I'm sorry, but in my experience, when a programmer tells me that code is correct because it runs fine some of the time, the most likely problem is that that code makes assumptions that are only true some of the time. And, especially when a programmer says his or her multithreaded code is correct because it works correctly when it is tested using a single thread, I have every reason to believe that the program is timing sensitive and any change to the system (added or reduced system load, new hardware, new software, etc.) may affect timing.

You are asking us to tell you why the assignment:

Code:

fetch_file=`sftp -b $batch_file $remote_user@$remote_IP`

is failing without showing us how batch_file, remote_user, and remote_IP are being set. We have no idea what is in remote_file_list_1, but if there are sometimes whitespace characters in that file, it is highly likely that any use of sftp_file_name will fail to do what you intended.

You are not showing us where the problem occurs; you are telling us a line of code sometimes sets a variable to an empty string based on three variables that may have been set incorrectly by code we can't see.

You have the script. If you're convinced it is working correctly and won't let us see what is going on, there isn't anything we can do to help you.

I wish you luck.