Tutorial: Simple Drobox-Based Server for iPad/iPhone

--D. Thiebaut (talk) 12:12, 20 May 2014 (EDT)

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The purpose of this tutorial is to setup a simple server to process photographs taken on a mobile device. Two devices must be linked by a shared Dropbox folder: a Mac running iOS and working as the server, and a mobile device on which the Dropbox app is installed. Whenever a new photo is deposited in the shared Dropbox folder on the mobile device, the image is replicated in the Mac's local Dropbox folder, and an iOS launchd process is automatically called to process the image and generate some new version of it. The resulting new version is automatically replicated in the Dropbox folder on the mobile device. Et voilà!
This type of setup can easily be modified to work in cases where programming in php/javascript is not possible or easy. The advantage of the method presented here is that the launchd script is run with the same privileges as the user, and has access to the the whole set of executable and file systems available to the logged in user.

Overview

This is the scenario we want to implement

1. we put a picture in a a Dropbox folder on our mobile device. Here we assume that the mobile device is an iPad but the idea would work on any mobile device where a Dropbox app is running.
2. Dropbox automatically syncs the picture onto a Dropbox folder on an OSX computer. We assume that the same user is logged in the two devices, the iPad and the OSX computer, so that the two Dropbox folders are automatically synchronized.
3. Once the file appears in the OSX computer, a launchd script discovers it and processes it in some way. For this tutorial we will simply generate a black and white version of it. This new image is automatically synchronized and replicated in the iPad's Dropbox folder.

Step 1: Create a Bash Script to Process the Image

This step should be performed on your Mac/OSX machine.

The first step is create a regular bash script that checks for the presence of a file in a given Dropbox folder, say inputDir, and if found, creates a monochrome version that is stored in a different Dropbox folder, say outputDir. The original file is removed from inputDir to prevent an endless processing of the file.

Source Code

Here's a the script, below:

#! /bin/bash
# makeMonochrome.sh
# D. Thiebaut
# checks the Dropbox folder's iPad directory inputDir
# for an image file. When found, transforms it to 
# black and white and saves the resulting version in 
# outputDir

inputDir=/whereEverYourDropboxFolderIsLocated/Dropbox/inputDir
outputDir=/whereEverYourDropboxFolderIsLocated/Dropbox/outputDir
log=/WhereEverYouWantToSaveTheLog/log.txt


# if there's a file in the inputDir folder, process it, then remove it
if ls ${inputDir}/*  &> /dev/null; then

    date > $log

    file=`ls -1 ${inputDir}/* | head -1`
    basename=${file##*/}
    newFile=${outputDir}/BW_${basename}
    echo "file    = $file" >> $log
    echo "newFile = $newFile" >> $log
    echo "convert $file -monochrome $newFile" >> $log

    # transform to black and white
    convert $file -monochrome $newFile

    # remove the file from the inputDir
    rm $file

    echo "makeMonochrome.sh done!" >> $log
    date >> $log

fi

The script is self explanatory. Make sure to replace the paths of inputDir, outputDir and log with valid paths on your system.

Testing

Make the script executable:

chmod +x makeMonochrome.sh

Then put a jpg or png file in the inputDir folder. Run the makeMonochrome.sh script from the command line. Verify that the result is a new file prefixed with BW_ in the outputDir folder of Dropbox.

Step 2: Control the Script with launchd

Mac OS/X supports a feature-rich system for launching scripts when various conditions occur. It is called launchd. A good tutorial on this system can be found here.
To create a launchd script requires very meticulous setup, and careful attention to details. The setup must be stored in a plist file in the user's ~/Library/LaunchAgents directory on the OS/X machine. We'll call this file makeMonochrome.plist.
One of the recurring problems with launchd scripts is that these scripts are run by the OS with the user's privileges, but without logging into the system, or using the user's environment. As a result, a script that runs perfectly fine from the command line may not run correctly as a launchd script because the launchd script gets a different default path. So we have to assign to the launchd script the user's default path.
Run this command and copy the list of paths generated:

echo $PATH
/opt/local/bin:/opt/local/sbin: . . .  /sw/bin:/sw/sbin:/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin

Save the resulting long string (do not use the one above, it is incomplete and won't work for you) in a safe place.
Create a new file, called makeMonochrome.plist with the code shown below:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
	<key>Label</key>
 	 <string>make.Monochrome</string>

        <key>Program</key>
         <string>/whatEverDirectoryYouScriptSitsIn/makeMonochrome.sh</string>

        <key>KeepAlive</key>
         <false/>

        <key>WatchPaths</key>
         <array>
           <string>/WhateverDirectionYourDropboxFolderSitsIn/Dropbox/inputDir</string>
         </array>

        <key>StandardOutPath</key>
         <string>/UseTheSamePathSpecifiedInMakeMonochrome.sh/log.txt</string>
        <key>StandardErrorPath</key>
         <string>/UseTheSamePathSpecifiedInMakeMonochrome.sh/log.txt</string>

        <key>EnvironmentVariables</key>
         <dict>
           <key>PATH</key>
            <string>pasteHereTheLongPathStringGeneratedInTheStepAbove</string>
         </dict>

</dict>
</plist>

Make sure you edit the file above with information relevant to your directory system, and paste your environment path in the PATH key string (last highlighted line above).

Activate the Launchd Script

Activation is very simple at this point. Assuming that you are in terminal and in the same directory where makeMonochrome.plist is located, simply type:

launchctl load  makeMonochrome.plist

If you get no output after you press ENTER, then everything worked fine!

Testing

Simply copy an image file in the inputDir folder of your Dropbox and see it quickly disappear once a black and white version of it will have been created in the outputDir folder of Dropbox. Verify that the black-and-white file is indeed there.

That's it!

You can now use your iPad and load images in the inputDir folder of your Dropbox app and get their black and white version available a few seconds later in the outputDir folder.