Re: [TUHS] First appearance of named pipes

[arnold@skeeve.com]

Hi Grant.

Grant Taylor via TUHS <tuhs@minnie.tuhs.org> wrote:

> The larger more onerous question is could I leverage exec to alter where 
> file descriptors 0 (STDIN), 1 (STDOUT), and 2 (STDERR) are set to, 
> including changing 1 to the value of a FIFO, and 0 of a subsequent 
> command to also be the value of the FIFO, thus have pipe like behavior 
> between two commands without using a pipe or redirection as in ">".

There's nothing preventing you from doing that. After the fork() and
before the exec(), just close() and dup() the relevant fds in the right
order and you're set.

> This has also gotten me to wonder about the possibility of having 
> multiple commands output to a file descriptor; 1 / 2 / other, that is 
> input to a separate command.  Sort of the opposite of tee, in a manner 
> of speaking.  I'll try to articulate:
>
> $ mkfifo test.fifo
> $ exec 3>&1
> $ exec 1> test.fifo
> $ for l in {a..z}; do echo $l; sleep 1; done &
> $ for L in {A..Z}; do echo $L; sleep 1; done &
> $ for n in {1..100}; do echo $n; sleep 1; done &
> $ exec 1>&3
> $ cat test.fifo

I don't think that this is any different from:

	(for l in {a..z}; do echo $l; sleep 1; done &
	 for L in {A..Z}; do echo $L; sleep 1; done &
	 for n in {1..100}; do echo $n; sleep 1; done &) | cat

which reduces to:

	(for l in {a..z}; do echo $l; sleep 1; done &
	 for L in {A..Z}; do echo $L; sleep 1; done &
	 for n in {1..100}; do echo $n; sleep 1; done &) > /some/file

(You might want to background that whole mess given that the final
pipeline will sleep for 100 seconds.)

> This seems special to me in that I have three processes (for loops) 
> writing into what is effectively the same pipe.

It's not any different than calling `stty -tostop' and then simply
backgrounding the three loops at the terminal.  Try it!

(This is the beauty of the Unix model, an fd is just a data sink,
we don't care where it goes.)

> I would be very eager to learn from anyone who is willing to teach me 
> pointers.  :-)

At the cost of tooting my own horn, I recommend my book "Linux Programming
by Example: The Fundamentals", which, despite the "Linux" in the title,
covers basic Unix programming, including file descriptor manipulation
of the sort under discussion here.

HTH,

Arnold