A vast majority of Linux systems these days are using systemd – a suite of programs aimed at managing and interconnecting different parts of the system. Systemd started replacing the init process back in 2014 and is now the first process that starts when most Linux systems boot. To get a quick peek, you can run a command like this, which verifies that process 1 is indeed systemd. On this system, two additional systemd processes are currently also 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 command below. The blank within the quotes is meant to prevent related processes like systemd-journald from showing up in 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 primary systemd process. The second and third are managing user (–user) sessions. In this case, one is associated with the GNOME display manager (gdm) and the other with a logged-in user.
If you look at all running systemd processes, you’re likely to see these. Each plays a role in managing system services. For example, system-journald collects and stores logging 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 command. You can view 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
I added the systemctl | head -1 in the command above to provide column headings.
For systemd, the word “UNIT” refers to any resource that the system knows how to operate and manage. To list those 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” doesn’t mean that a service is running. And “running” doesn’t necessarily mean that it’s enabled. Each means something different. “Enabled” means that the system will run the service on the next boot (whether or not it’s running now). Once you enable a service, you still need to manually start it if you want it to run right away, or you can reboot the system and it will start automatically.
The “running” status means that the process is actually running. If it’s not also enabled, it won’t restart when you reboot.
In the commands below, we can see that the web service is both 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 view a lot 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
Alternately, you can simply determine whether 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-up
The systemctl command provides different details on system processes than the ps command. Where ps only lists processes which are running, systemctl lists which services are known, which can be managed by systemd and whether services are enabled.
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