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Getting Unison file-sync working on Android

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While Daniel Roggen has done a great job in porting unison and associated utilities to Android in a simple, free Android App, the actual synchronization step is left to either (a) the user, at the command line, or (b) using his for-pay sync App.

Not that I object to earning money by selling apps, but 'just because', here are steps that worked for me on my Android phone (which was 'rooted' simply by changing the settings in Settings-Security-Superuser).

Enabling Android Debugging

This makes development of scripts much easier, and uploading/downloading files while doing so.

On a local 'real' machine, plug in the device, and find the idVendor from watching tail -f /var/log/messages. Then create a new udev rules file in (for instance)/etc/udev/rules.d/51-android.rules :

# /etc/udev/rules.d/51-android.rules
SUBSYSTEM=="usb", ATTRS{idVendor}=="0bb4", MODE="0666"

And reload the ruleset :

udevadm control --reload-rules

Thereafter, adb commands will work :

adb -d shell

Dry run through

These instructions were also helpfully given in the Unison App (but having them here makes them much easier to implement).

Go into the right directory :

adb -d shell
su
export HOME=/data/data/net.danielroggen.unison/files/
cd $HOME

Check that unison runs

(one should also check out what version is running on the server, so that they can be matched)

./unison-2.45.28 -version
./unison-2.40.102 -version

Generate key pair

./ssh-keygen -t rsa -f orsa.key -P ""

and then inspect the public key, so that it can be uploaded to the server's .ssh/authorized_keys :

more orsa.key.pub

(remember to pop the orsa.key.pub into the right place on the server...)

Now test the sync command works

Here, you'll need to choose suitable entries for :

  • a (writeable) local directory for the sync'd folder

  • the server's sync'd folder

  • the server's domain (here 'unison.example.com')

  • the user running unison on the server (here 'unison')

  • the server's port (here '23456')

./unison-2.40.102 -auto -fat -sshargs "-i orsa.key -p 23456" -sshcmd ./ssh /storage/sdcard0/ToRead ssh://unison@unison.example.com/ToRead

Running the script on the phone

Having checked all that works, the final script (with comments) can be assembled on the local 'real' machine. Mine (stored in ~/.unison/android/ToRead.sh) looks like :

# Execute this using 'sh', when root

export HOME=/data/data/net.danielroggen.unison/files/
cd $HOME

# check that unison runs (and also check on server, to match)
#./unison-2.45.28 -version
#./unison-2.40.102 -version

# Generate key pair
#./ssh-keygen -t rsa -f orsa.key -P ""
#more orsa.key.pub
# Upload public key into server's ".ssh/authorized_keys"

./unison-2.40.102 -auto -fat -sshargs "-i orsa.key -p 23456" -sshcmd ./ssh /storage/sdcard0/ToRead ssh://unison@unison.example.com/ToRead

# adb -d push ~/.unison/android/ToRead.sh /storage/sdcard0/ToRead.sh

Load the script onto the phone

Choose a suitable place for the script to live, and execute on the local 'real' machine :

adb -d push ~/.unison/android/ToRead.sh /storage/sdcard0/ToRead.sh

Finally, the script can be run using a terminal program on the Android device :

su
cd /storage/sdcard0
sh ToRead.sh

All done!

Of course, the file 'ToRead.sh' can easily be adapted to other sync'd folders, so that you can have multiple sync'd folders being independently updated (or even put into a combined file).

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