How to determine if a file is in use

[Donald Russell]

Another system uses FTP to drop files in a directory for me to process.
I have a bash script to process the incoming files. The script is started by
cron periodically.

There's a problem if the FTP transfer is still in progress because the
process begins reading the file even though it isn't complete yet.

> From a bash script, is there a way to tell if the file is still being
written to?
I was looking at the lsof command, which will tell me if the file is opened
or not, so that's a possibility... but it sure seems awkward for the task.

I could also configure the ftp server to lock files being written, but that
seems to be discouraged. (based on man vsftpd.conf)

Basically, what I want is something like
Can I get an exclusive read on file x?
No - skip that file, go onto the next one
Yes - start processing that file
(I'm not concerned about the possible race condition there... I have other
protections for that)

Thanks for any suggestions...