I - Intro

Continuing from the previous post about GNU/Linux system startup, this time I’ll tell you about services.

How many times did I struggle with the “print spooler” service in Windows! If you worked in IT starting in Help Desk, Service Desk, or similar positions in corporate environments… you’ve suffered through it too -_-

As for Linux… well, the concept is similar but of course it has its differences.

Warning! If you keep reading, you’ll keep learning ;)

Services, Systemd, Systemctl, Sysvinit? Let’s go!

II /sbin/init

Once the real filesystem is mounted, the kernel executes the binary located at /sbin/init (typically nowadays it’s a link to systemd), this /sbin/init process will always have ID=1. It’s the first user process of the system and initiates other subprocesses.

The init process determines and launches the services and processes needed to complete system boot. Traditionally, System V Unix (SysVinit) was used. It had different runlevels where each contained sets of scripts to start and stop services. Its modern contemporary counterpart systemd has backward compatibility in this regard and supports runlevels through runlevel targets. It emulates the telinit program to work with runlevels, so if you’re reading this from any GNU/Linux distro with systemd, you can try them to feed your curiosity:

    –> RHEL “Runlevels”.

    –> Linux Manual Page - man

Most non-kernel processes in the OS have their root in this process, except those processes initiated directly by the Kernel to handle internal Kernel details.

init is responsible for keeping the system running and shutting it down cleanly, and also for managing login services when users connect and disconnect.

The init can vary depending on the Linux version and distro, as I mentioned before, the most modern and widely used today is systemd.

It’s possible to run another init by specifying it in the system configuration or from the bootloader.

How to know which init I’m using? With the following command. Notice that /usr/sbin/init points to systemd.

ls -l $(which init)

proyectoleeti@fedora42:~$ ls -l $(which init)
lrwxrwxrwx. 1 root root 22 sep  6 21:00 /usr/sbin/init -> ../lib/systemd/systemd

III Services

A service is a program or process that runs in the background (like a daemon).

The systemd init is more modern than SysVinit and is the most widely used currently. It’s responsible for loading and starting processes as well as starting and stopping services during OS execution proper. Some examples:

Apache web server httpd.

Managing printers with CUPS cupsd.

It’s common to manage a service through the systemctl command, for example, to enable or disable services at OS startup:

  • systemctl enable httpd

  • systemctl disable httpd

To start or stop it:

  • systemctl start httpd

  • systemctl stop httpd

IV Systemd

Systemd uses .service files unlike SysVinit which uses bash scripts.

systemd characteristics:

  • Organizes all daemons in their own cgroups (control groups), contributing to better process traceability.

  • Boots faster than previous init systems.

  • Has a focus on parallelization.

  • Uses socket and D-Bus activations to start services.

  • Offers on-demand daemon startup.

  • Maintains mount points and automounting.

  • Can replace SysVinit without losing compatibility with its scripts.

  • Compatibility with SysVinit commands.

Reviewing official documentation is always considered a good practice and is a tool that’s less valued every day (compared to “googling” and asking AI). For example, Fedora’s official documentation provides a command equivalence table between SystemD and SysVinit and much more:

Example of coexistence with SysVinit commands even on a system with Systemd, on Fedora 42:

Notice that the same action is executable with systemctl (systemd) and with service (sysVinit), first to check the service status with sshd and then to start/stop it with start and stop:

systemctl status sshd

proyectoleeti@fedora42:~$ systemctl status sshd
○ sshd.service - OpenSSH server daemon
     Loaded: loaded (/usr/lib/systemd/system/sshd.service; disabled; preset: disabled)
    Drop-In: /usr/lib/systemd/system/service.d
             └─10-timeout-abort.conf
     Active: inactive (dead)
       Docs: man:sshd(8)
             man:sshd_config(5)

sudo systemctl start sshd
systemctl status sshd

proyectoleeti@fedora42:~$ sudo systemctl start sshd

proyectoleeti@fedora42:~$ systemctl status sshd
● sshd.service - OpenSSH server daemon
     Loaded: loaded (/usr/lib/systemd/system/sshd.service; disabled; preset: disabled)
    Drop-In: /usr/lib/systemd/system/service.d
             └─10-timeout-abort.conf
     Active: active (running) since Mon 2025-09-15 12:04:20 CEST; 4s ago
 Invocation: 5d416aa670044ec4a863c36ee1fb69d3
       Docs: man:sshd(8)
             man:sshd_config(5)
   Main PID: 40530 (sshd)
      Tasks: 1 (limit: 15357)
     Memory: 1M (peak: 1.5M)
        CPU: 17ms
     CGroup: /system.slice/sshd.service
             └─40530 "sshd: /usr/sbin/sshd -D [listener] 0 of 10-100 startups"

sudo service sshd stop
systemctl status sshd
service sshd status

proyectoleeti@fedora42:~$ sudo service sshd stop
Redirecting to /bin/systemctl stop sshd.service

proyectoleeti@fedora42:~$ systemctl status sshd
○ sshd.service - OpenSSH server daemon
     Loaded: loaded (/usr/lib/systemd/system/sshd.service; disabled; preset: disabled)
    Drop-In: /usr/lib/systemd/system/service.d
             └─10-timeout-abort.conf
     Active: inactive (dead)
       Docs: man:sshd(8)
             man:sshd_config(5)

proyectoleeti@fedora42:~$ service sshd status
Redirecting to /bin/systemctl status sshd.service
○ sshd.service - OpenSSH server daemon
     Loaded: loaded (/usr/lib/systemd/system/sshd.service; disabled; preset: disabled)
    Drop-In: /usr/lib/systemd/system/service.d
             └─10-timeout-abort.conf
     Active: inactive (dead)
       Docs: man:sshd(8)
             man:sshd_config(5)

V systemctl

System Control or systemctl is a command to manage units.

A unit is a configuration file that tells systemd how to manage a system resource. It describes how to start, stop, reload, or monitor a system component.

These files are usually located in /etc/systemd/system or /usr/lib/systemd/system.

They represent a system task or component, among others: Services, Timers, Sockets, Mounts, etc. The most common are services and sockets:

  • A .service is a traditional service that runs in the background, like sshd, bluetooth, etc.

  • A socket defines a network or UNIX socket. For example, a network socket (ip + port).

Systemd can listen on that socket and only when there’s a connection, start the associated service. Useful for saving system resources. This is called socket activation.

Some commands require sudo (elevated user privileges), others don’t, depending on the scope.

The command structure is as follows:

systemctl [options] command [name]

Here are some very common day-to-day command examples.

  • See the status of all processes controlled by systemd:

    • systemctl

  • Show all available services:

    • systemctl list-units -t service -all

  • List only active services:

    • systemctl list-units -t service

  • Various ways to start a service:

    • systemctl start bluetooth

    • systemctl start bluetooth.service

    • systemctl start /usr/lib/systemd/system/bluetooth.service

  • To stop it:

    • systemctl stop bluetooth

  • To enable/disable a service at system boot:

    • systemctl enable sshd.service

    • systemctl disable sshd.service

See some of the commands in action, this time in an Ubuntu 22.04 environment:

🐧 You’ve noticed that this post contains more command content than the previous one. It’s not an accident: in the next one we’ll complement with a hands-on, that is, a lab to practice everything on your own system.

Until the next post! 🐧