find pipe to less - why only correct lines are left when I press "up" key?

Question

I by accident find a good way to filter useful lines of find: just pipe it to less and when it shows (END) at the bottom, press up arrow key and only correct lines are left. No Permission denied, no symbolic link errors, nothing else.

find / -name foo | less

But, why? Cannot find an answer about why this behaviour. less magically filters out non-result lines?

Answer 1

find with no “action” applies its default -print action, which outputs the full file name to standard output. Errors go to standard error.

The pipe operator only redirects standard output; so only “correct” file names are sent to less, everything else goes to standard error, which is your terminal. less also writes to your terminal, so you’ll initially see both file names and errors on your screen; but when you scroll up in less (or invoke any other action which causes it to update the screen), the errors will be overwritten by less’s updates since less is only aware of the input it’s seen from find’s standard output.

To page through the complete output in less, you need to redirect standard error too:

find / -name foo 2>&1 | less

To completely ignore errors, redirect it to the bit bucket instead:

find / -name foo 2>/dev/null | less

Answer 2

This has nothing to do with less itself. It's just that there are two output streams: standard output (stdout) and standard error (stderr). Error messages, as you would expect, go to stderr, while regular output goes to stdout. The pipe, by default, only captures stdout and ignores stderr. Since find's errors are in stderr, these are not sent to less and this is why it looks like less is filtering out the errors.