Question

I know 0 , 1, 2 are STDIN , STDOUT and STDERR file descriptors.
I am trying to understand redirection.
‘>’ means dump to a file
‘>>’ means append

But what does ‘>&’ do ?
Also what is the step by step process for the following commands ?

command > file 2>&1       
command > file 2<&1       

Answer 1

NUMBER1>&NUMBER2 means to assign the file descriptor NUMBER2 the file descriptor NUMBER1.

That means, to execute dup2 (NUMBER2, NUMBER1).

command > file 2>&1

Bash process the command line, it finds first the redirection >file, it changes stdout to be written to file, then continue to process and finds 2>&1, and changes stderr to be written to stdout (which is file in this moment) .

command > file 2<&1       

this is the same, but 2<&1 redirects stderr to read from stdout. Because nobody reads from stderr, this second redirection normally has no effect.

However, bash treats this special case doing the same as for 2>&1, so executing dup2 (1, 2).

What does "2<&1" redirect do in Bourne shell?