Answer by Kamil Maciorowski for How to truncate the streaming output of a redirect?

Solution

At the moment I have genrtr > genrtr.log and with a cron I tried to use > genrtr.log but it doesn't work.

This approach can easily be fixed by using genrtr >> … instead of genrtr > … (while still using > in cron to truncate the file).

The difference is explained in this other answer of mine which can be summarized by the following statement:

>> is essentially "always seek to end of file" while > maintains a pointer to the last written location.

Read the linked answer, it totally matches your case.

Side note

Also rm genrtr.log doesn't help because then the process stops updating the file.

Strictly it does not stop updating the file. The file gets unlinked from the directory, but it's still open, still written to, it still consumes more and more space in the filesystem.

Any new file is a different file even if it takes the same pathname. The process does not update the new file because it never opens it, it doesn't even notice it. The file descriptor used by the process still leads to the old (deleted) file.