[OpenIndiana-discuss] shell script mutex locking or process signaling

[Bob Friesenhahn]

On Thu, 30 May 2013, Edward Ned Harvey (openindiana) wrote:

> I'm looking around, and not finding any great answers.  So far, 
> using mkdir, it's easy to see there exists a way to do mutex 
> locking, and you could easily write your PID into the subdir that 
> was just created; unfortunately, the problem is when a script gets 
> killed, leaving the stale lock.  So I'm looking for something better 
> than "mkdir" to use for locking.

Hard links from an existing file are a robust atomic "locking" 
mechanism.  These have been used on mail servers for several tens of 
years such as in /var/mail.  The shell supports a signal trap 
mechanism so you can handle delivery of a signal and remove the hard 
link.

The main thing which is lacking is a way to wait for a lock beyond 
simple polling.

Bob
-- 
Bob Friesenhahn
bfriesen at simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/