The content of the \/etc\/default\/grub<\/strong> file looks like this: <\/p>\n\n\n\n Kernel options that are common to each entry are defined on the GRUB_CMDLINE_LINUX<\/strong> line. For example, if you like to see the full detailed boot messages, delete rhgb quiet<\/strong>. If you like to see the standard boot messages, just delete rhgb<\/strong>. Changes to the setting is made effective by executing the following command: <\/p>\n\n\n\n On UEFI-based systems, the command will be grub2-mkconfig -o \/boot\/efi\/EFI\/centos\/grub.cfg<\/strong> <\/p>\n\n\n\n To list all the menu entries that will be displayed at system boot, issue the following command: <\/p>\n\n\n\n Or alternatively: <\/p>\n\n\n\n The file name \/etc\/grub2.cfg<\/strong>\n is a symbolic link to the grub.cfg file, whose location is architecture\n dependent. It is better to use absolute paths when accessing to a file,\n as in the example above, especially when repairing a system. The \ndefault entry is defined by the GRUB_DEFAULT<\/strong> line in the \/etc\/default\/grub<\/strong> file. However, if the GRUB_DEFAULT<\/strong> line is set as saved<\/strong>, the parameter is stored in the \/boot\/grub2\/grubenv<\/strong> file. It may be viewed by: <\/p>\n\n\n\n Another useful entry in the \/etc\/default\/grub<\/strong> file is: <\/p>\n\n\n\n Together with GRUB_DEFAULT=saved<\/strong>,\n it makes sure that the currently used boot menu entry is automatically \nmarked for the next boot — useful e.g. on a dual-boot system where \nupdating Windows needs a number of reboots. <\/p>\n\n\n\n The \/boot\/grub2\/grubenv<\/strong> file cannot be manually edited. Use the following command instead: <\/p>\n\n\n\n Note that the first entry in the output of the awk command, above, is index number 0. <\/p>\n\n\n\n Now reboot the system. <\/p>\n\n\n\n The «Linux 0-rescue-…» menu entry is to boot the system in rescue mode. This is equivalent to the single-user mode. <\/p>\n\n\n\n In\n addition, with CentOS 7, an emergency mode is also available. In this \nlatter mode, a shell is provided immediately after systemd has started. \nNo other processes will exist and the root filesystem will be mounted as\n read-only. No other filesystems will be mounted. <\/p>\n\n\n\n To\n boot into emergency mode, enter the setup edit display by typing «e» in\n the grub2 menu. Then add «systemd.unit=emergency.target» to the end of \nthe kernel option. <\/p>\n\n\n\nGRUB_TIMEOUT=5\nGRUB_DEFAULT=saved\nGRUB_DISABLE_SUBMENU=true\nGRUB_TERMINAL_OUTPUT=\"console\"\nGRUB_CMDLINE_LINUX=\"crashkernel=auto rhgb quiet\"\nGRUB_DISABLE_RECOVERY=\"true\"<\/pre>\n\n\n\n
[root@host ~]# grub2-mkconfig -o \/boot\/grub2\/grub.cfg\nGenerating grub configuration file ...\nFound linux image: \/boot\/vmlinuz-3.10.0-229.14.1.el7.x86_64\nFound initrd image: \/boot\/initramfs-3.10.0-229.14.1.el7.x86_64.img\nFound linux image: \/boot\/vmlinuz-3.10.0-229.4.2.el7.x86_64\nFound initrd image: \/boot\/initramfs-3.10.0-229.4.2.el7.x86_64.img\nFound linux image: \/boot\/vmlinuz-3.10.0-229.el7.x86_64\nFound initrd image: \/boot\/initramfs-3.10.0-229.el7.x86_64.img\nFound linux image: \/boot\/vmlinuz-0-rescue-605f01abef434fb98dd1309e774b72ba\nFound initrd image: \/boot\/initramfs-0-rescue-605f01abef434fb98dd1309e774b72ba.img\ndone<\/pre>\n\n\n\n
2. How to Define the Default Entries<\/h2>\n\n\n\n
[root@host ~]# awk -F\\' '$1==\"menuentry \" {print i++ \" : \" $2}' \/etc\/grub2.cfg\n0 : CentOS Linux 7 (Core), with Linux 3.10.0-229.14.1.el7.x86_64\n1 : CentOS Linux 7 (Core), with Linux 3.10.0-229.4.2.el7.x86_64\n2 : CentOS Linux 7 (Core), with Linux 3.10.0-229.el7.x86_64\n3 : CentOS Linux 7 (Core), with Linux 0-rescue-605f01abef434fb98dd1309e774b72ba<\/pre>\n\n\n\n[root@host ~]# grep \"^menuentry\" \/boot\/grub2\/grub.cfg | cut -d \"'\" -f2<\/pre>\n\n\n\n
[root@host ~]# grub2-editenv list\nsaved_entry=CentOS Linux (3.10.0-229.14.1.el7.x86_64) 7 (Core)<\/pre>\n\n\n\n
GRUB_SAVEDEFAULT=true<\/pre>\n\n\n\n
[root@host ~]# grub2-set-default 2\n[root@host ~]# grub2-editenv list\nsaved_entry=2<\/pre>\n\n\n\n
3. Rescue Mode and Emergency Mode<\/h2>\n\n\n\n
![\":-)\"\/](\"https:\/\/wiki.centos.org\/moin_static1910\/memodump\/img\/smile.png\")
<\/figure>\n\n\n\n