not to nitpick but the cache file is supposed to never be deleted as it will be created for each new combo of host/interface, but whatever you need, just highlighting the comment of the dev : 

so if you delete the file each time, the check does pretty much nothing, (as it need previous information to know how much traffic happened between the 2 checks)

and if the date in the history is too old it will start anew automatically

i’m more focus on this part from your problem : 

it seems you have 2 underscore in your file name, so you may have modified something in the scrtipt between the username and the suffix

and the original message in the code is full of english error and does not match what you pasted, you message does not appear anywhere in the original code:

could you please try the original file where there is not deletion, and check/diff your modification for syntax errors?

there is also a debug mode with “-v” which will make a logfile in /tmp, maybe there will be more information

Hello and thank you for your answer,

I took the original file and here is the error I have and the content of the target file: 

I'll look at the log file later and get back to you
thanks again

This is the content of the log file : 

For these tests, I downloaded and did not modify the check_traffic.sh file

I'll look at your comment about the double underscore, I actually hadn't noticed. I don't know where it comes from, I'll let you know if I find anything, thanks!

Thanks, I solved the problem!

The $Username variable that uses 'id --user --name' was apparently not returning a value in this one.
So I used 'whoami' and it seems to work now!

Thanks a lot for your answer, I couldn't have found it without this one! :)