Chapter 11. 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'

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

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

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• 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.

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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

Procedure

• 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
  • 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

    Expand

    Table 11.2. 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).

    Show more

• Verify that a particular service unit is running:

  $ systemctl is-active <name>.service

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

  $ systemctl is-enabled <name>.service

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  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

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  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]

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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
  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 the Root access.

Procedure

• Start a system service in the current session:

  # *systemctl start <systemd_unit> *

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  Replace <systemd_unit> 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 sendmail service, and you attempt to start the postfix service, systemd first automatically stops sendmail, 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

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  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

• You have the Root access.

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).

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

• 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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  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

• You have Root access.

Procedure

• Verify whether 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 <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 Root access.

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.

• Optional: To prevent that the unit can be accidentally started by an administrator or as a dependency of other units, mask the service:

  # systemctl mask <name>.service

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• 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

• You have 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!

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  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

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• In RHEL 8, you can schedule a shutdown by using the time argument. This also gives users warning that a system shutdown has been scheduled.

• Shut down the system and power off the machine at a certain time:

  # shutdown --poweroff hh:mm

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  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

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  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

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Prerequisites

• Root access

• Shut down the system and power off the machine:

  # systemctl poweroff

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

  # systemctl halt

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Prerequisites

• You have Root access.

Procedure

• Restart the system:

  # systemctl reboot

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• Suspend
• Hibernate
• Hybrid Sleep
• Suspend-then-hibernate

Prerequisites

• You have Root access.

• Suspend Suspending saves the system state in RAM and with the exception of the RAM module, powers off most of the devices in the machine. When you turn the machine back on, the system then restores its state from RAM without having to boot again. Because the system state is saved in RAM and not on the hard disk, restoring the system from suspend mode is significantly faster than from hibernation. However, the suspended system state is also vulnerable to power outages. To suspend the system, run:

  # systemctl suspend

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• Hibernate Hibernating saves the system state on the hard disk drive and powers off the machine. When you turn the machine back on, the system then restores its state from the saved data without having to boot again. Because the system state is saved on the hard disk and not in RAM, the machine does not have to maintain electrical power to the RAM module. However, as a consequence, restoring the system from hibernation is significantly slower than restoring it from suspend mode. To hibernate the system, run:

  # systemctl hibernate

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• Hybrid sleep This combines elements of both hibernation and suspending. The system first saves the current state on the the hard disk drive, and enters a low-power state similar to suspending, which allows the system to resume more quickly. The benefit of hybrid sleep is that if the system loses power during the sleep state, it can still recover the previous state from the saved image on the hard disk, similar to hibernation. To hibernate and suspend the system, run:

  # systemctl hybrid-sleep

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• Suspend-then-hibernate This mode first suspends the system, which results in saving the current system state to RAM and putting the system in a low-power mode. The system hibernates if it remains suspended for a specific period of time that you can define in the HibernateDelaySec parameter. Hibernation saves the system state to the hard disk drive and shuts down the system completely. The suspend-then-hibernate mode provides the benefit of conserving battery power while you are still able to quickly resume work. Additionally, this mode ensures that your data is saved in case of a power failure. Suspend and then hibernate the system:

  # systemctl suspend-then-hibernate

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