Linux 7 Boot Process

1. As soon as the machine is powered on, the system firmware(either UEFI or BIOS) runs a Power On Self Test (POST) and starts initialising some hardware.
2. The system firmware then searches for a bootable device, which is either configured in UEFI boot firmware or by searching for Master Boot Record(MBR) on all disks in the order configured in BIOS.
3. The system firmware reads a boot loader from disk and passes control to the boot loader(grub2)
4. The boot loader loads it's configuration from disk and presents user with a menu of possible configurations to boot.
5. The boot loader then loads the kernel and initramfs from disk and place them in memory. An initramfs is a gziped cpio archive containing kernel modules for all hardware necessary at boot, init scripts and more. The initramfs containes an entire usable system by itself.
6. The bootloader hands control of the system to the kernel.
7. The kernel initializes all hardware for which it can find driver in the initramfs, then executes /sbin/init from initramfs as PID 1. The initramfs contains a working copy of systemd as /sbin/init, as well as udev daemon in RHEL 7.
8. The systemd instance from initramfs executes all units for initrd.target. This includes mounting the actual root file system on /sysroot
9. The kernel root file system is switched from the initramfs root file system to system root file system that was previously mounted on sysroot. systemd then re-executes itself using copy of systemd installed on the system.
10. systemd then looks for a default target, then starts(and stops) units to comply with the configuration for that target, solving dependancies between units automatically.