, mask the service:

  # systemctl mask <name>.service

  This command replaces the /etc/systemd/system/name.service file with a symbolic link to /dev/null, rendering the actual unit file inaccessible to systemd.

• The graphical.target unit for starting a graphical session, starts system services such as the GNOME Display Manager (gdm.service) or Accounts Service (accounts-daemon.service), and also activates the multi-user.target unit.
• Similarly, the multi-user.target unit starts other essential system services such as NetworkManager (NetworkManager.service) or D-Bus (dbus.service) and activates another target unit named basic.target.

Prerequisites

• Root access

Prerequisites

• Root access

Procedure

• To enter the rescue mode, change the current target in the current session:

  # systemctl rescue
  
  Broadcast message from root@localhost on pts/0 (Fri 2023-03-24 18:23:15 CEST):
  
  The system is going down to rescue mode NOW!

  Note

  This command is similar to systemctl isolate rescue.target, but it also sends an informative message to all users that are currently logged into the system.

  To prevent systemd from sending a message, enter the following command with the --no-wall command-line option:

  # systemctl --no-wall rescue

• You can schedule a shutdown by using the time argument. This also gives users warning that a system shutdown has been scheduled.
• You can cancel the shutdown.

• Shut down the system and power off the machine at a certain time
• Shut down and halt the system without powering off the machine
• Cancel a pending shutdown

Prerequisites

• Root access

• Specify the time at which you want to shut down the system and power off the machine:

  # shutdown --poweroff hh:mm

  Where hh:mm is the time in the 24-hour time notation. To prevent new logins, the /run/nologin file is created 5 minutes before system shutdown.

  When you use the time argument, you can notify users logged in to the system of the planned shutdown by specifying an optional wall message, for example shutdown --poweroff 13:59 "Attention. The system will shut down at 13:59".

• Shut down and halt the system after a delay, without powering off the machine:

  # shutdown --halt +m

  Where +m is the delay time in minutes. You can use the now keyword as an alias for +0.

• Cancel a pending shutdown:

  # shutdown -c

Prerequisites

• Root access

• Shut down the system and power off the machine:

  # systemctl poweroff

• Shut down and halt the system without powering off the machine:

  # systemctl halt

• After installing new software or updates
• After making changes to system settings
• When troubleshooting system issues

Prerequisites

• Root access

Procedure

• Restart the system:

  # systemctl reboot

Prerequisites

• Root access

• Suspend the system:

  # systemctl suspend

• Hibernate the system:

  # systemctl hibernate

• Hibernate and suspend the system:

  # systemctl hybrid-sleep

• Suspend and then hibernate the system:

  # systemctl suspend-then-hibernate