GRUB & Systemd
GRUB
GRUB (GRand Unified Bootloader): Configurable bootloader program.
Versions:
- GRUB Legacy (1999)
- GRUB2 (2005): A complete rewrite of the legacy GRUB.
We can configure and show a menu-based system where you can choose which Kernel or chainloader to boot. It is also possible to edit the menus on the fly or give direct commands from a command line, all before booting into a Kernel.
We can also configure splash logo, colors, etc… of the GRUB menu.
And of course, we can change Kernel startup parameters like which systemd.unit to keep as target, which initramfs to use, etc…
The GRUB configuration is possible by editing files in /etc/grub.d directory.
Systemd
Hated because: doesn’t store logs in text files and violates UNIX principles of KISS (keep it simple stupid) since it does many things beyond what is sufficient for a single tool. Most distros today use it anyways!
Made around units. A unit can be a service, an action, a group of service enabling a certain functionality on the system (target), etc…
It is goal-driven. It acheives a goal by loading a target, which in turn can also have dependencies (other targets or services) that needs to be loaded, so those dependencies are also loaded before the goal is declared “acheived”.
Use systemctl command to config units and journalctl command for logs.
Runlevels and Boot Targets
System V - Runlevels: stages in which the system can go into. Ex - Shutdown, single-user mode, reboot, etc…
Systemd - Boot targets: units that we can start, stop, configure to start after each other, etc…
Some Systemd boot targets:
poweroff.target - shutdown system
rescue.target - single user mode
multi-user.target - multiuser with networking
graphical.target - multiuser with networking and GUI
reboot.target - restart
The default boot goal of default.target usually points to the graphical.target