10 handy systemd commands: A reference

Poor systemd has had its share of detractors, but it seems to be here to stay for Linux admins so we might as well get used to it. This handy systemd command reference will help you keep your sanity when trying to perform normal administrative tasks. So, until we get something that's more usable, palatable, and desirable than systemd, please enjoy this list of ten handy commands for your convenience. These commands are in no particular order of importance or relevance.

List unit files

From the systemd man page: A unit file is a plain text ini-style file that encodes information about a service, a socket, a device, a mount point, an automount point, a swap file or partition, a start-up target, a watched file system path, a timer controlled and supervised by systemd, a resource management slice, or a group of externally created processes.

$ systemctl list-unit-files

UNIT FILE                                     STATE   
proc-sys-fs-binfmt_misc.automount             static  
dev-hugepages.mount                           static 
dev-mqueue.mount                              static 
proc-sys-fs-binfmt_misc.mount                 static  
sys-fs-fuse-connections.mount                 static  
sys-kernel-config.mount                       static  
sys-kernel-debug.mount                        static  
tmp.mount                                     disabled
brandbot.path                                 disabled
systemd-ask-password-console.path             static  
systemd-ask-password-plymouth.path            static  
systemd-ask-password-wall.path                static  
session-1.scope                               static  
arp-ethers.service                            disabled
auditd.service                                enabled 
autovt@.service                               enabled 

<many more entries>

Of course, you can always pipe to grep to see just the enabled services.

$ systemctl list-unit-files |grep enabled

auditd.service                                enabled 
autovt@.service                               enabled 
crond.service                                 enabled 
dbus-org.fedoraproject.FirewallD1.service     enabled 
dbus-org.freedesktop.nm-dispatcher.service    enabled 
firewalld.service                             enabled 
getty@.service                                enabled 
irqbalance.service                            enabled 
kdump.service                                 enabled 
lvm2-monitor.service                          enabled 

<many more entries>

These unit files, located under /lib/systemd/system, are roughly equivalent to the legacy init scripts that were located under /etc/rc.d/init.d. In fact, if you or your software installation create init scripts, a corresponding systemd unit file is mapped for you. A further explanation is given by /etc/rc.d/init.d/README:

You are looking for the traditional init scripts in /etc/rc.d/init.d,
and they are gone?

Here's an explanation on what's going on:

You are running a systemd-based OS where traditional init scripts have
been replaced by native systemd services files. Service files provide
very similar functionality to init scripts. To make use of service
files simply invoke "systemctl", which will output a list of all
currently running services (and other units). Use "systemctl
list-unit-files" to get a listing of all known unit files, including
stopped, disabled and masked ones. Use "systemctl start
foobar.service" and "systemctl stop foobar.service" to start or stop a
service, respectively. For further details, please refer to
systemctl(1).

Note that traditional init scripts continue to function on a systemd
system. An init script /etc/rc.d/init.d/foobar is implicitly mapped
into a service unit foobar.service during system initialization.

Thank you!

Further reading:
        man:systemctl(1)
        man:systemd(1)
        http://0pointer.de/blog/projects/systemd-for-admins-3.html
        http://www.freedesktop.org/wiki/Software/systemd/Incompatibilities

As you can see, init.d has been removed in favor of systemd. It's here to stay until someone comes up with something better. (I'm hoping that someone is rapidly working on a replacement.)

List units

Listing active units displays a lot of useful information about your loaded and active services. The output is too detailed to demonstrate here, but try the following command on your system to see what I mean.

$ systemctl list-units

The status fields are great to see, but the description field is the most useful to me. It provides detailed information about your services.

Start a service

To get a service name, list your unit files and grep for the one you want. Then use the systemctl command to start your service. I'm using firewalld as the example.

$ sudo systemctl start firewalld

Surprisingly, or maybe not so surprisingly, there's no response from starting, stopping, or restarting a service. To check the status of a service, you must use the status command.

Checking a service status

To check a service's status, use the systemctl status service-name command.

$ sudo systemctl status sshd
[sudo] password for khess: 
● sshd.service - OpenSSH server daemon
   Loaded: loaded (/usr/lib/systemd/system/sshd.service; enabled; vendor preset: enabled)
   Active: active (running) since Wed 2020-04-29 07:44:57 CDT; 2h 17min ago
     Docs: man:sshd(8)
           man:sshd_config(5)
 Main PID: 1055 (sshd)
   CGroup: /system.slice/sshd.service
           └─1055 /usr/sbin/sshd -D

Apr 29 07:44:57 centos7 systemd[1]: Starting OpenSSH server daemon...
Apr 29 07:44:57 centos7 sshd[1055]: Server listening on 0.0.0.0 port 22.
Apr 29 07:44:57 centos7 sshd[1055]: Server listening on :: port 22.
Apr 29 07:44:57 centos7 systemd[1]: Started OpenSSH server daemon.
Apr 29 07:51:35 centos7 sshd[1396]: Accepted password for khess from 192.168.1.85 port 56769 ssh2

I like systemd's status because of the detail given. For example, in the above listing, you see the full path to the unit file, the status, the start command, and the latest status changes.

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Stop a service

Stopping a running service is as easy as starting one.

$ sudo systemctl stop firewalld

Again, you see no response from issuing this command. Issue a service status to check your success or failure.

Restarting a service

If you want to stop and start a service without issuing two commands (sysadmins are a lazy lot, after all), issue a restart.

$ sudo systemctl restart firewalld

No system response is displayed.

System restart, halt, and shutdown

These three tasks are typical ones that sysadmins need to know and are now under the control of systemd.

Reboot

There are multiple ways to restart your systems, but the old standby, reboot, is actually a link to the systemctl command. I assume that since it works, it links the systemctl command with the reboot switch added as follows:

$ sudo systemctl reboot

The same link applies to the halt and to the shutdown commands.

Shutdown and halt

It doesn't matter how you used to do it with halt -p or shutdown now or whatever, the universal command is now:

$ sudo systemctl poweroff

This command powers down the system.

Set services to run at boot time

You're used to the chkconfig command to enable your services to run at boot time and under a particular runlevel. Well, those days too are gone, and they have been usurped by the ubiquitous systemctl command.

Enabling a service to run at boot time

To set any service to start at boot, issue the following command. I'm using firewalld as the example service.

$ sudo systemctl enable firewalld

Disabling a service from running at boot time

To prevent any service from starting at boot time, issue:

$ sudo systemctl disable firewalld

The firewalld service will not start at boot time.

Wrap up

This brief but handy systemd/systemctl reference guide should take some of the pain out of dealing with systemd. That's the theory, at least. And, as you'll often see in my articles or hear me say out loud, "Everything works on paper." Be sure to let me know on Twitter what you think of my articles and also to suggest new topics.

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