I am using webmin to update a file for multiple users. Since many users can update the same file at the same time, one user's change may be overwritten by another user.

Webmin doesn't have any option to lock a file.

Is there a way in OS itself to force the lock, make the change and unlock. I tried using rcs, but that I couldn't make it work.

Any ideas/hints/suggestions appreciated.

Webmin has had file locking for quite sometime (~2005ish?) in many modules. You might wish to review if the module is already performing locking or not, there are a lot of different methods... flock, dotfile, etc..

I worked with Webmin Support, but they dont have the file locking I am looking. There is a module called file locking, but what it does is, make sure two processes doesn't write the same file corrupt, BUT it allows overwriting.

Avoiding corruption is all that file locking can do... as soon you as you release the lock another user can change the file anyway!
Either you want to avoid corruption, or you want to limit who is allowed to change the file (ever). The latter is a different qn and is solved using permissions.

I am not sure how permissions can solve what I am looking for. May be I did not explain correctly.

1. user1 and user2 have access to /tmp/file1
2. Created a custom file editor in webmin so that can edit the files thru browser
3. User1 and User2 open /tmp/file1 at the same time and make changes. When they commit changes, whoever saves last will overwrite previous ones changes.

Now, I want to prevent that. When the file is opened by user1 and user2 tries to open the file, I want to throw some user friendly message.

Kindly let me know if you need more information.


thanks

In that case, your custom editor needs to use a locking protocol, and check target file before opening it.
As its a custom program, the protocol can be anything you like.
What lang did you use?

The "custom file editor" is a module in Webmin. I did not write it.