how does `ls "*" 2>&1 | tee ls.txt` work?

Question

I was finding a way to save all output to a file and print it.

And the command like the following does work perfectly!

ls "*" 2>&1 | tee ls.txt

But I think I don't understand it well.

And I tried ls "*" | tee ls.txt. It doesn't work. The error message was not saved into ls.txt.

Also I tried ls "*" 1>&2 | tee ls.txt. It behaved some strange. What happened?

Answer 1

2>&1 says "redirect stderr (2) to wherever stdout (1) goes". 1 and 2 are the file descriptors of stdout and stderr respectively.

When you pipe ls output to tee, only stdout goes to tee (without 2>&1). Hence, the error messages are not saved into ls.txt.