As you remember from the previous posts, our goal was to install three OSes; Raspian, RetroPie, and Openlec, move all root filesystem to a fast USB key and install a script that will magically reboot into an OS of our choice. In this final blog post of the series, I will wrap up the triple-boot Raspberry Pi install by showing how to remotely run a script — located on the Raspberry Pi — from another system.

If you missed Part 1 and Part 2, I suggest clicking the links and following the series in order. Meet me back here to complete this project.

Overview

To accomplish this feat, the following will need to be done.

• Generate SSH Keys (make a passwordless SSH Connection between your desktop and the Raspberry Pi)
• Upload the public key to the user’s home directory (Yes, for each OS)
• Copy the script to the user’s home directory (again, needs to be done for each OS)
• Grant sudo no password permission for the Pi user

I will be using the Mac OS as my desktop for this example. Generating the keys should be similar on Linux and probably quite different on Windows (quite frankly, don’t do it on Windows — find a Linux box or install one via VirtualBox. I’ll leave that to you…

Create SSH Keys

cd ~/
mkdir .ssh
chmod go-rwx .ssh
cd .ssh
ssh-keygen -b 1024 -t rsa -f id_rsa -P ""
cd .ssh ; ls -la

You should now see yours SSH keys, the private key is id_rsa and the public key is id_rsa.pub. Important thing to note is that the private key will need to stay where it is.

 -rw-------   1 bobby  staff   887B Mar  1 11:35 id_rsa 
 -rw-r--r--   1 bobby  staff   239B Mar  1 11:35 id_rsa.pub

Copy SSH Keys

Firstly, copy the contents of id_rsa.pub file to a temporary location. This is a sample.

ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAAAgQC5IBdipImrYb0jm4i9BlSiim7SnkvW/6K6eSHP+OH1o2Aesg82EaLwE8mTayn7ftrAdEi4zc+UMK3APex0STfxZ8VPABvSj0+7H0LX1hCeyGIS+ing8CEKMS70j8ItJV2EUSlDs91K17+/6VI91e5pjVI3CFkI228bb8cmNhuE6w== bobby@desktop.localdomain

With that done, will we need to boot into each OS and copy this public key into a file called authorized_keys in the user’s ~/.ssh directory. Consult the guide below for the login details of each OS (assuming you haven’t change the default passwords).

So login into the first OS, pick Raspbian.

cd ~/.ssh
touch authorized_keys
nano authorized_keys
(Copy contents of id_rsa.pub)
cd ~
chmod 700 .ssh
chmod 600 .ssh/authorized_keys

Repeat for RetroPie and OpenELEC. Keep in mind that OpenELEC does not have a Pi user, instead uses root and you will have to enable the SSH daemon via the welcome wizard on first boot up of OpenELEC.

You can test whether the keys are working by simply attempting to login via ssh from your desktop. If it doesn’t prompt you for a password, success!

ssh pi@<IP_ADDRESS_OF_RPI>

One more thing, if you have a problem logging into OpenELEC without a password, as I did, do the following from within the OpenELEC SSH shell (after logging in with a password).

chown root:root /storage
chmod go-w /storage

Copy Script

If the above works, all that’s left to do is copy the below script to each OS. I chose to simply copy it to the user’s home directory as change_boot.sh.

If you did not follow Step 2 of Part 2 (i.e.: did not move the boot partitions to a smaller SD card) then use this file.

#!/bin/bash

DEVICE="/dev/mmcblk0p3"
MOUNT_POINT="/tmp/noobs"
NOOBS_CONF="noobs.conf"
SUDO=''

if [ "$#" -ne 1 ]; then
    echo "Usage: $0 <raspbian|retropie|openelec>"
    exit 1
else
    PARTITION=$1
fi

if [ "$(id -u)" != "0" ]; then
    echo "Not root user, using sudo"
    SUDO='sudo'
fi

case $PARTITION in
    raspbian)
        PARTITION=5
        ;;
    retropie)
        PARTITION=7
        ;;
    openelec)
        PARTITION=9
        ;;
    *)
        echo "Invalid partition, exiting..."
        exit 1
esac

