flock file descriptor argument
 

Hi all,

I'm trying to understand the flock command and got stuck trying to understand the following code example:

(
flock -n 9 || exit 1
# ... commands executed under lock ...
) 9>/var/lock/mylockfile

I understand most of the above code but confused about the number 9. I know it's a file descriptor from the man page and I understand the famous usage of file descriptors like "/dev/null 2>&1" but what's the significance of file descriptor 9 in the above code? Or is it just arbitrary and I can just use number 10 or 199?

The code opens mylockfile as file descriptor 9, then applies the flock command on that file descriptor.

Any number is fine. I suppose a comparably high number avoids possible clashes with existing file descriptors. The man page I just read uses 200.

Note that the last line is executed first: Before the script within parentheses is executed, the shell opens mylockfile.

Thank you for the response, it makes sense now