Systemd vs sysVinit - Initializing the system

Akshay — Wed, 20 Dec 2023 07:02:57 +0000

Motivation behind this post - my first post here. In this post I'll try to explain the system initializer in linux and the difference between sysVinit and systemd.

ESSENTIAL BACKGROUND

What is the `init` process?

To answer this, we need to look a little bit at the booting process.

Boot sequence

The boot process is everything that happens from the time you press the power button until you are presented with a login screen. It generally follows this sequence:



1. BIOS/UEFI locates and executes the bootloader (or UEFI can load a kernel directly).
2. Bootloader(e.g. GRUB2 loads the desired kernel into memory).
3. Kernel starts the init process (pid = 1).
4. init process manages overall system initialization : systemd( or sysVinit, upstart, etc).
5. Then the display manager takes over the control (the login screen).

This post focuses on point #4 of the boot sequence.

The init process essentially means loading all the services and daemons required by the user after login, to be loaded into the RAM and get the system up and ready.

Now, various system initializers are available - systemd, sysVinit, upstart, openRC, etc. In modern systems - systemd is the default system initializer, however some distros like MXLinux still prefer sysVinit (an older guy in the game).

What does systemd do?

The systemd is an init and service manager. It is the first user-level process that runs until the PC is shutdown. It's process ID id 1.

It does the following tasks:

Once kernel starts the init process, systemd handles all the later stages of the boot process.
It configures the environment by loading all the services and daemons needed to login the user.
It allows managing services, devices, sockets, mount points, swap areas, etc.

Few notes about systemd:

Starts all services parallely.
Can start daemons without restarting the running service.
It is the universal parent of all processes.

What is sysVinit?

sysVinit was initially created for UNIX systems, it later got adapted in Linux. Till RHEL5, RedHat used this as its default system initializer. Thereafter, they switched to systemd.

Few notes about sysVinit:

It loaded services in kind of a synchronous fashion as against parallel in systemd.
A next service can be loaded only after the current one being loaded is timed out.

One final question beginners might have is...

What is systemctl and how is it related to systemd?

systemctl is a command line utility to manage services, get the status of services.
systemd on the other hand is a daemon which keeps running in the background until the machine is shut down. systemctl is simply an interface to access the services provided by systemd.

Finally, if you found this topic interesting, I had highly recommend you to check out this reddit discussion.

If you find any mistakes, feel free to comment. Also, if you want to add any points, the comment section is all yours.

ERROR : "System has not been booted with systemd as init system (PID 1). Can't operate."

Akshay — Tue, 19 Dec 2023 18:57:23 +0000

I encountered this error when I was setting up docker on my recently installed MXLinux machine.
This fix is likely to work on Debian-based distros.

The command was simply to check if docker service is up or not.

sudo systemctl status docker

It gave me the following error:

OR Is it really an error that needs fixing?

Try this command:

sudo service docker status

Explain this to me

Temporary fix

I figured out a way to fix this temporarily by editing the grub command line arguments by adding the parameter
init=/lib/systemd/systemd besides splash quite. You can locate this on a line starting with the linux keyword. However, this turns out to be a temporary fix. So at every reboot, I needed to repeat this procedure of adding this init parameter to the grub edit menu.

Permanent fix

A little bit of time into fixing this and I learnt that those parameters are actually referenced from the /etc/default/grub file. This file is essentially a key-value pair required to generate the desired configuration during the booting process.

From the /etc/default/grub file, the key(variable) of our attention is GRUB_CMDLINE_LINUX_DEFAULT. This variable is passed to the kernel at boot-time. The famous quiet splash attributes to disable the log messages and enabling the splash screen are values to this variable.

Now, we simply have to add the required library that will load the systemd process at every boot by default. Thus, we add a new attribute i.e. the path to the systemd file to the above mentioned variable as follows. Assuming quiet splash already exist there

GRUB_CMDLINE_LINUX_DEFAULT="splash quiet init=/lib/systemd/systemd"

The final step is to reboot the system.

sudo reboot

Try the systemctl command again:

See, it works on my machine ;)

Forem: Akshay