The RHEL 7 boot process begins with the BIOS or UEFI firmware initializing hardware and searching for the boot loader. The boot loader then loads the kernel and initramfs into memory, where initramfs provides drivers and systemd to execute units like mounting the actual root file system. Finally, systemd switches the root file system to the hard disk and starts the default target to present the graphical or text login screen.

1. RHEL 7 Boot Process
Team ElectroMech
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2. RHEL7 Boot Process ...
Boot Loader Grub23. Read and passes control to boot loader
Boot Loader Menu4. Read config file from disk and present boot menu
Loads kernel and initramfs5. Load kernel & initramfs in memory, initramfs provides
drivers and entire usable systemd
BIOS UEFI1. POST and initialize some hardware
MBR UEFI Firmware2. MBR or UEFI firmware search boot loader
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3. RHEL7 Boot Process
systemd(from initramfs) execute
all units initrd.target8. initrd.target includes mounting actual root f/s on
/sysroot, mounts /etc/fstab
systemd switch rootfs to harddisk9. root f/s will be switched to /sysroot and reexecute
using hardidisk systemd
systemd looks & start default
target, GUI or text login10. systemd start default target configured in file or CLI
and present login screen.
Boot Loader pass on control to kernel6. Option specified on CLI or config file /etc/grub.d
/etc/default/grub /boot/grub/grub.cfg
Execute /sbin/init systemd PID 17. Kernel initializes all h/w with initramfs, execute
/sbin/init = systemd (working copy) from initramfs as
PID 1 and udev daemon
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