kernel postinstall script to update systemd-boot on debian/ubuntu

zz-update-systemd-boot

#!/bin/bash
#
# /etc/kernel/postinst.d/zz-update-systemd-boot
# 0755 root:root
#
# This is a simple kernel hook to populate the systemd-boot entries
# whenever kernels are added or removed.
#

set -euo pipefail

ESP=/boot/efi                   # bootctl --print-esp-path
MACHINE=/ubuntu                 # FIXME: use the machine-id ?
ESP_MACHINE=${ESP}${MACHINE}
ESP_LOADER_ENTRIES=$ESP/loader/entries
CMDLINE=`< /etc/kernel/cmdline`

# List of kernels
KERNELS=()
FIND="find /boot -maxdepth 1 -name 'vmlinuz-*' -type f -print0 | sort -rz"
while IFS= read -r -u3 -d $'\0' LINE; do
    KERNEL=$(basename "${LINE}")
    KERNELS+=("${KERNEL:8}")
done 3< <(eval "${FIND}")

# There has to be at least one kernel.
if [ ${#KERNELS[@]} -lt 1 ]; then
    echo -e "\e[2msystemd-boot\e[0m \e[1;31mNo kernels found.\e[0m"
    exit 1
fi

mkdir -p $ESP_MACHINE
      
# Copy the latest kernel files to a consistent place so we can keep
# using the same loader configuration.
LATEST="${KERNELS[@]:0:1}"
echo -e "\e[2msystemd-boot\e[0m \e[1;32m${LATEST}\e[0m"
for FILE in initrd.img vmlinuz; do
    cp "/boot/${FILE}-${LATEST}" "$ESP_MACHINE/${FILE}"
    cat <<- EOF > $ESP_LOADER_ENTRIES/ubuntu.conf
	title   Ubuntu ${LATEST}
	linux   $MACHINE/vmlinuz
	initrd  $MACHINE/initrd.img
	options $CMDLINE
	EOF
done

# Success!
exit 0

Tested on Ubuntu 22.04 on a Scaleway instance. To migrate from Grub to systemd-boot I did this:

1. shutdown instance
2. make a snapshot image
3. power-on
4. apt purge grub* && apt autoremove --purge
5. bootctl install
6. create /etc/kernel/cmdline - just a plain file, mostly a copy paste from /proc/cmdline except any initrd= or BOOT= or similar. mine is just root=PARTUUID=c1789075-d188-466c-b2a8-af2a999514f9 ro console=tty1 console=ttyS0 panic=-1
7. create /etc/kernel/postinst.d/zz-update-systemd-boot (see above, root:root, chmod 0755)
8. run it: /etc/kernel/postinst.d/zz-update-systemd-boot
9. reboot
10. if all is good that's it
    • if all is not good, restore from the snapshot that you took at 2)