A Detailed Look at the Boot Process in RHEL 7

A Detailed Look at the Boot Process in RHEL 7

In this tutorial we will see a detailed look at the Boot Process in RHEL 7.

Red Hat Enterprise Linux 7 Booting Process

RHEL goes through the boot process when the system is powered up or reset, with the boot process lasting until all enabled services are started and a login prompt appears on the screen.

The following diagram shows the high level stages of a typical Linux boot process:


  • BIOS stands for Basic Input/output System
  • Performs some system integrity checks
  • Searches, loads, and executes the boot loader program.
  • It looks for boot loader in floppy, CD-ROMs, or hard drive. You can press a key (typically F12 of F2, but it depends on your system) during the BIOS startup to change the boot sequence.
  • Once the boot loader program is detected and loaded into the memory, BIOS gives the control to it.
  • So, in simple terms BIOS loads and executes the MBR boot loader.


  • MBR stands for Master Boot Record.
  • It is located in the 1st sector of the bootable disk. Typically /dev/hda, or /dev/sda
  • MBR is less than 512 bytes in size. This has three components as shown:
    • Primary boot loader info in 1st 446 bytes
    • Partition table info in next 64 bytes
    • MBR validation check in last 2 bytes.
  • It contains information about GRUB2 (GRUB or LILO in old systems).
  • So, in simple terms MBR loads and executes the GRUB2 boot loader.


  • The default bootloader program used on RHEL 7 is GRUB 2. GRUB stands for Grand Unified Bootloader. GRUB 2 replaces the older GRUB bootloader also called as legacy GRUB.
  • The GRUB 2 configuration file is located at /boot/grub2/grub.cfg (Do not edit this file directly).
  • GRUB 2 menu-configuration settings are taken from /etc/default/grub when generating grub.cfg.
  • Sample /etc/default/grub file:

  • If changes are made to any of these parameters, you need to run grub2-mkconfig to re-generate the /boot/grub2/grub.cfg

  • GRUB2 searches the compressed kernel image file also called as vmlinuz in the /boot
  • GRUB2 loads the vmlinuz kernel image file into memory and extracts the contents of the initramfs image file into a temporary, memory-based file system (tmpfs).
  • The initial RAM disk (Initrd) is an initial root file system that is mounted before the real root file system.

About Initramfs  –  /etc/dracut.conf

  • The job of the initial RAM file system is to preload the block device modules, such as for IDE, SCSI, or RAID, so that the root file system, on which those modules normally reside, can then be accessed and mounted.
  • The initramfs is bound to the kernel and the kernel mounts this initramfs as part of a two-stage boot process.
  • The Dracut utility creates initramfs whenever a new kernel is installed.
  • Use the lsinitrd command to view the contents of the image created by dracut:

  • So, in simple terms GRUB2 just loads and executes Kernel and initramfs images.


  • Linux Kernel is the central core of the OS and it is the first program loaded on the system starts up. While system starting kernel loads all the necessary Kernel Modules and Drives from initrd.img to load system first process systemd in Linux 7.
  • The kernel starts the systemd process with a process ID of 1 (PID 1) as shown below:


  • Systemd process is the first process ID (PID 1) to run on Linux 7 systems, it initializes the system and launches all the services that were once started by the traditional init (/etc/init.d) process. Systemd process reads the configuration file of /etc/systemd/system/default.target, then its load the OS in targeted target.
  • This tells systemd to start everything in the /usr/lib/systemd/system/basic.target before starting the other multi-user services.
  • systemd brings the system to the state defined by the system target, performing system initialization tasks such as:
  • Setting the host name
  • Initializing the network
  • Initializing SELinux based on its configuration
  • Printing a welcome banner
  • Initializing the system hardware based on kernel boot arguments
  • Mounting the file systems, including virtual file systems such as the /proc file system
  • Cleaning up directories in /var
  • Starting swapping
  • systemd uses ‘targets’ instead of runlevels. By default, there are two main targets:
    • multi-user.target: analogous to runlevel 3
    • target: analogous to runlevel 5

Note: Refer Man pages for more details: info grub2

Boot, Reboot and Shutdown

To power off or reboot a running system from the command line, we can use the systemctl command.

