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.serviceYou can leave .service suffix. For example, to start the apache server on Ubuntu:
$ sudo systemctl start apache2To stop a running service:
$ sudo systemctl stop name.service
So, to stop the apache server on Ubuntu:
$ sudo systemctl stop apache2Restarting and Reloading services
To restart a running service, use restart command:
$ sudo systemctl restart name.serviceAnd where the only reloading configuration file is required
$ sudo systemctl reload name.serviceEnabling and Disabling Services
If you want a service to start automatically at system boot, use enable command:
$ sudo systemctl enable name.serviceTo disable a service from starting at system boot:
$ sudo systemctl disable name.serviceDisabling does not stop a running service.
Viewing service status
To view information about a service:
$ sudo systemctl status name.serviceThis 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 apache2is
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
activeTo 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 helpAvailable unit types:
service
socket
target
device
mount
automount
swap
timer
path
slice
scopeTo list all installed units, use list-unit-files
$ sudo systemctl list-unit-filesUNIT 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
apache-htcacheclean@.service disabled
apache2.service enabled
apache2@.service disabled
apparmor.service enabled
apport-autoreport.service static
apport-forward@.service static
apport.service generated
apt-daily-upgrade.service static
apt-daily.service static
atd.service enabled
autovt@.service 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.
