Viewing enabled and running services on Linux with systemctl

The vast majority of Linux systems these days use systemd – a set of programs designed to manage and interconnect different parts of the system. Systemd began to replace the in that process in 2014 and is now the first process that starts when most Linux systems boot. For a quick look, you can run a command like this, which verifies that process 1 is actually systemd. In this system, two systemd The processes are also currently running.

$ ps -C systemd
    PID TTY          TIME CMD
      1 ?        00:00:59 systemd	<===
   1244 ?        00:00:00 systemd
  54429 ?        00:00:00 systemd

To see a little more detail, try the following command. The white space between the quotes is meant to prevent related processes like systemd-journald to appear on the list.

$ ps -ef | grep "systemd " | grep -v grep
root    1   0 0 Jul17 ?    00:00:59 /usr/lib/systemd/systemd --system --deserialize 30
gdm  1244   1 0 Jul17 ?    00:00:00 /usr/lib/systemd/systemd --user
shs  5429   1 0 Jul19 ?    00:00:00 /usr/lib/systemd/systemd --user

The first process listed (with –system) is the principal systemd process. The second and third are the admin user (–Username) sessions. In this case, one is associated with the GNOME display manager (gdm) and the other with a registered user.

If you look at everything running systemd processes, you will likely see these. Each plays a role in managing the system services. For instance, system-journald collects and stores log data.

/usr/lib/systemd/systemd
/usr/lib/systemd/systemd-journald
/usr/lib/systemd/systemd-udevd
/usr/lib/systemd/systemd-oomd
/usr/lib/systemd/systemd-resolved
/usr/lib/systemd/systemd-homed
/usr/lib/systemd/systemd-machined
/usr/lib/systemd/systemd-logind
/usr/lib/systemd/systemd-userdbd
/usr/lib/systemd/systemd
systemd-userwork

To view or control systemd services, use the systemctl I send. You can see the running processes with a command like this:

$ systemctl | head -1; systemctl | grep running | head -11
 UNIT                               LOAD   ACTIVE SUB       DESCRIPTION
 proc-sys-fs-binfmt_misc.automount  loaded active running   Arbitrary Executable File Formats File System
 cups.path                          loaded active running   CUPS Scheduler
 init.scope                         loaded active running   System and Service Manager
 session-13.scope                   loaded active running   Session 13 of User shs
 session-6.scope                    loaded active running   Session 6 of User shs
 session-c1.scope                   loaded active running   Session c1 of User gdm
 abrt-journal-core.service          loaded active running   Creates ABRT problems from coredumpctl messages
 abrt-oops.service                  loaded active running   ABRT kernel log watcher
 abrt-xorg.service                  loaded active running   ABRT Xorg log watcher
 abrtd.service                      loaded active running   ABRT Automated Bug Reporting Tool
 accounts-daemon.service            loaded active running   Accounts Service

Add the systemctl | head -1 in the above command to provide column headings.

To systemd, the word “UNIT” refers to any resource that the system knows how to operate and manage. To list the ones that are enabled, you can use a command like this:

$ systemctl list-unit-files --state=enabled | head -15
UNIT FILE STATE VENDOR PRESET
cups.path enabled enabled
abrt-journal-core.service enabled enabled
abrt-oops.service enabled enabled
abrt-vmcore.service enabled enabled
abrt-xorg.service enabled enabled
abrtd.service enabled enabled
accounts-daemon.service enabled enabled
atd.service enabled enabled
auditd.service enabled enabled
avahi-daemon.service enabled enabled
bluetooth.service enabled enabled
chronyd.service enabled enabled
crond.service enabled enabled
cups.service enabled disabled

Note that “enabled” does not mean that a service is running. And “running” doesn’t necessarily mean it’s enabled. Each one means something different. “Enabled” means that the system will run the service on the next boot (whether it is running now or not). Once you enable a service, you still have to start it manually if you want it to run immediately, or you can reboot the system and it will start automatically.

The “running” status means that the process is actually running. If it is not also enabled, it will not reboot when you reboot.

In the following commands, we can see that the web service is enabled and running:

$ systemctl list-unit-files | head -1; systemctl list-unit-files | grep http
UNIT FILE                        STATE           VENDOR PRESET
httpd.service                    enabled         disabled  <== enabled
httpd@.service                   disabled        disabled
httpd.socket                     disabled        disabled

$ systemctl | grep running | grep http
  httpd.service     loaded active running   The Apache HTTP Server <== running

You can see much more information related to this service by asking for its status:

$ systemctl status httpd.service
● httpd.service - The Apache HTTP Server
     Loaded: loaded (/usr/lib/systemd/system/httpd.service; enabled; vendor preset: disabled)
     Active: active (running) since Sat 2021-07-17 18:21:44 EDT; 1 week 1 day ago
       Docs: man:httpd.service(8)
   Main PID: 876 (httpd)
     Status: "Total requests: 154; Idle/Busy workers 100/0;Requests/sec: 0.000204; Bytes served/sec:   0 B/sec"
      Tasks: 213 (limit: 7072)
     Memory: 20.3M
        CPU: 1min 58.761s
     CGroup: /system.slice/httpd.service
             ├─   876 /usr/sbin/httpd -DFOREGROUND
             ├─394234 /usr/sbin/httpd -DFOREGROUND
             ├─394235 /usr/sbin/httpd -DFOREGROUND
             ├─394236 /usr/sbin/httpd -DFOREGROUND
             └─394237 /usr/sbin/httpd -DFOREGROUND

Jul 25 00:00:07 dragonfly systemd[1]: Reloaded The Apache HTTP Server.
Jul 25 00:00:08 dragonfly httpd[876]: Server configured, listening on: port 80

Alternatively, you can simply determine if a particular service is active and / or enabled with commands like these:

$ systemctl is-active httpd.service
Active
$ systemctl is-enabled httpd.service
Enabled

Wrap

The systemctl The command provides different details about the system processes that the P.S. I send. Where P.S. it only lists the processes that are running, systemctl lists which services are known, which can be managed by systemd and if the services are enabled.

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