From GRUB to a target: how the system boots

Not running Starting usually ~20 s

Step 8 of 8 · summary · 2 min

Summary

The boot goes in stages: firmware → GRUB → the kernel with its initramfs → systemd → the boot target, on a server — multi-user.target.
systemd-analyze measures the kernel and the services, blame how long each unit took to start, critical-chain whom the boot waited for.
After= and Before= set the order, Wants= and Requires= a dependency; together they mean "start it and wait for it".
A service gets into the boot through a target — WantedBy= and enable — and systemd-analyze verify checks the unit in advance.
The boot at the start of the lesson took longer than usual because of the network wait: the wait service was first in blame, and in critical-chain it sat under network-online.target, which nginx and the terminal waited for.

Ubuntu 24.04