(February 2017)
Minimally, linux only needs three things to boot:
The initramfs is the “initial RAM filesystem”. It’s a cpio (“copy in and out”) archive of a self-contained root filesystem.
Normally, linux maintains an in-memory cache of pages of data read from the storage device; these pages can be discarded or written back to disk if linux needs to free RAM. With a tmpfs, linux effectively mounts this in-memory cache as a filesystem. The rootfs is a special instance of tempfs populated by the initramfs contents.
On boot, Linux copies the contents of the intitramfs cpio archive into the built-in rootfs.
Linux looks for a file called init in the rootfs, and executes it as PID 1.
Init must complete certain tasks (e.g. leading crypto fs drivers) before linux over-mounts rootfs with the real/final root filesystem.
The initramfs must contain all device drivers and tools to mount the final root file system.
loads kernel initramfs kernel
boot –> kernel –> extracts –> fills –> runs –> init loads drivers, loader image initramfs rootfs “init” over-mounts real root /
Get a bootable image on a USB stick, like the recovery/install image for the distribution.
The Debian installer has a “recovery” mode selectable from the boot menu;
this is the same as typing rescue at the boot: prompt.
Boot from recovery media.
Mount the machine’s real root filesystem somewhere (e.g. /mnt).
Use chroot to swap out the recovery rootfs with the machine’s real root filesystem.
Make repairs.
In more detail:
If using the Debian recovery mode, the menu should present a list of partitions from which to choose the root.
If not, find the real root filesystem.
The /sbin/ of our recovery rootfs should contain fdisk.
fdisk -l shows the partitions.
# cd /
# mount -t ext4 /dev/sda2 /mnt
# mount -t proc proc /mnt/proc
# mount -t sysfs sys /mnt/sys
# mount -o bind /dev /mnt/dev
If /boot is on a different partition from /, and we need to fix it (e.g., working with GRUB, performing a kernel upgrade, etc.), mount that partition too:
# mount -t ext4 /dev/sda1 /mnt/boot
Do the same for other filesystems (/var, /usr) on separate partitions to which we’ll need access.
Finally:
# chroot /mnt /bin/bash
http://superuser.com/questions/111152/whats-the-proper-way-to-prepare-chroot-to-recover-a-broken-linux-installation https://www.debian.org/releases/stable/i386/ch08s07.html.en https://wiki.debian.org/GrubEFIReinstall https://www.kernel.org/doc/Documentation/filesystems/ramfs-rootfs-initramfs.txt https://en.wikipedia.org/wiki/Linux_startup_process https://news.ycombinator.com/item?id=13622301 https://en.wikipedia.org/wiki/Initramfs http://serverfault.com/questions/275988/what-is-rootfs https://en.wikipedia.org/wiki/List_of_Linux_distributions_that_run_from_RAM http://www.linuxfromscratch.org/blfs/view/svn/postlfs/initramfs.html