How does this UNIX command function? cp 2>&1 | tee err.txt?

Question

I wanted to capture the error into a file as well as display the error in stdout. My friend gave me this command. I know that tee captures the output/error into a file but really, how does the given command work?

Answer 1

You understood Tee only half of what it does, it stands for T-bone, it copies and T-boning a stream of input into 2 streams of identical outputs, one stdout, the other err.txt as the command says.

Answer 2

In unix, 0,1 and 2 are file descriptors corresponding to stdin (standard input), stdout (standard output) and stderr (standard error).

2>&1 is redirecting standard error into standard output. So, along with standard output and standard error, everything will be present in err.txt