Script to update running libre presentation from Dropbox. Written for a Raspberry PI.

drop_and_play.bash

#!/bin/bash

# Drop And Play
# A script for updating a document from a dropbox URL.
# It checks the etag, if it has changed, it downloads and deploy the new document.
# Call this from Cron.
#
# Originally written for Raspberry PI running a Libre Office impress slideshow
# in an infinite loop (called play.odp)

# Marji Cermak, April 27, 2015


# The shared dropbox URL:
DOCO_URL='https://www.dropbox.com/x/SOMEPATH/test.txt?dl=0'


# The file we download from dropbox must be this type:
PLAY_FILE_TYPE='OpenDocument Presentation'

# The current document (symlink to a versioned file):
PLAY=~/play.live

# Required space in the PWD.
# Stop if there is less to prevent filling up the disk.
MIN_FREE_SPACE=1000000

# The directory we keep the document versions:
DOWNLOAD_DIR=~/drop_and_play

if [ ! -e $DOWNLOAD_DIR ]; then
  echo "ERR: Directory $DOWNLOAD_DIR does not exist"
  exit 1
fi

# TODO: etag could be read from the currently deployed document filename.
ETAG_FILE="${DOWNLOAD_DIR}/etag"

# A semaphore preventing this script to do any changes, if it exists.
# Can be used for sanity - if there is an error, stop this script from
# downloading again and again.
STOP_FILE=/tmp/stop_drop_and_play


# If this file exists, it means we deployed a new version of the document and
# the application might need a restart.
RESTART_REQUIRED=/tmp/restart_play


# check we are not running already: http://mywiki.wooledge.org/BashFAQ/045
exec 9>/tmp/update_lock
if ! flock -n 9  ; then
  echo "Another instance of this script running.";
  exit 1
fi

# read the currently deployed etag:
if [ -f ${ETAG_FILE} ]; then
  ETAG_LOCAL=$(<${ETAG_FILE})
else
  echo "Warn: no ETAG_FILE found."
fi

# A shared dropbox URL seems to always redirect to a one-time unique URL,
# but the etag stays the same if the file has not changed
ETAG_LINE=$(curl --silent --location --head ${DOCO_URL} | grep ^etag | tr -d '\r')
ETAG=`echo ${ETAG_LINE} | awk '/etag:/ { print $2 }'`
echo "DEBUG: local version: ${ETAG_LOCAL}, remote version: ${ETAG}."
if [ "${ETAG_LOCAL}" == "${ETAG}" ]; then
  echo "DEBUG: Local and remote versions are identical. Exit."
  exit 0
fi

# Check free space, so we don't fill up
FREE_SPACE=$(df -P . | awk 'NR==2 {print $4}')
if [ "${FREE_SPACE}" -lt "${MIN_FREE_SPACE}" ]; then
  echo "ERR: not enouch free space (${FREE_SPACE} free). Exit."
  exit 1
fi

# Check we are allowed to download
if [ -e "${STOP_FILE}" ]; then
  echo "ERR: not allowed to carry on"
  exit 1
fi
# dropbox has different etag then the local version.
# Download the new version
#
NEW_PLAY="${DOWNLOAD_DIR}/play-${ETAG}.odp"
if [ -e "${NEW_PLAY}" ]; then
  echo "ERR: NEW_PLAY file already exists. $NEW_PLAY. assert."
  touch "${STOP_FILE}"
  exit 1
fi

echo "WARN: Downloading new version ${ETAG}."
curl -o "${NEW_PLAY}" --location ${DOCO_URL}
RV=$?
if [ $RV -ne 0 ]; then
  echo "ERR: failed downloading via curl, RETVAL ${RV}"
  touch "${STOP_FILE}"
  exit 1
fi

if [ ! -f "${NEW_PLAY}" ]; then
  echo "ERR: new downloaded file does not exist. assert."
  touch "${STOP_FILE}"
  exit 1
fi

FILE_TYPE=$(file -b "${NEW_PLAY}")
if [ "${FILE_TYPE}" != "${PLAY_FILE_TYPE}" ]; then
  echo "ERR: retrieved unexpected file type: ${FILE_TYPE}"
  touch "${STOP_FILE}"
  exit 1
fi

# TODO: check the new document size

# Successful download. Replace the old link with a new one.
ln -sf "${NEW_PLAY}" "${PLAY}"
echo "${ETAG}" > $ETAG_FILE
echo "${ETAG}" > $RESTART_REQUIRED
echo "DEBUG: new version installed. App restart is needed."
exit 0