How to Manage Systemd Services with Systemctl?

How to Manage Systemd Services with Systemctl?

The default system and service manager for most Linux distributions now is systemd.

The systemd process replaces the SysV init. It runs as the first process after the kernel boot and is responsible for bringing the Linux host up to the state where it can be used. It is responsible for starting and managing the services, mounting filesystems, managing hardware, generating the login prompt, and much more.

A key benefit over SysV is that systemd starts as many services as possible in parallel, thus speeding up the startup process, and that brings up the login screen faster.

Units

The items that are managed by the systemd are called units. The unit files are located in /lib/systemd/system.

Service Units

For service management, the target units are service units, which have unit files with a suffix of .service.

Managing systemd services

The command to manage systemd units is systemctl.

Starting and Stopping Services

To start a systemd service, use the systemctl start command:

$ sudo systemctl start name.service

You can leave .service suffix. For example, to start the apache server on Ubuntu:

$ sudo systemctl start apache2

To stop a running service:

$ sudo systemctl stop name.service

So, to stop the apache server on Ubuntu:

$ sudo systemctl stop apache2

Restarting and Reloading services

To restart a running service, use restart command:

$ sudo systemctl restart name.service

And where the only reloading configuration file is required

$ sudo systemctl reload name.service

Enabling and Disabling Services

If you want a service to start automatically at system boot, use enable command:

$ sudo systemctl enable name.service

To disable a service from starting at system boot:

$ sudo systemctl disable name.service

Disabling does not stop a running service.

Viewing service status

To view information about a service:

$ sudo systemctl status name.service

This will show you the status of the service and the first few lines of the log file. So, while the service is running is, the output of

sudo systemctl status apache2

is

apache2.service - The Apache HTTP Server    Loaded: loaded (/lib/systemd/system/apache2.service; enabled; vendor preset: enabled)   Drop-In: /lib/systemd/system/apache2.service.d            └─apache2-systemd.conf    Active: active (running) since Tue 2020-05-19 22:11:36 UTC; 4 days ago   Process: 116002 ExecReload=/usr/sbin/apachectl graceful (code=exited, status=0/SUCCESS) Main PID: 104165 (apache2)     Tasks: 55 (limit: 1024)    CGroup: /system.slice/apache2.service            ├─104165 /usr/sbin/apache2 -k start            ├─116006 /usr/sbin/apache2 -k start            └─116007 /usr/sbin/apache2 -k start May 19 22:11:36 ubuntu18 systemd[1]: Starting The Apache HTTP Server... May 19 22:11:36 ubuntu18 systemd[1]: Started The Apache HTTP Server. May 21 06:25:01 ubuntu18 systemd[1]: Reloading The Apache HTTP Server. May 21 06:25:01 ubuntu18 systemd[1]: Reloaded The Apache HTTP Server. May 22 06:25:01 ubuntu18 systemd[1]: Reloading The Apache HTTP Server.

To check if a service is active:

$ sudo systemctl is-active name.service

So, while the apache2 service is running, the output of systemctl is-active command is :

$ sudo systemctl is-active apache2 active

To check if a service is enabled:

$ sudo systemctl is-enabled name.service.

Viewing System State

All commands you have seen till now are used to manage a single service. When you want an overview of the system state, use the following set of commands:

To see all unit types

$ sudo systemctl -t help
Available unit types: service socket target device mount automount swap timer path slice scope

To list all installed units, use list-unit-files

$ sudo systemctl list-unit-files
UNIT FILE STATE proc-sys-fs-binfmt_misc.automount static -.mount generated boot-efi.mount generated dev-hugepages.mount static dev-mqueue.mount static mnt.mount generated proc-sys-fs-binfmt_misc.mount static sys-fs-fuse-connections.mount static sys-kernel-config.mount static sys-kernel-debug.mount static acpid.path enabled apport-autoreport.path enabled systemd-ask-password-console.path static systemd-ask-password-plymouth.path static systemd-ask-password-wall.path static session-161.scope transient accounts-daemon.service enabled 

The output has only two columns Unit File and State. The state will usually be enabled, disabled, static or masked.

  • Static: This means the unit cannot be enabled, performs a one-off action, or is a dependency of another unit and cannot be run by itself.
  • Masked:A unit listed as masked means it is completely unstartable, as it is linked to /dev/null.This is called masking the unit. This prevents the service from being started, manually or automatically.

List all installed services

The systemctl list-unit-files command with -t or –type service filter shows the state of installed services only.

$ sudo systemctl list-unit-files -t service 
UNIT FILE STATE accounts-daemon.service enabled acpid.service disabled apache-htcacheclean.service disabled [email protected] disabled apache2.service enabled [email protected] disabled apparmor.service enabled apport-autoreport.service static [email protected] static apport.service generated apt-daily-upgrade.service static apt-daily.service static atd.service enabled [email protected] enabled blk-availability.service enabled bootlogd.service masked bootlogs.service masked 

To see all active service units, use list-units with -t service filter

$ sudo systemctl list-units -t service 
UNIT LOAD ACTIVE SUB DESCRIPTION accounts-daemon.service loaded active running Accounts Service apache2.service loaded active running The Apache HTTP Server apparmor.service loaded active exited AppArmor initialization apport.service loaded active exited LSB: automatic crash report generation atd.service loaded active running Deferred execution scheduler blk-availability.service loaded active exited Availability of block devices cloud-config.service loaded active exited Apply the settings specified in cloud-con cloud-final.service loaded active exited Execute cloud user/final scripts cloud-init-local.service loaded active exited Initial cloud-init job (pre-networking) cloud-init.service loaded active exited Initial cloud-init job (metadata service console-setup.service loaded active exited Set console font and keymap cron.service loaded active running Regular background program processing dae 

The output has the following columns :

  • UNIT: The systemd service unit name
  • LOAD: Shows whether the unit definition was properly read and loaded
  • ACTIVE: Describes if the unit is active.
  • SUB: Low-level activation state of the unit, giving more detailed information about the unit. This will vary by unit type.
  • DESCRIPTION: The service unit’s description.

Conclusion

I hope this gives you an idea about using systemctl to manage services on Linux. If interested in learning more, you can check out this Linux Mastery course.

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