Concoctions of Uselessness

For something I was working on needed to keep two folders in sync with eachother. Especially, one of the folders was the write&read folder and the other was only a read folder. That is the latter will be untouched by the user the only thing that write to the folder should be the sync program. After looking at the idiosyncrasies of rsync, i settled to use Unison[1].

$ apt-get install unison

~/mypictures » ls -l

and just wanted to share to share it across to another folder ~/mypicsbacked. All I had to do is run the following command

But once I pressed the RETURN on that command, I get this...

This message is shown becasue this is the first time I am synchronizing these folders, as the reading through this will suggest and never I bothered about all the dhcp crap as this synchronization was inside the same machine. Yeah thats right, unison can take care of remote synchronization as well, worked pretty well, but scoping that out off this article. All I did was press the spacebar. Then you should see some thing like this.

I guess that f was for "force", but I was wrong after trying about a bit I figured that "?" will give a short help. And it turns out that "f" stands for "Follow unison's recommendations", so I did. And ended up with this,

Now I just pressed the "?" again...

And then keyed in a "y" to complete the process, of syncing. 

All this to and fro of prompts and answers between unison and me really did not auger well for the solution that I intended to figure out. So I had to figure out a silent way of doing this. I had not look no further that "-help" option and found exactly what I needed.

So my final command turned out to be

This was only half the problem solved. I had to figure out how do I run this every time something changed in ~/mypictures how to I get that to the ~/mypicsbacked. I was looking for some kinda notifier on file changes to the source directory that, could trigger a process to unison. It turned out there is something called "incrond"[2], a very similar utility to cron but works using the inotify module, a process can be triggered whenever there are any filesystem changes on a specific directory. The definition for what needs to run for what filesystem event have to be defined in an incrontab.

# apt-get install incron

And immediately tried incrontab, but it turns out that I cannot run this command unless I have authorized users to run this command. I looked at the /etc/incron.conf, and apperently there is a file /etc/incron.allow where I had to list the user who are allowed to run the incrontab or rather the user who are allowed to use incron. So I went ahead and edited /etc/incron.allow and myself.

# service incron restart
$ incrontab -e

There was nothing in the file, to help me out. I figured out that you need to specify the folder that needs looking into, and what changes need to monitored, and the command that need to run. Thats pretty simple.

So my incron command looked something like this...

The IN_CREATE, IN_DELETE and IN_MODIFY are the inotity masks for the signify, create, delete and modify operations respectively. Also I choose to give absolute paths to avoid any ambiquity. Now to check my setup works well, I copy a file in the origin directory into itself with a different name.

And evidently this should appear on the synced destination folder, ~/mypicsbacked

Well, you can see that worked. Now I assure you this is just the beginning of using a great tool, would be exploring to see how to I get the unison to work with S3, and also explore its myriad of option in the coming days. But I have scratched my initial itch.