# Locking a process **URL:** \ **Category:** Ruby\ **Created:** [June 30, 2009, 3:17am UTC](https://www.ruby-forum.com/t/locking-a-process/171285 "2009-06-30T03:17:53Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![judge](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/9de0a6/32.png) [@judge](https://www.ruby-forum.com/u/judge)\ **Post date:** [June 30, 2009, 3:17am UTC](https://www.ruby-forum.com/t/locking-a-process/171285/1 "2009-06-30T03:17:53Z")
I am in the process of developing a few scripts that: 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 I figure that for 2 & 3, the easiest way to make sure they work is to have them run in a cron job once a minute (or five). (Wake up, check to see if there’s any work, either do it or quit.) The problem is that I need to make sure that if the script starts while another copy of the script is running, it quits. I can work out how to create lock files, stick in PIDs to make sure the script is really there, etc. But it seems like someone must have tackled this problem already (IIRC, I ran across a Perl module that back in my misspent youth). Does anyone know of a gem (or just some canonical code) that handles this easily? Paul PS I’ve also thought about using ‘daemonize’, but I’d still have to worry about locking. ---
**Author:** ![judge](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/9de0a6/32.png) [@judge](https://www.ruby-forum.com/u/judge)\ **Post date:** [June 30, 2009, 3:37am UTC](https://www.ruby-forum.com/t/locking-a-process/171285/2 "2009-06-30T03:37:11Z")
On 30 Jun 2009, at 02:17, Paul A. wrote: > The problem is that I need to make sure that if the script starts > > PS I’ve also thought about using ‘daemonize’, but I’d still have to > worry about locking. This is not for the faint-hearted, but if you grab the Ruby Plumber’s Guide from the link in my sig you’ll find an example of using POSIX semaphores with syscall for process synchronisation on slide 41. This is a work in progress so you’ll need to look up some syscall numbers for your platform, however if you’re not in a hurry I’m hoping to have a ruby/dl version in the next version (for Rails Underground in July) which will avoid this limitation. Ellie ## Eleanor McHugh Games With Brains [http://slides.games-with-brains.net](http://slides.games-with-brains.net) raise ArgumentError unless @reality.responds\_to? :reason ---
**Author:** ![judge](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/9de0a6/32.png) [@judge](https://www.ruby-forum.com/u/judge)\ **Post date:** [June 30, 2009, 4:39am UTC](https://www.ruby-forum.com/t/locking-a-process/171285/3 "2009-06-30T04:39:53Z")
Paul A. wrote: > The problem is that I need to make sure that if the script starts while > another copy of the script is running, it quits. I can work out how to > create lock files, stick in PIDs to make sure the script is really > there, etc. But it seems like someone must have tackled this problem > already (IIRC, I ran across a Perl module that back in my misspent youth). What I do for this is simply rename the process: abort(“already running”) if `pgrep -f my-wonderful-process` != “” $0 = “my-wonderful-process” ---
**Author:** ![judge](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/9de0a6/32.png) [@judge](https://www.ruby-forum.com/u/judge)\ **Post date:** [June 30, 2009, 6:52pm UTC](https://www.ruby-forum.com/t/locking-a-process/171285/4 "2009-06-30T18:52:19Z")
11:39am, Daniel DeLorme wrote: > > Cool, thanks. I like this. Of course, it’s still possible to have a race condition, but it should be pretty unlikely in my case. Paul ---
**Author:** ![judge](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/9de0a6/32.png) [@judge](https://www.ruby-forum.com/u/judge)\ **Post date:** [July 16, 2009, 8:27pm UTC](https://www.ruby-forum.com/t/locking-a-process/171285/5 "2009-07-16T20:27:21Z")
> The problem is that I need to make sure that if the script starts while > PS I’ve also thought about using ‘daemonize’, but I’d still have to worry > about locking. Just a followup: I’ve taken a closer look at ‘daemons’, and it actually will only allow one instance of the script to run at once. That combined with a cron job to make sure the script is still running, and I’m good to go on this front. Paul ---
**Author:** ![judge](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/9de0a6/32.png) [@judge](https://www.ruby-forum.com/u/judge)\ **Post date:** [June 30, 2009, 10:46am UTC](https://www.ruby-forum.com/t/locking-a-process/171285/6 "2009-06-30T10:46:53Z")
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.