Update `systemd-boot` after kernel update

zz-update-systemd-boot

#!/bin/bash
#
# This is a simple kernel hook to populate the systemd-boot entries
# whenever kernels are added or removed.
#
# Modified from https://blobfolio.com/2018/replace-grub2-with-systemd-boot-on-ubuntu-18-04/

# The UUID of your disk.
# Note: if using LVM, this should be the LVM partition.
UUID=$(findmnt -n / -o UUID)

# Any additional flags you wish to set.
DEFAULT_FLAGS=""

# Distribution pretty name.
. /etc/os-release

# Our kernels.
KERNELS=()
FIND="find /boot -maxdepth 1 -name 'vmlinuz-*' -type f -not -name '*.dpkg-tmp' -print0 | sort -Vrz"
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

rm -rf /boot/efi/loader/entries/ubuntu*.conf
rm -rf /boot/efi/ubuntu
mkdir /boot/efi/ubuntu

# Copy kernel and write systemd-boot entry.
create_entry() {
    NAME="$1"
    SUFFIX="$2"
    FLAGS="$3"

    # Set default flags.
    if [ -z "${FLAGS}" ]; then
        FLAGS="${DEFAULT_FLAGS}"
    fi

    # Set title of boot entry.
    if [ -n "$NAME" ]; then
        TITLE="${PRETTY_NAME} (${NAME})"
        NAME="-${NAME}"
    else
        TITLE="${PRETTY_NAME}"
    fi

    echo -e "\e[2msystemd-boot\e[0m \e[1;32m${TITLE}\e[0m ${FLAGS}"

    # Find real kernel.
    REAL_KERNEL=$(realpath /boot/vmlinuz${SUFFIX} | cut -c 15-)
    if [ -n "$REAL_KERNEL" ]; then
        REAL_SUFFIX="-$REAL_KERNEL"
    else
        REAL_SUFFIX="$SUFFIX"
    fi

    # Copy kernel and initramfs.
    for FILE in initrd.img vmlinuz config System.map; do
        cp "/boot/${FILE}${REAL_SUFFIX}" "/boot/efi/ubuntu/${FILE}${REAL_SUFFIX}"
    done

    # Create systemd-boot entry.
    cat << EOF > /boot/efi/loader/entries/ubuntu${NAME}.conf
# This file is generated by zz-update-systemd-boot. Do not edit.
# Instead, edit /etc/kernel/postinst.d/zz-update-systemd-boot and rerun it.
title   ${TITLE}
linux   /ubuntu/vmlinuz${REAL_SUFFIX}
initrd  /ubuntu/initrd.img${REAL_SUFFIX}
options root=UUID=${UUID} ro ${FLAGS}
EOF
}

LATEST="${KERNELS[@]:0:1}"

create_entry "" "-${LATEST}"
create_entry nohz_full "-${LATEST}" "isolcpus=1-63,65-127 nohz_full=1-63,65-127"

for VERSION in "${KERNELS[@]}"; do
    create_entry "${VERSION}" "-${VERSION}"
done

# Success!
echo -e "\e[2m---\e[0m"
exit 0