Question

mkfifo fifo
echo 1 >fifo
blocks on last command.

I find a explanation of this in gnu libc

However, it has to be open at both ends simultaneously before you can proceed to do any input or output operations on it. Opening a FIFO for reading normally blocks until some other process opens the same FIFO for writing, and vice versa.

But

>mkfifo fifo  
>exec 5<>fifo  
>echo 1 >&5

doesn’t block and works well. Again AFAIK shells use a dup2 to make redirections but
how does this makes a difference?

Actually I find the answer when I formulated the question. I’ll post it below.

Answer 1

Well the answer is short.

From my linux distro man 7 fifo

Under Linux, opening a FIFO for read and write will succeed both in blocking and nonblocking mode. POSIX leaves this behavior undefined.

So it’s due to <> in redirection.