redirect to the same file as the source file processed by the command

Question

1. Redirection is not allowed here. Is cat not allowing? But isn't redirection independent of any command running with it?

   $ cat bk > bk
   cat: bk: input file is output file
   

2. Why does the following redirect empty the file:

   $ less  bk > bk
   

   awk also works in the similar way as less in regard to redirection. awk is the one with which I actually found the problem, while the above examples are meaningless

In general, what are the right way to use redirect?

Answer 1

The problem is that the output redirection kills the file before less is even started:

open("file", O_WRONLY|O_CREAT|O_TRUNC, 0666) = 3
dup2(3, 1)                        = 1
close(3)                          = 0
execve("/usr/bin/less", ["less", "-WNS", "file"], [/* 102 vars */]) = 0

The normal output redirection overwrites the file. The O_TRUNC (see man 2 open) deletes the file content.