"reset" a ubuntu server to a minimal state without doing a full reinstall

reset_to_minimal.sh

#!/bin/bash
set -e

echo "=== Step 1: Preserve critical configs ==="
# Backup SSH and network configs
cp -r /etc/ssh /root/ssh_backup
cp -r /etc/netplan /root/netplan_backup 2>/dev/null || true
cp /etc/network/interfaces /root/interfaces_backup 2>/dev/null || true

echo "=== Step 2: Stop and disable non-essential services ==="
KEEP_SERVICES="ssh networking systemd-networkd systemd-resolved ufw cron rsyslog"

for service in $(systemctl list-units --type=service --state=running --no-legend | awk '{print $1}'); do
    svc_name=$(echo "$service" | sed 's/.service//')
    if ! echo "$KEEP_SERVICES" | grep -qw "$svc_name"; then
        echo "Disabling: $svc_name"
        systemctl stop "$service" 2>/dev/null || true
        systemctl disable "$service" 2>/dev/null || true
    fi
done

echo "=== Step 3: Mark only essential packages ==="
# Define essential packages to keep
ESSENTIAL="ubuntu-minimal openssh-server openssh-client netplan.io \
           systemd udev apt dpkg bash coreutils util-linux \
           iproute2 iputils-ping net-tools sudo"

# Mark everything as auto-installed first
apt-mark auto $(apt-mark showmanual) 2>/dev/null || true

# Mark essentials as manually installed (protected)
apt-mark manual $ESSENTIAL

echo "=== Step 4: Remove unnecessary packages ==="
apt-get autoremove --purge -y

echo "=== Step 5: Clean up package cache ==="
apt-get clean
apt-get autoclean

echo "=== Step 6: Remove snap packages (optional) ==="
for snap in $(snap list 2>/dev/null | awk 'NR>1 {print $1}' | grep -v snapd); do
    snap remove --purge "$snap" 2>/dev/null || true
done

echo "=== Step 7: Clear logs ==="
journalctl --rotate
journalctl --vacuum-time=1s
find /var/log -type f -name "*.log" -exec truncate -s 0 {} \;
find /var/log -type f -name "*.gz" -delete

echo "=== Step 8: Clear temp files ==="
rm -rf /tmp/* /var/tmp/*

echo "=== Step 9: Restore SSH config (safety net) ==="
cp -r /root/ssh_backup/* /etc/ssh/
systemctl restart ssh

echo "=== Step 10: Reset hostname and hosts file (optional) ==="
# Uncomment below if you want to reset hostname
# hostnamectl set-hostname ubuntu-server

echo "=== Done! Rebooting in 10 seconds... ==="
echo "Press Ctrl+C to cancel reboot"
sleep 10
reboot

chmod +x reset_to_minimal.sh
sudo bash reset_to_minimal.sh

Check SSH is running

systemctl status ssh

Check network is up

ip addr show

Check what packages remain

dpkg --get-selections | grep -v deinstall | wc -l