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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.