19 boot procedure grub2-mkconfig system.unit = emergency.target | /etc/default/grub

By OK -

  1. Power on self test (POST)
  2. boot device (hard disk)
  3. grub 2
  4. kernel & initrd
  5. root fs mounting
  6. systemd



Grub

/etc/default/grub - is the most important configuration file

[svn@svn ~]$ cat /etc/default/grub
GRUB_TIMEOUT=5
GRUB_DISTRIBUTOR="$(sed 's, release .*$,,g' /etc/system-release)"
GRUB_DEFAULT=saved
GRUB_DISABLE_SUBMENU=true
GRUB_TERMINAL_OUTPUT="console"
GRUB_CMDLINE_LINUX="crashkernel=auto rd.lvm.lv=cl/root rd.lvm.lv=cl/swap rhgb quiet"
GRUB_DISABLE_RECOVERY="true"
[svn@svn ~]$


/etc/grub.d/ - additional configuration file

Updating GRUB 2 configuration on BIOS systems

The grub2 packages contain commands for installing a bootloader and for creating a bootloader configuration file.
grub2-install will install the bootloader - usually in the MBR, in free unpartioned space, and as files in /boot. The bootloader is installed with something like:
grub2-install /dev/sda
grub2-mkconfig will create a new configuration based on the currently running system, what is found in /boot, what is set in /etc/default/grub, and the customizable scripts in /etc/grub.d/ . A new configuration file is created with:
[root@localhost ~]# cat /etc/default/grub 
GRUB_TIMEOUT=5
GRUB_DISTRIBUTOR="$(sed 's, release .*$,,g' /etc/system-release)"
GRUB_DEFAULT=saved
GRUB_DISABLE_SUBMENU=true
GRUB_TERMINAL_OUTPUT="console"
GRUB_CMDLINE_LINUX="crashkernel=auto rd.lvm.lv=cl/root rd.lvm.lv=cl/swap rhgb quiet"
GRUB_DISABLE_RECOVERY="true"


grub2-mkconfig -o /boot/grub2/grub.cfg
The configuration format has evolved over time, and a new configuration file might be slightly incompatible with the old bootloader. It is therefore a good idea to first run grub2-install whenever you would need to run grub2-mkconfig.

================
Access boot menu by pressing Escape
system.unit = emergency.target as a boot option start troubleshooting mode
================





================================
modifying grub2 parameters

 [bit@localhost ~]$ cat /etc/default/grub 
GRUB_TIMEOUT=5
GRUB_DISTRIBUTOR="$(sed 's, release .*$,,g' /etc/system-release)"
GRUB_DEFAULT=saved
GRUB_DISABLE_SUBMENU=true
GRUB_TERMINAL_OUTPUT="console"
GRUB_CMDLINE_LINUX="crashkernel=auto rd.lvm.lv=cl/root rd.lvm.lv=cl/swap rhgb quiet"
GRUB_DISABLE_RECOVERY="true"

[bit@localhost ~]$
==========
rhgb quiet - hides booting details

after changed parameters grub2-mkconfig should be run and system rebooted




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