From: Brian Candler <b.candler@...>
Date: 2009-06-30T17:46:55+09:00
Subject: Re: locking a process

If you're running under a Unix-like system, then you can do something as 
simple as this:

File.open("/tmp/mylock","w") do |f|
  raise "Already running" unless f.flock(File::LOCK_EX | File::LOCK_NB)
  ... rest of code
end

Might even be able to combine the open and lock using IO.sysopen, I 
haven't dug into that. Only works if all processes are on the same 
machine of course.

However I don't think it's necessary for your job here:

> 1) pull images off a CF card on my laptop
> 2) move the images to a server
> 3) rename the images on the server according to the EXIF info

All your script needs to do is to 'grab' each file by renaming it using 
File.rename, e.g. File.rename(fn, "#{fn}.working"). If this is 
successful then it can be sure it has the file. If it fails, then it 
means some other program grabbed the file first, so it can skip. However 
if your script is aborted midway through it may leave some files in this 
grabbed state, so they would need renaming back again manually.

Another option is to forget cron, and just start the program in the 
background; it processes whatever files it can, sleeps for 300 seconds, 
and then loops around. This will consume RAM on your machine while the 
process sleeps though.
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