Unattended single disk resize

README.md

1. Create /usr/share/initramfs-tools/hooks/parttools as above
2. Run update-initramfs -u
3. Reboot
4. Type e on the Grub2 boot menu entry
5. Add break=mount to the kernel line
6. In initramfs, run this (assuming /dev/sda1 is the to-be-resized disk):

e2fsck -f /dev/sda1
resize2fs /dev/sda1 20G
fdisk /dev/sda
# print
p
# delete part 1
d
1
# new primary part 1, start as suggested, end +20G
n
p
1
<return>
+20G
# make part 1 bootable
a
1
# new primary part 3, start as suggested, end as suggested
n
p
3
<enter>
<enter>
# write
w

usr_share_initramfs-tools_hooks_parttools

#!/bin/sh

set -e

PREREQ=""

# Output pre-requisites
prereqs()
{
        echo "$PREREQ"
}

case "$1" in
    prereqs)
        prereqs
        exit 0
        ;;
esac

. /usr/share/initramfs-tools/hook-functions

copy_exec /sbin/parted /sbin
copy_exec /sbin/resize2fs /sbin
copy_exec /sbin/fdisk /sbin
copy_exec /sbin/blockdev /sbin
copy_exec /usr/bin/realpath
copy_exec /usr/bin/dirname