systemd-boot update script

systemd-boot does not automatically regenerate entry configuration files like update-grup or grub-mkconfig. So you can use below script for Gentoo Linux.

#!/bin/bash
#
# This is a simple kernel hook to populate the systemd-boot entries
# whenever kernels are added or removed.
#

# The UUID of your disk.
# Note: if using LVM, this should be the LVM partition.
UUID="CHANGEME"

# Intel microcode file name
MCODE="CHANGEME"

# Any rootflags you wish to set. For example, mine are currently
# "subvol=@ quiet splash intel_pstate=enable".
ROOTFLAGS="CHANGEME"

# Our kernels.
KERNELS=()
FIND="find /boot -maxdepth 1 -name 'vmlinuz-*' -type f -not -name '*.old' -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

# 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"
cat << EOF > /boot/loader/entries/gentoo.conf
title   Gentoo Linux
linux   /vmlinuz-${LATEST}
initrd  /${MCODE}
initrd  /initramfs-${LATEST}.img
options root=UUID=${UUID} rw ${ROOTFLAGS}
EOF
#done

# Copy any legacy kernels over too, but maintain their version-
# based names to avoid collisions.
if [ ${#KERNELS[@]} -gt 1 ]; then
        LEGACY=("${KERNELS[@]:1}")
        for VERSION in "${LEGACY[@]}"; do
            echo -e "\e[2msystemd-boot\e[0m \e[1;32m${VERSION}\e[0m"
            cat << EOF > /boot/loader/entries/gentoo-${VERSION}.conf
title   Gentoo Linux ${VERSION}
linux   /vmlinuz-${VERSION}
initrd  /${MCODE}
initrd  /initramfs-${VERSION}.img
options root=UUID=${UUID} rw ${ROOTFLAGS}
EOF
        done
fi

printf "\n"
printf "Updating systemd-boot firmware \n"
bootctl update

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

This script forked from here.