Sync EFI System Partition to redundant copy if necessary

sync-esps.sh

#!/bin/sh

# Keep two different EFI System Partitions (ESP) in sync, because it's 2021 and
# EFI still doesn't support software RAID properly. The idea being to add boot
# entries for both:
#
# $esp1/grubx64.efi
#
# …and:
#
# $esp2/grubx64.efi
#
# …using efibootmgr, and then either can be used to boot from.
#
# This script is then called after every package update to see if the main ESP
# ($esp1) needs to be synced to the secondary one.
#
# You configure that by making a file like /etc/apt/apt.conf.d/100sync-esps
# containing:
#
# DPkg::Post-Invoke { "/usr/local/sbin/sync-esps.sh"; };
#
# That will run it after every time an apt tool (any of them) calls "dpkg".
#
# Based on script by Sven Hartge:
# https://www.mail-archive.com/debian-user@lists.debian.org/msg762787.html

esp1="/boot/efi/EFI/debian/"
esp2="/boot/efi2/EFI/debian/"

if [ -d "$esp1" ] && [ -d "$esp2" ]; then
    if ! diff -rq "$esp1" "$esp2"; then
        printf "%s differs from %s; syncing…\\n" "$esp1" "$esp2"
        rsync -rv "$esp1" "$esp2"
        printf "Done.\\n"
        exit 0
    fi
else
    printf "I don't seem to have both ESPs (%s and %s) mounted\\n" \
        "$esp1" "$esp2"
    printf "Run %s again once you've sorted it out.\\n" "$0"
    exit 1
fi