Modern computer
systems are complex combinations of hardware and software. Starting from an
undefined, powered down state to running system with a login prompt requires
large number of pieces of hardware and software to work together. The
following list gives a high-level overview of the task involved for physical
x86_64 system booting Red Hat Enterprise Linux 7. The list of x86_64 virtual
machine is a roughly the same, but some of the hardware specific steps handled
in software by the hypervisor.
How to Reset the Forgotten Root Password of Red Hat Linux 7 using Commands
- The machine is powdered on. The system firmware runs on Power On Self Test (POST), and starts to initialize some of the hardware.
- The system firmware searches for a bootable device, either configured in the UEFI boot firmware or by searching for a Master boot record (MBR) on all disks, in the order configured in the BIOS.
- The system firmware reads from a
boot loader from disk, then passes control of the system to the boot loader.
How to Extend Logical Volume in Redhat Linux using command line
On a Red Hat enterprise Linux 7 system, this will typically be grub2. - The boot loader loads its configuration from disk, and presents the user with a menu of possible configuration to boot.
- After the user has made a choice, the boot loader loads configured kernel and initramfs from disk and places them in memory. An initramfs is a gzip-ed cpio archive contains kernel modules for all hardware necessary at boot, init script, and more. On Red Hat Enterprise Linux 7, the initramfs contains an entire usable system by itself.
- The boot loader hands control of the system over the corner, passing in any options specified on the kernel command line in the boot loader, and the location of the initramfs in memory.
- The kernel initializes all hardware for which it can find a driver in the initramfs, then executes /sbin/init from the initramfs as PID 1. on Red Hat Enterprise Linux 7, the initramfs contains a working copy of systemd as /sbin/init, as well as udev daemon.
- The systemd instance from the initramfs executes all units for the initrd.target target. This includes mounting the actual root file system on /sysroot.
- The kernel root file system is switched from initramfs root file system to the system root file system that was previously mounted on /sysroot. systemd then executes itself using the copy of systemd installed on this system.
- systemd looks for a default target, either passed in from the kernel command line or configured on the system, then starts units to comply with the configuration for that target, solving dependencies between units automatically. systemd target is a set of units that should be activated to reach a desired system state. these targets will include at least a text based login for a graphical login screen being spawned.
What is boot, reboot and shutdown
To power off or reboot a running system from the command line,
administrators can use the systemctl command.
Change Default Server Language in CentOS using i18n File
systemctl power off will stop all running services, unmount all
file systems and then power down the system.
systemctl reboot will stop all running services, unmount all
file systems, and then reboot the system.
