11 Mar
2020
11 Mar
'20
8:02 a.m.
Hi Grant.
Grant Taylor via TUHS <tuhs(a)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