CentOS - Jul 2011 - Lock files in scripts

I was about to ask here how to do proper locking in a bash script when I 
found a page that addressed my objections to the race conditions I was 
finding in most sample code:

<http://www.davidpashley.com/articles/writing-robust-shell-scripts.html>

I just wanted to pass on the link to anyone else that needs this.

One thing not addressed is how to deal with an orphaned lock file (eg. if 
the system crashes with the lock held). He stores the PID in the lock file, 
so one could look up the matching process and see if it's the script
that's
expected to create it. Otherwise one should be safe in deleting the lock 
file and trying again.

--On Wednesday, July 13, 2011 10:16:12 AM -0700 Kenneth Porter
<shiva at sewingwitch.com> wrote:
> I was about to ask here how to do proper locking in a bash script when I 
> found a page that addressed my objections to the race conditions I was 
> finding in most sample code:
> 
>
<http://www.davidpashley.com/articles/writing-robust-shell-scripts.html>
> 
> I just wanted to pass on the link to anyone else that needs this.
> 
> One thing not addressed is how to deal with an orphaned lock file (eg. if 
> the system crashes with the lock held). He stores the PID in the lock
> file,  so one could look up the matching process and see if it's the
> script that's  expected to create it. Otherwise one should be safe in
> deleting the lock  file and trying again.
If you're concerned about an OS crash, you can distinguish that
case by putting your lock files into either a directory that is 
cleaned up by the system on boot (like /var/lock) or to put it
in a tmpfs filesystem (such as if you have /tmp mounted as tmpfs).
If you're using the latter, then watch for other users playing
silly-bugger with your lock files to try to compromise or crash
your system, such as making /tmp/mylockfile a symlink to /etc/passwd
so that you can trash it when you try to write to it.
(See the last paragraph on avoiding it.)

If you're concerned about a script crash, the trap (with 0 1 2 15)
takes care of that.

However, although I like the trap mechanism for dealing with cleaning
up temporary files (especially those files or directories containing
temporary files created by mktemp(1)), I don't think that it's the
right tool for lock files.  Instead, have a look at flock(1) to avoid
the race conditions.  (See flock(2) about its unsuitability for locks
on NFS filesystems, though.)

Devin