if [ ! -d "$MOUNT_POINT" ]; then
    echo "Creating mount point $MOUNT_POINT"
    $SUDO mkdir "$MOUNT_POINT"
fi

if mount | grep "$MOUNT_POINT" > /dev/null; then
    echo "${MOUNT_POINT} is already mounted"
else
    echo "${MOUNT_POINT} is not mounted, mounting..."
    $SUDO mount $DEVICE "${MOUNT_POINT}"
fi

if [ ! -f "${MOUNT_POINT}/${NOOBS_CONF}" ]; then
    echo "${MOUNT_POINT}/${NOOBS_CONF} does not exist, exiting..."
else
    echo "${MOUNT_POINT}/${NOOBS_CONF} exists, good..."
    $SUDO sed -i.bak "s,^\(default_partition_to_boot\s*=\s*\).*$,\1${PARTITION}," ${MOUNT_POINT}/${NOOBS_CONF}
fi

if mount | grep "$MOUNT_POINT" > /dev/null; then
    $SUDO umount "${MOUNT_POINT}"
    $SUDO rmdir "${MOUNT_POINT}"
fi

$SUDO reboot

Else, use this file.

#!/bin/bash

DEVICE="/dev/mmcblk0p3"
MOUNT_POINT="/tmp/noobs"
NOOBS_CONF="noobs.conf"
SUDO=''

if [ "$#" -ne 1 ]; then
    echo "Usage: $0 <raspbian|retropie|openelec>"
    exit 1
else
    PARTITION=$1
fi

if [ "$(id -u)" != "0" ]; then
    echo "Not root user, using sudo"
    SUDO='sudo'
fi

case $PARTITION in
    raspbian)
        PARTITION=5
        ;;
    retropie)
        PARTITION=6
        ;;
    openelec)
        PARTITION=7
        ;;
    *)
        echo "Invalid partition, exiting..."
        exit 1
esac

if [ ! -d "$MOUNT_POINT" ]; then
    echo "Creating mount point $MOUNT_POINT"
    $SUDO mkdir "$MOUNT_POINT"
fi

if mount | grep "$MOUNT_POINT" > /dev/null; then
    echo "${MOUNT_POINT} is already mounted"
else
    echo "${MOUNT_POINT} is not mounted, mounting..."
    $SUDO mount $DEVICE "${MOUNT_POINT}"
fi

if [ ! -f "${MOUNT_POINT}/${NOOBS_CONF}" ]; then
    echo "${MOUNT_POINT}/${NOOBS_CONF} does not exist, exiting..."
else
    echo "${MOUNT_POINT}/${NOOBS_CONF} exists, good..."
    $SUDO sed -i.bak "s,^\(default_partition_to_boot\s*=\s*\).*$,\1${PARTITION}," ${MOUNT_POINT}/${NOOBS_CONF}
fi

if mount | grep "$MOUNT_POINT" > /dev/null; then
    $SUDO umount "${MOUNT_POINT}"
    $SUDO rmdir "${MOUNT_POINT}"
fi

$SUDO reboot

The difference between the two files is simply where the boot partition lies.

Sudo No Password Permission

Because the script needs root access (sudo) to do its thing, and since sudo prompts for a password, we must relax this restriction. The following must be done with both Raspbian and RetroPie and NOT OpenELEC (as the default user is already root).

sudo visudo

If not already there, add the following at the end of the file.

pi ALL=(ALL) NOPASSWD: ALL

Save and close.

All that remains now is how to use the script. Actually that’s quite easy. Assume the Raspberry Pi is running and currently in Raspbian, and assume you’d like to now use RetroPie. From your desktop, do the following.

ssh pi@<IP_ADDRESS_OF_RPI> "./change_boot.sh retropie"

Boot to OpenELEC.

ssh pi@<IP_ADDRESS_OF_RPI> "./change_boot.sh openelec"

Remember, if OpenELEC is currently running, the user is root.

ssh root@<IP_ADDRESS_OF_RPI> "./change_boot.sh raspbian"

Oh, every subsequent reboot will always boot into the last OS selected.

Conclusion

Well, I hope you truly enjoyed this series and invite you to leave a comment if you have questions. Thanks for reading!