#
# How to make Raspberry Pi to operate with read-only filesystem to save the sdcard (temporary files to RAM memory etc)
#
# Do everything as root after clean install
#
# This guide is great and somewhat similar:
# https://hallard.me/raspberry-pi-read-only/
#

# Update everything first and installing some convenient tools
sudo bash
apt update
apt upgrade -y

# (OPTIONAL) Enable SSH server for easier remote control and check the IP address for SSH access
systemctl enable ssh
systemctl start ssh
ip addr show

# (OPTIONAL) Installing some useful tools (some may be installed already)
apt install lsof dnsutils creen htop vim nano net-tools jq bat zip unzip ncdu p7zip-full -y

# (OPTIONAL) Setting timezone
timedatectl set-timezone Europe/Helsinki

# Setting /tmp as 200 MB RAM drive
echo 'tmpfs /tmp tmpfs defaults,size=200M 0 0' >> /etc/fstab

# Edit /etc/fstab with nano editor and change these two mount points to to read only:
PARTUUID...something here leave alone... /boot           vfat    defaults,ro          0       2
PARTUUID...something here leave alone... /               ext4    defaults,noatime,ro  0       1


# For boot, disable some common services causing IO. Disable swap memory
# edit /etc/rc.local with nano editor and replace the contents to:

#!/bin/bash

systemctl stop syslog.socket
systemctl stop systemd-journald
systemctl stop systemd-journald.socket
systemctl stop systemd-journald-dev-log.socket
systemctl stop rsyslog.service
pkill rsyslogd
echo 0 > /proc/sys/vm/swappiness
swapoff -a

# create symbolic links to tmp from other temporary directories and files:
cd /var
rm -rf lock run spool tmp
ln -s /tmp lock
ln -s /tmp run
ln -s /tmp spool
ln -s /tmp tmp

cd /var/lib
rm -rf dhcp dhcpcd
ln -s /tmp dhcp
ln -s /tmp dhcpcd

rm -f /var/lib/systemd/random-seed
ln -s /tmp/random-seed /var/lib/systemd/random-seed

# Usually all crons can be removed (taking backups to /root). Use find command first to verify what you are removing:
find /etc/cron*
mkdir /root/cronbackups
cp -a /etc/cron* /root/cronbackups/
rm -f /etc/cron.d/*
rm -f /etc/cron.hourly/*
rm -f /etc/cron.daily/*
rm -f /etc/cron.weekly/*
rm -f /etc/cron.monthly/*

# Edit /lib/systemd/system/systemd-random-seed.service file with nano and add:
#
# ExecStartPre=/bin/echo "" > /tmp/random-seed  under the service section
#
# It should look something like this:

[Service]
Type=oneshot
RemainAfterExit=yes
ExecStartPre=/bin/echo "" > /tmp/random-seed
ExecStart=/lib/systemd/systemd-random-seed load
ExecStop=/lib/systemd/systemd-random-seed save

# Reboot the system, login again and check if the filesystem is RO:
systemctl reboot
...
sudo bash
touch /testfile # this should give read-only filesystem error


#
# Optional tools
#

#
# Example Bash script to remount filesystem to read-only
#
# readonly.bash
#

#!/bin/bash

echo 'Running now: mount -o remount,ro,noatime / ; mount -o remount,ro,noatime /boot'
mount -o remount,ro,noatime / ; mount -o remount,ro,noatime /boot
#sync; mount -f -o remount,ro,noatime / ; mount -o remount,ro,noatime /boot # possibly dangerous with -f (force) option
# use this to see open processes: lsof / | awk '$4 ~ /[0-9].*w/'

#
# Example Bash script to remount filesystem to read and write
#
# readandwrite.bash
#

#!/bin/bash

echo 'Running now: mount -o remount,rw,noatime / ; mount -o remount,rw,noatime /boot'
mount -o remount,rw,noatime / ; mount -o remount,rw,noatime /boot