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Code:

#!/bin/sh

# This script installs RockPro64 devicetree files into /boot when new kernels
# are installed. It should be copied into /etc/kernel/{postinst.d,postrm.d}/
# and named with a "zz-" prefix (e.g. zz-rockpro64-dtbs).
# https://www.debian.org/doc/manuals/debian-kernel-handbook/ch-update-hooks.html

set -e
exec </dev/null >&2  # Don't use stdin or stdout

version="$1"
pkgname=${DPKG_MAINTSCRIPT_PACKAGE:-kernel package}
if [ -z "$version" ]; then
    echo "W: $pkgname did not pass a version number; exiting."
    exit 0
fi

eval set -- "$DEB_MAINT_PARAMS"
action="$1"

srcdir=/usr/lib/linux-image-${version}/rockchip
fname=rk3399-rockpro64-v2.dtb
dirname=dtbs-${version}
destsubdir=/boot/$dirname/rockchip

if [ "$action" = configure ]; then

    if ! [ -f "$srcdir/$fname" ]; then
        echo "W: $pkgname did not provide $srcdir/$fname; exiting."
        exit 0
    fi

    echo "I: installing $destsubdir/$fname"
    mkdir -p "$destsubdir"
    cp "$srcdir/$fname" "$destsubdir/$fname"

    if [ -L /boot/dtbs ] && [ "$(readlink /boot/dtbs)" != "$dirname" ]; then
        #echo "I: moving /boot/dtbs symlink to /boot/dtbs.old"
        rm -f /boot/dtbs.old
        mv /boot/dtbs /boot/dtbs.old
    fi
    if ! [ -L /boot/dtbs ]; then
        #echo "I: creating /boot/dtbs symlink"
        ln -s "$dirname" /boot/dtbs
    fi
fi

if [ "$action" = remove ]; then

    if [ -L /boot/dtbs ] && [ "$(readlink /boot/dtbs)" = "$dirname" ]; then
        #echo "I: removing /boot/dtbs symlink"
        rm /boot/dtbs

        if [ -L /boot/dtbs.old ]; then
            #echo "I: moving /boot/dtbs.old symlink to /boot/dtbs"
            mv /boot/dtbs.old /boot/dtbs
        fi
        #todo: look for candidates to become .old, so removing two new kernels
        #      in a row won't leave the system without a dtbs symlink
    fi

    if [ -f "$destsubdir/$fname" ]; then
        echo "I: removing $destsubdir/$fname"
        rm "$destsubdir/$fname"
        rmdir "$destsubdir" || true
        rmdir "/boot/$dirname" || true
    fi
fi