Manage services on Centos Linux

On Centos Linux (in this case version 8), the command systemctl allows administration of services on Linux. The version of systemctl in use is displayed with command

systemctl --version

systemd 239 (239-58.el8)

+PAM +AUDIT +SELINUX +IMA -APPARMOR +SMACK +SYSVINIT +UTMP +LIBCRYPTSETUP +GCRYPT +GNUTLS +ACL +XZ +LZ4 +SECCOMP +BLKID +ELFUTILS +KMOD +IDN2 -IDN +PCRE2 default-hierarchy=legacy

Check status of services

systemctl status httpd

systemctl status containerd

systemctl status kubelet

systemctl list-unit-files

Background services is list with

systemctl list-jobs

View service information

systemctl show httpd

systemctl show containerd

Start and stop a service

systemctl start httpd

systemctl stop httpd

On some services, there is the command to restart or reload. Reload, reads the updated configuration for a service without stopping the service.

systemctl start httpd

systemctl reload httpd

Boot target

On linux, the run levels describe what the server should do after a startup. Where runlevel and the numeric equivalent of target. Here is a list of runlevel and in brackets are the systemctl commands for it.

Runlevel 0 - poweroff.target (systemctl isolate poweroff.target)

Runlevel 1 - rescue.target  (systemctl isolate rescue.target)

Runlevel 2 - text based multi-user.target without network  (systemctl isolate runlevel2.target)

Runlevel 3 - text based multi-user.target with network  (systemctl isolate runlevel3.target)

Runlevel 5 - graphical graphical.target  (systemctl isolate graphical.target)

Runlevel 6 - reboot.target (systemctl isolate reboot.target)

Default boot target is set by /etc/systemd/system/default.target and can be easily viewed with the command 'ls'.

Or the command systemctl get-default

multi-user.target

View available targets

systemctl list-units --type target --all

To change a default boot target,

systemctl set-default multi-user.target

Troubleshooting

List dependencies of the service

systemctl list-dependencies httpd

Unit files are list as

systemctl list-unit files

When a service is mask, it cannot be started until it is unmask. This can be done with

systemctl unmask httpd