  • systemctl poweroff will stop all running services, unmount all file systems (or remount them read-only when they cannot be unmounted), and then power down the system.
  • systemctl reboot will stop all running services, unmount all file systems, and then reboot the system.

Note:  systemctl halt and halt are also available to stop the system, but unlike their poweroff equivalents, these commands do not power off the system; they bring a system down to a point where it is safe to manually power it off.

Managing GRUB

The /etc/default/grub File

The /etc/default/grub configuration file defines directives that govern how GRUB behaves at boot time. Values defined in this file are used to regenerate the /boot/grub2/grub.cfg file, which controls the behavior of GRUB at boot time. Any changes made to the grub file will only take effect after the grub2-mkconfig utility has been executed.

The /boot/grub2/grub.cfg File

This is the main configuration file that controls the behavior of GRUB at boot time. This file is located in the /boot/grub2 directory on BIOS-based systems and in the /boot/efi/EFI/redhat directory on UEFI-based systems. This file can be regenerated manually with the grub2-mkconfig utility, or it is automatically regenerated when a new kernel is installed. During this process, any manual changes made to this file are lost.

Here is how you would run this utility to reproduce the grub.cfg file on BIOS and UEFI systems, respectively:

When this utility runs, it uses the settings defined in the /etc/default/grub file and in the helper scripts located in the /etc/grub.d/ directory to regenerate this file for kernels located in the /boot directory. Here is the list of default helper scripts located in the /etc/grub.d/ directory:

The first script, 00_header, sets the GRUB environment; the 10_linux script searches for all installed kernels on the same disk partition; the 30_os-prober searches for the presence of other operating systems; and the 40_custom and 41_custom scripts are for us to add any customization to the new GRUB configuration file.

The grub.cfg file contains menuentry blocks for each installed kernel. Each block begins with a title and includes the names of the kernel and RAM disk image files, their location with respect to /boot, and several options and modules to be loaded. These menu entry titles are displayed at the time of system boot and you can choose one of them to boot.

If a new kernel is added to the system, existing kernel entries will remain in this file and can be chosen in the GRUB menu at startup to boot.

Important Directives:

  • To troubleshoot a broken grub2 configuration, an administrator will need to understand the syntax of /boot/grub2/grub.cfg Actual bootable entries are encoded inside menuentry blocks. In these blocks, linux16 and initrd16 lines point to the kernel to be loaded from disk and the initramfs to be loaded.
  • The set root lines inside those blocks do not point to the root file system for the RHEL 7 system, but instead point to the file system from which grub2 should load the kernel and initramfs files. The syntax is (harddrivepartition) where hd0 is the first hard drive in the system, hd1 is the second etc… the partitions are indicated as msdos1 for the 1stMBR partition or gpt1 for the 1st GPT partition on that drive.

Selecting a systemd target (runlevel in older version)

A systemd target is a set of systemd units that should be started to reach a desired state. The most important of these targets are listed in the following table:

  • To view all available targets

  • To view all targets installed on disk

  • Selecting a target at runtime

On a running system, we can choose to switch to a different target using the systemctl isolate command as shown.

Isolating a target will stop services not required by that target and start any required services that have not yet been started.

Note: Not all targets can be isolated. Only targets that have AllowIsolate=yes set in their unit files can be isolated.

Setting a default target

When the system starts, and control is passed over to systemd from the initramfs, systemd will try to activate the default.target target. Normally, the default.target target will be a symbolic link (in /etc/systemd/system) to either graphical.target or multi-user.target.

Instead of editing this symbolic link by hand, the systemctl tool comes with two commands to manage this link: get-default and set-default as shown below:

  • To view current default target, run: systemctl get-default, you will see

  • To set a default target, run:

I hope this article will help you understanding the basic components of booting process and important files related to grub. let me know if you have any questions or concerns!!

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A Detailed Look at the Boot Process in RHEL 7
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