Re: [TUHS] First appearance of named pipes

[Grant Taylor via TUHS <tuhs@minnie.tuhs.org>]

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On 3/10/20 1:29 AM, arnold@skeeve.com wrote:
> Absolutely:
> 
> 	$ mkfifo the_fifo
> 	$ ls -l  the_fifo
> 	prw-rw-r-- 1 arnold arnold 0 Mar 10 09:28 the_fifo
> 	$ echo foo > the_fifo & sleep 1 ; cat the_fifo
> 	[1] 3721
> 	foo
> 	[1]+  Done                    echo foo > the_fifo
> 
> As you stated, not that you'd want to do that, but you can.

Thank you for your reply Arnold.

As I was reading your reply, I realized that I did not fully convey the 
question that I was still mulling over in my head.  (More in a moment.)

This thread is one of about three things happening in my life that have 
to do with pipes, FIFOs, and file descriptors.  I managed to articulate 
the simpler of the questions while reading Noel's email.

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 ">".

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

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

After having mulled this over for a few days and typing this out, I 
realize that the "pipe" is really just a fifo and that in this case the 
fifo is a named pipe on the file system.  I could do the same thing with 
a file.  Historically I would have done the same thing with a file.  But 
now I realize that the file is not required and that I can use a fifo 
which is in memory and never hits the disk.  (Save for creating the name 
interface to the pipe / fifo.)

At least, I think that's all accurate.

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



-- 
Grant. . . .
unix || die


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