Taking output of one program into input of another one in Bash

Yes, we can loop output of a pipeline back to its input using a FIFO. Here's how.

First, let's create a fifo (named pipe):

$ mkfifo pipe

Let's also, for demonstration purposes, create shell functions to use in the pipeline:

$ p1() { while read a; do sleep 0.1; echo "$((a+1))"; done; }
$ p2() { while read b; do echo "$((b+1))"; done | tee /dev/stderr; }

Now, let's run them with p1 feeding its output to p2 which sends its output to pipe which is fed back to the input of p1:

$ { echo 1; cat pipe; } | p1 | p2 >pipe
3
5
7
9
11
13
^C

The loop would go on forever but I terminated it with a ctrl-C first. p1 increments its input by one and p2 increments its input by one. The output we see comes from the stderr stream of p2.

How it works

{ echo 1; cat pipe; } provides the input to p1. It provides a 1 as a seed value and then supplies whatever it receives from pipe.

p1 works as follows:

while read a
do
    sleep 0.1
    echo "$((a+1))"
done

It reads from its input, waits 0.1 seconds, and writes an incremented value to its stdout. The purpose of the short wait is so that the output does not scroll by too fast on the screen.

p2 works as follows:

while read b
do
    echo "$((b+1))"
done | tee /dev/stderr

Just like p1, it reads from stdin and writes an incremented value to stdout. So that we can see what's going on on the terminal, this also sends a copy of the output to stderr.