It does manage to get the output of two cisco commands (show log, and show interface description), then simply throws that info to the screen.

If you're looking to see what interfaces are up/down, perhaps a "show ip int brief" (or similar command depending on the device and IOS version) might be more helpful?

But it looks like you might be attempting to grasp at asking how to filter through the log file, looking for up/down messages. I would suggest reading up on regular expressions and the grep command, or you could do it line by line like this:

while(@show_log_output) {
    if (/(up|down)/i) {
        print $_;
    }
}
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