Grub2 Configuration (boot menu)

Modify default boot OS,  in su mode:


1. Find Windows menu entry

cat /boot/grub2/grub.cfg | grep Windows


2. Set Windows as default OS

grub2-set-default "Windows 7 (loader) (on /dev/sda1)"


3. Verity

grub2-editenv list

4. Generate new grub config file

grub2-mkconfig -o /boot/grub2/grub.cfg

5. Reboot

reboot

### GRUB Bootloader Usage and Troubleshooting in Linux Systems #### Understanding GRUB Configuration Issues When encountering issues where GRUB loads but drops into the rescue shell without any errors, this situation might occur due to a missing or incorrectly placed `grub.cfg` file. Specifically, when installing GRUB2 UEFI with the `--boot-directory` option, if the `grub.cfg` is absent, such problems arise. Additionally, changes in the boot partition's number, which is hardcoded into the `grubx64.efi`, can also lead to similar failures[^1]. For instance, suppose one encounters this issue while working within a UEFI environment. In that case, verifying the presence of `grub.cfg` under `/boot/grub/` or ensuring the correct identification of the boot partition becomes crucial. #### Utilizing GRUB Menuentries for Booting ISO Images Directly from Hard Drive Another aspect involves utilizing GRUB menu entries to boot ISO images directly from a hard drive instead of creating physical media like CDs/DVDs. This method applies particularly to distributions like Ubuntu and certain utility ISOs along with other Linux operating systems. By configuring appropriate GRUB configurations, users eliminate the necessity for external storage mediums entirely[^2]. An example configuration snippet could look something like: ```bash menuentry "Ubuntu Live ISO" { set root=(hd0,msdos1) loopback loop /path/to/ubuntu.iso linux (loop)/casper/vmlinuz boot=casper iso-scan/filename=/path/to/ubuntu.iso quiet splash --- initrd (loop)/casper/initrd.lz } ``` This script sets up a new entry named `"Ubuntu Live ISO"` allowing direct booting from specified paths on local drives. #### Handling Multi-Distribution Scenarios Through GRUB Menus In environments hosting multiple distributions alongside each other—such as having both Fedora and Ubuntu installed—the GRUB startup menus play an essential role. After rebooting, selecting options through these menus enables switching between different OS installations seamlessly. For example, choosing the Ubuntu option presented during initial boot leads further into its own specific bootloader interface before finally loading selected kernels based upon user preferences or predefined timeouts[^3]. #### Adjustments Post-Recovery Mode Boots It is worth noting special conditions following boots into recovery modes or instances where previous attempts at starting failed. Under those circumstances, rather than adhering strictly to configured time limits defined inside `/etc/default/grub`, interactive selections become mandatory until valid choices are made by end-users[^5]. --- --related questions-- 1. How does changing the boot order affect how GRUB handles multi-boot scenarios? 2. What steps should be taken to ensure proper installation of GRUB onto removable USB devices intended for live sessions? 3. Can custom scripts automate updates to GRUB configurations whenever system packages change significantly? 4. Is there support for secure boot mechanisms provided natively within modern versions of GRUB? 5. Which tools outside of standard package managers offer advanced diagnostics capabilities concerning GRUB-related issues?
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