Pi Presentation

This project enables you to quickly turn a RaspberryPi running Raspbian into a display of content stored in a format that LibreOffice's Impress can open and display.

Installation

To install the presentation software follow these four steps:

sudo apt-get install git
git clone https://github.com/jjshoe/pi_presentation.git /tmp/pi_presentation
sudo cp /tmp/pi_presentation/30-usb.rules /etc/udev/rules.d/30-usb.rules
sudo cp /tmp/pi_presentation/pi_presentation.pl /usr/local/bin/pi_presentation.pl

You should double check your permissions and ownership:

sudo chown root:root /etc/udev/rules.d/30-usb.rules /usr/local/bin/pi_presentation.pl
sudo chmod 644 /etc/udev/rules.d/30-usb.rules
sudo chmod 744 /usr/local/bin/pi_presentation.pl

USB Drive Layout

Pi Presentation looks for any USB storage device it can mount. After it mounts the device it looks for a directory in the root of your USB device called: presentation. Inside the presentation directory it looks for a presentation file name that begins wih current. Any file ending LibreOffice Impress supports should work.

Technical

UDEV

Udev rules control what happens when a new device is discovered on the dbus. For a full understanding of udev rules check out Daniel Drake's great tutorial.

First Rule

KERNEL=="sd?[0-9]", SUBSYSTEMS=="usb", ACTION=="add", RUN=="/usr/local/bin/pi_presentation.pl %k start", ENV{UDISKS_PRESENTATION_HIDE}="1"

Explanation

Match any kernel device we see that starts with sd, followed by any single character, followed by a single number 0 through 9. It should be from the USB subsystem, and we only want to act when the device is added to the system. The run command is called with two options, %k and start. The start option is used by the perl script to denote the flow of code to take. The variable %k is specific to udev, and stands for the device that was added. Example values for %k are sda1 sdc3 and sdd2. The last pice ENV{UDISKS_PRESENTATION_HIDE}="1" is an environment variable we set for the running program. It's a way to tell the presentation system to ignore the fact that a disk was added. This prevents a popup from coming up asking you what you would like to do with this device, however this also stops it from being mounted.

Second Rule

KERNEL=="sd?[0-9]", SUBSYSTEMS=="usb", ACTION=="remove", RUN=="/usr/local/bin/pi_presentation.pl %k stop", ENV{UDISKS_PRESENTATION_HIDE}="1"

Explanation

The KERNEL SUBSYSTEMS and ENV sections all match the first rule. The ACTION has changed to remove and the RUN section passes in stop instead of start. That means that this rule matches when the device is removed, and we let the script know to stop the presentation.

Script

The script that gets called by udev, pi_presentation.pl, is a perl script. It has six variables at the top that can be tweaked to match your RaspberryPi linux distribution specifics. After defining these variables it takes two main program flows, start and stop.

Variables

$mount_path Where your USB device should be mounted (/media/PI_PRESENTATION) $mount_cmd Where to find the command to mount devices (/bin/mount) $umount_cmd Where to find the command to un-mount devices (/bin/umount) $libreoffice_Cmd Where to find the command to launch LibreOffice (/usr/bin/libreoffice) $pid_path Where to write out the PID of the running scripts (/var/run/pi_presentation.pid) $x_user The name of the user running X (pi)

Start

1. Fork
   1. Parent Process
      1. Make the directory $mount_path (default: /media/PI_PRESENTATION) where we will mount the device
      2. Set ourselves as the parent to a process group
      3. Mount the device read only
      4. Launch LibreOffice's Impress to play our presentation
   2. Child Process
      1. Write out process ID to $pid_path (default: /var/run/pi_presentation.pid)

Stop

1. Read the process ID in from $pid_path (default: /var/run/pi_presentation.pid)
2. Send a TERM (-15) to our process ID. Since we setup a process group this takes care of all children.
3. Delete $pid_path (default: /var/run/pi_presentation.pid)
4. Repeatedly attempt to un-mount $mount_path (default: /media/PI_PRESENTATION) the device until succesful