Update LXC Debian 12 to 13 checking if the container can be upgraded

upgrade_debian_12_to_13.sh

#!/bin/bash
set -euo pipefail

#based on https://gist.github.com/aerodomigue/0320ec3dffbc60caeaee8a6fc83cb470, thank you aerodomigue

# --- Safety check: only run on Debian 12 (bookworm) ---
if [ ! -r /etc/os-release ]; then
    echo "ERROR: /etc/os-release not found, cannot verify OS. Aborting." >&2
    exit 1
fi

. /etc/os-release

if [ "${ID:-}" != "debian" ]; then
    echo "ERROR: This is not Debian (detected ID='${ID:-unknown}', PRETTY_NAME='${PRETTY_NAME:-unknown}')." >&2
    echo "This script only supports upgrading Debian 12 (bookworm) to 13 (trixie). Aborting." >&2
    exit 1
fi

if [ "${VERSION_CODENAME:-}" != "bookworm" ]; then
    echo "ERROR: Expected Debian codename 'bookworm', found '${VERSION_CODENAME:-unknown}'." >&2
    echo "This container is either already upgraded or on an unsupported version. Aborting." >&2
    exit 1
fi

echo "Detected Debian 12 (bookworm). Proceeding with upgrade to Debian 13 (trixie)..."

# Update source from bookworm to trixie
sed -i 's/bookworm/trixie/g' /etc/apt/sources.list
[ -d /etc/apt/sources.list.d ] && sed -i 's/bookworm/trixie/g' /etc/apt/sources.list.d/*.list 2>/dev/null || true

apt-get update

# Upgrade to trixie
DEBIAN_FRONTEND=noninteractive apt-get -o Dpkg::Options::="--force-confold" dist-upgrade -y

# Disable services that break in LXC / containers (harmless if not present)
systemctl disable --now systemd-networkd-wait-online.service || true
systemctl disable --now systemd-networkd.service || true
systemctl disable --now ifupdown-wait-online || true

# Install ifupdown2 (better networking stack for LXC/VMs) — only if it's actually available
if apt-cache show ifupdown2 >/dev/null 2>&1; then
    apt-get install -y ifupdown2
else
    echo "WARNING: ifupdown2 not available in the repos — re-enabling systemd-networkd instead so networking still works." >&2
    systemctl enable --now systemd-networkd.service || true
    systemctl enable --now systemd-networkd-wait-online.service || true
fi

# Cleanup
apt-get autoremove --purge -y
apt-get clean

reboot