Chapter 9: Services and the boot process

Managing services with systemd

Almost everything useful a server does, such as serving web pages, keeping time, accepting SSH logins or running scheduled jobs, is done by a service: a program that runs in the background without a terminal. On RHEL, a program called systemd starts, stops and watches them. systemctl is how you talk to it.

Explain what systemd and a unit are, and tell service units from other unit types.
Start, stop, restart, reload, enable, disable and mask a service, and say which command acts on "now" and which on "boot".
Read systemctl status, list units, and find failed ones.

systemd and units

systemd is the first process the kernel starts (PID 1). It reads configuration files called units, each describing one thing it manages. The type is the file's suffix:

Unit type	Manages	Example
`.service`	A background program	`sshd.service`, `crond.service`
`.target`	A group of units that mark a state of the system	`multi-user.target`
`.socket`	A listening network or local socket that starts a service on demand	`dbus.socket`
`.timer`	A schedule that starts another unit	`dnf-makecache.timer`
`.mount`	A mounted file system	`boot-efi.mount`

You can leave out .service when you type a service name: systemctl status crond means crond.service.

Asking about a service

[root@servera ~]# systemctl status crond
● crond.service - Command Scheduler
     Loaded: loaded (/usr/lib/systemd/system/crond.service; enabled; preset: enabled)
     Active: active (running) since Sat 2026-10-03 16:36:40 UTC; 4s ago
   Main PID: 642 (crond)
      Tasks: 1 (limit: 5820)
     Memory: 1.0M (peak: 1.3M)
        CPU: 6ms
     CGroup: /system.slice/crond.service
             └─642 /usr/sbin/crond -n

Oct 03 16:36:40 servera.lab.example.com systemd[1]: Started Command Scheduler.
Oct 03 16:36:40 servera.lab.example.com crond[642]: (CRON) STARTUP (1.5.7)

Read it from the top:

Line	Tells you
`●` / `○`	A green dot: active. A hollow dot: inactive. Red: failed.
`Loaded:`	Where the unit file is, and whether it is enabled (starts at boot)
`Active:`	The current state and since when
`Main PID`, `CGroup`	The processes that belong to it
log lines	The last few journal lines, to see what it said

For scripts, ask a single question and use the answer: systemctl is-active NAME prints active or inactive, systemctl is-enabled NAME prints enabled, disabled or masked, and both set the exit status.

Now versus at boot

This is the central idea of the lesson. Every service has two independent properties:

Two separate questions: Select a combination to see what it means.

Command	Changes	Affects
`systemctl start NAME`	Starts it	Now only
`systemctl stop NAME`	Stops it	Now only
`systemctl restart NAME`	Stops, then starts	Now
`systemctl reload NAME`	Asks it to re-read its configuration, without stopping	Now
`systemctl enable NAME`	Starts it at every boot	Boot only
`systemctl disable NAME`	Stops it from starting at boot	Boot only
`systemctl enable --now NAME`	Both	Boot and now
`systemctl disable --now NAME`	Both	Boot and now
`systemctl mask NAME`	Makes it impossible to start	Always

enable works by creating a symbolic link, which you can see in its output:

[root@servera ~]# systemctl disable crond
Removed "/etc/systemd/system/multi-user.target.wants/crond.service".
[root@servera ~]# systemctl enable crond
Created symlink /etc/systemd/system/multi-user.target.wants/crond.service → /usr/lib/systemd/system/crond.service.

mask goes further than disable: a disabled service can still be started by hand or by another service that needs it, but a masked one cannot.

[root@servera ~]# systemctl mask bluetooth
Created symlink /etc/systemd/system/bluetooth.service → /dev/null.
[root@servera ~]# systemctl start bluetooth
Failed to start bluetooth.service: Unit bluetooth.service is masked.

Listing units

[root@servera ~]# systemctl list-units --type=service --state=running
[root@servera ~]# systemctl list-unit-files --type=service
[root@servera ~]# systemctl --failed
  UNIT LOAD ACTIVE SUB DESCRIPTION
0 loaded units listed.

list-units shows what is loaded now; list-unit-files shows what is installed and whether each is enabled; --failed is the first thing to run when something is wrong.

Try it

Check your understanding

A service is running, but after a reboot it is gone. What was missing?
Which command shows whether a unit is currently running?
What is the difference between restart and reload?
A masked service ...
Which command lists every unit that failed?
Where do administrator-created and modified unit files belong?