Shell – How to manually boot Arch Linux from preboot emergency shell

arch linuxbootmkinitcpioshelluefi

I broke an Arch Linux installation. Let's say I made a typo in the PARTUUID string. The result is that the boot process can't mount the root file system and stops at the mkinitcpio preboot stage with the error message:

Your are now being dropped into an emergency shell.

Regression #1: mount + exec

blog.entwicklerseite.de has an example how to manually boot from rootfs, like:

# mount /dev/sda2 /new_root/
# exec /usr/bin/switch_root /new_root /sbin/init

That brings these messages to the console output:

Trying to run as user instance, but the system has not been booted with systemd. 
Trying to continue (this will most likely fail) ... 
...
ERROR: Failed to mount the real root device. 
Bailing out, you are now on your own. Good luck.
...

And a kernel panic.

Regression #2: set root + mount + exec

# root=/dev/sda2
# mount /dev/sda2 /new_root/
# exec /usr/bin/switch_root /new_root /sbin/init

No change in the output with that version.

With which commands can I continue booting (manually) from the Arch Linux emergency shell?

Best Answer

Later I found a posting on the Arch Linux forum, in thread "Arch fails to mount my root partition".

The solution to boot Arch Linux from the emergency shell is even less typing:

# mount /dev/sda2 new_root
# exit

Related Solutions

Linux – How to boot into root btrfs file system with minimal initramfs without udev hook

Cause

In version 23, the mkinitcpio resolve_device() function is called only once. When at execution time the drive labels are not yet read, blkid can't lookup the kernel drive (/dev/...) name for the requested label.

Solution

By adding the "without-udev" hook, as listed below, the resolve_device function is left untouched. Though standard available mkinitcpio functionality to ~~override the mount_handler~~ add a run_hook is used to poll until blkid returns a value, or (a timeout of) 10 seconds has passed. Thus the "udev" hook can be removed from mkinitcpio config.

Notes

This solution was created with the help of falconindy.
There was an error message in the early boot phase involving fsck. To remove that message the without-udev hook has been rewritten to use a run_hook instead of a ~~mount_handler~~. The newer code is even shorter.

$ cat /usr/lib/initcpio/hooks/without-udev
#!/bin/ash
# Minimal initramfs files are created without udev.
# This hooks provides a polling disk mount replacement for udev.
# Udev hook can be removed, resulting in smaller initramfs files.

run_hook () {
    local dev timeout sleepval device=$root
    # if udev is running then exit
    [ "$udevd_running" -eq 1 ] && return
    # try for (timeout * sleepval =) 10 seconds to handle slow (USB) devices
    timeout=1000
    sleepval=0.01

    case $device in
        # label to resolve, when resolved the kernel block device also exists
        UUID=*|LABEL=*|PARTUUID=*|PARTLABEL=*)
            while [ $timeout -gt 0 ]; do
                timeout=$((timeout - 1))
                dev=$(blkid -lt "$device" -o device)
                [ -n "$dev" ] && timeout=0 || sleep $sleepval
            done
            ;;
        # kernel named block device, poll for existence
        /dev/*)
            while [ $timeout -gt 0 ]; do
                timeout=$((timeout -1))
                if [ -b "$device" ]; then
                    dev=$device
                    timeout=0
                else
                    sleep $sleepval
                fi
            done
            ;;
    esac
}

# vim:set syntax=sh:

$ cat /usr/lib/initcpio/install/without-udev
#!/bin/bash

build() {
    add_runscript
}

help() {
    cat <<HELPEOF
This hook provides support for booting without the "udev" hook,
including support for UUID, LABEL, PARTUUID, PARTLABEL.
HELPEOF
}

# vim: set ft=sh ts=4 sw=4 et:

Linux – /sbin/init does not exist

Following on from my comment, what I'd recommend you do to correct it is to reboot your system with the livecd (or whatever method you used to boot into the install environment) and redo these steps of the install process:

If it still isn't working after this, I'd suggest checking what filesystem is actually being mounted by the bootloader. Perhaps giving a list of what files are present in the /usr/bin directory would allow us to help you further.

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