# The way to restrict a ruby program to just once occurrence o **URL:** \ **Category:** Ruby\ **Created:** [November 7, 2006, 11:09am UTC](https://www.ruby-forum.com/t/the-way-to-restrict-a-ruby-program-to-just-once-occurrence-o/75078 "2006-11-07T11:09:08Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![Glenn\_S](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/bbce88/32.png) [@Glenn\_S](https://www.ruby-forum.com/u/Glenn_S)\ **Post date:** [November 7, 2006, 11:09am UTC](https://www.ruby-forum.com/t/the-way-to-restrict-a-ruby-program-to-just-once-occurrence-o/75078/1 "2006-11-07T11:09:08Z")
I’m rewriting a vb app to ruby (in about a quarter of the code!). If I ran the same ruby program twice in parallel, the second instance should exit. In VB6 I could do this: ``` If App.PrevInstance Then Exit Sub ``` Any ideas how this is done in ruby? Ta muchly ---
**Author:** ![Glenn\_S](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/bbce88/32.png) [@Glenn\_S](https://www.ruby-forum.com/u/Glenn_S)\ **Post date:** [November 7, 2006, 3:20pm UTC](https://www.ruby-forum.com/t/the-way-to-restrict-a-ruby-program-to-just-once-occurrence-o/75078/2 "2006-11-07T15:20:02Z")
Never mind, solved it (unless anybody knows a better way). It’s a windows app, so: require ‘Win32API’ SW\_HIDE = 0 WS\_OVERLAPPED = 0 WS\_DISABLED = 0x08000000 WS\_MINIMIZED = 0x20000000 def already\_running # # Not sure of any other way to do this. # Create a window with the title “i4FileProcessorHiddenWindow”. When we come to run this program again, # if there is already a window called “i4FileProcessorHiddenWindow” then abort that instance. # In this way, only one instance of the script should run. # ``` findwindow = Win32API.new('user32','FindWindow', ['p','p'], 'L') return true if findwindow.call("STATIC", ``` “i4FileProcessorHiddenWindow”) != 0 ``` createwindow = Win32API.new('user32', 'CreateWindowEx', ['l', 'p', ``` ‘p’, ‘l’, ‘i’, ‘i’, ‘i’, ‘i’, ‘l’, ‘l’, ‘l’, ‘P’], ‘l’) dwStyle = (WS\_DISABLED | WS\_MINIMIZED) cw = createwindow.call( WS\_OVERLAPPED, ‘STATIC’, ‘i4FileProcessorHiddenWindow’, dwStyle, 0, 0, 0, 0, 0, 0, 0, 0 ) show = Win32API.new(‘user32’, ‘ShowWindow’, [‘L’, ‘L’], ‘L’) show.call(cw, SW\_HIDE) ``` return false ``` end On 07/11/06, Glenn S. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > Ta muchly – All the best Glenn Aylesbury, UK ---
**Author:** ![Glenn\_S](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/bbce88/32.png) [@Glenn\_S](https://www.ruby-forum.com/u/Glenn_S)\ **Post date:** [November 7, 2006, 3:30pm UTC](https://www.ruby-forum.com/t/the-way-to-restrict-a-ruby-program-to-just-once-occurrence-o/75078/3 "2006-11-07T15:30:06Z")
Glenn S. wrote: > Ta muchly > > ------=\_Part\_1672\_14945421.1162889756916– Though not very “elegant”, here’s one approach… When the script starts up, run the following line of code, within the BEGIN block of a BEGIN…RESCUE…ENSURE construct. def main() begin lockFile = File.new(“semaphore.lck”, File::CREAT|File::EXCL) rescue Errno::EEXIST puts(“Script is already running”) rescue Exception =\> ex puts(ex.message()) ensure unless lockFile.nil? lockFile.close() File.delete(“semaphore.lck”) end end end main() The first instance of the script will create and open the “semaphore.lck” file. Running another instance of the script will cause the preceeding line of code to throw a Errno::EEXIST error, which is easy enough to trap. In that case, display a message to the user. The ENSURE block will need to close and delete the semaphore file so that, in the case of an abend, the file is not left hanging out there. Otherwise, this will prevent the script from running again until someone manually deletes the file. ---
**Author:** ![Glenn\_S](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/bbce88/32.png) [@Glenn\_S](https://www.ruby-forum.com/u/Glenn_S)\ **Post date:** [November 7, 2006, 5:12pm UTC](https://www.ruby-forum.com/t/the-way-to-restrict-a-ruby-program-to-just-once-occurrence-o/75078/4 "2006-11-07T17:12:27Z")
You could also take a leaf out of unix tradition and simply write a small file containing the current process ID into a temporary directory. When your program starts, it checks for this file, and if a corresponding running Ruby process is found then it can exit (perhaps with a log message or something). If not, then the starting instance would seem to be the only one. - james ---
**Author:** ![Glenn\_S](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/bbce88/32.png) [@Glenn\_S](https://www.ruby-forum.com/u/Glenn_S)\ **Post date:** [November 7, 2006, 3:34pm UTC](https://www.ruby-forum.com/t/the-way-to-restrict-a-ruby-program-to-just-once-occurrence-o/75078/5 "2006-11-07T15:34:06Z")
Is it some kind of GUI app? I don’t know what toolkit you are using, but I do remember from my days as a wxPython jockey that the wxWidgets toolkit came with a api for doing exactly this: wx.SingleInstanceChecker. I imagine that there’s a win32 API that is used to do it, and that should be available to most other toolkits you might be using (e.g. I’m sure Java Swing has something similar). I imagine that such an API should be available even if it isn’t a GUI app (perhaps through Win32API) and I imagine that using this would be slightly cleaner than doing your own version. Admittedly this isn’t much use as all I’m saying is, “yeah, there is a way to do this” without providing much details… Sorry Muz ---
**Author:** ![Glenn\_S](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/bbce88/32.png) [@Glenn\_S](https://www.ruby-forum.com/u/Glenn_S)\ **Post date:** [November 7, 2006, 10:50pm UTC](https://www.ruby-forum.com/t/the-way-to-restrict-a-ruby-program-to-just-once-occurrence-o/75078/6 "2006-11-07T22:50:30Z")
I thought that, the problem being what if the other ruby program crashed without removing the file? I guessed the file would remain. I suppose the check against whether or not the existing pid was a ruby program would mostly remove this problem, but it seemed imperfect. Then again, who’s to say my solution is…! On 07/11/06, James A. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > On 11/7/06, Murray S. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > > > Sorry > > > > > ``` > > > # if there is already a window called > > > dwStyle = (WS_DISABLED | WS_MINIMIZED) > > > > > > ``` > > > > > > > I’m rewriting a vb app to ruby (in about a quarter of the > > > > Any ideas how this is done in ruby? > > > > Glenn – All the best Glenn Aylesbury, UK