Repairing Linux – edit files or reinstall grub (x86)

 Even if you're working with Linux it could happen that a system doesn't come up again. In the most cases the problem is between the monitor and the keyboard - the user itself....... .

 Let's take an example. An user is encrypting harddisks and usbsticks using LUKS (please see Good LUKS 1 and Good LUKS 2). Within the /etc/fstab entry he's working with the device names instead of the UUIDs. Usually the system wouldn't come up because the device names are not assigned in the right order. The error message could look like in the following screenshot:

Don't panic! In this case you could login as root with the appropriate password. But - you're in a read-only mode. First we've to change in a writeable mode and then modify the /etc/fstab file . To change into the writeable mode we got two possibilities. The fastest is use the mount command and to modify the /etc/fstab file immediately.

Sometimes it is necessary to add the option -n (--no-mtab) for example when /etc is on a read-only filesystem.

The other way is to boot up another Linux client and access the "damaged" instance (in this example I'm using KNOPPIX which is beeing installed on a 8 GB USB stick). The advantage is that you could use a lot of repair tools such as gparted, fdisk, mc....

By the way - it's recommended to determine some options at the bootmenu of KNOPPIX. Specify the appropriate values for example: knoppix64 (64 Bit mode), lang=ch (language Swiss German) and desktop=lxde (choose your preferred desktop environment). Once we're up with our KNOPPIX we open a terminal and request some information.

As we know the root directory '/` is on the encrpyted device sda2. Get the UUID value of sda2.

Now, we're able to open the encrypted disk.

To get detailed information then it's good idea to use dmsetup.

Are there some volume groups and which ones?

Yes, we got one named 'fedora'. Activate the volume group and get the logical volumes.

As next step we've to mount some important directories - at minimum /root, /boot, /dev, /sys and /proc. On KNOPPIX '/mnt' is empty - so mount them into '/mnt'.

After the successful mount, give back the 'damaged' instance to the root - changing the root into /mnt:

Now you're back and could modify your files or install grub again. After done all modifications leave the chroot mode entering 'exit', umount the appropriate mountpoints and reboot the whole system.

Additonal topic: Re-install grub

It may be necessary to re-install grub. 

Usually you've to make an update typing 'update-grub'.

Within Fedora we've to use the command grub2-mkconfig:

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