When your Ubuntu stops booting for some reason and you need to gain access to it, you can simply start a LiveCD, mount the root filesystem and then chroot into the system.

Boot From LiveCD

The easiest way is to download Ubuntu desktop and ‘burn it’ to a USB stick. The Ubuntu Desktop installer is a live-cd which means you can boot off your USB stick, open terminal and start fixing.

Mount Root Filesystem

When the USB stick boots up, close the installer and start gparted. This will give you the overview of all block devices you have, and you’ll be able to mount them easily. First, let’s focus on mounting the root filesystem; we’ll mount /boot and EFI later on, based on your /etc/fstab.

Also, see LUKS, LVM and Linux Boot for command-line tools which print your block devices.

LUKS/LVM

If you use LUKS for encryption, open the device via

$ sudo cryptsetup open /dev/xyz6 xyz6_crypt

TODO LVM

Mounting Root Filesystem

Run

$ sudo mount /dev/mapper/xyz6_crypt /mnt

Note: If you’re using btrfs, chances are you’ll see folders /mnt/@ and /mnt/@home. Those are subvolumes and you need to mount them in a different way:

$ sudo umount /mnt
$ sudo mount -o subvol=@ /dev/mapper/xyz6_crypt /mnt
$ sudo mount -o subvol=@home /dev/mapper/xyz6_crypt /mnt/home

LUKS revisited

If you’re using LUKS: Check out /mnt/etc/crypttab: if it says something else than xyz6_crypt, you’ll need to start over:

• unmount /mnt/boot/efi, /mnt/boot, /mnt/home and finally /mnt
• close LUKS: sudo cryptsetup close xyz6_crypt
• open LUKS with the proper name from your crypttab via sudo cryptsetup open /dev/xyz6 my_crypt_thingy_from_crypttab

Mounting Everything Else

cat /mnt/etc/fstab and mount all other filesystems (except for swap). Chances are there is at least /boot/efi (and /boot if your root is encrypted). If your fstab says UUID=xyz, just mount /dev/disk/by-uuid/xyz. Don’t forget the options, especially subvol in case of btrfs.

chroot

Mount some more stuff and chroot:

$ sudo mount -t proc proc /mnt/proc
$ sudo mount -t sysfs sys /mnt/sys
$ sudo mount -o bind /dev /mnt/dev
$ sudo mount -t devpts pts /mnt/dev/pts
$ sudo chroot /mnt

You’re now chrooted in your existing system. You can perform any maintenance as required, by using command-line.