How to make a pipe loop in bash

Question

Assume that I have programs P0, P1, ...P(n-1) for some n > 0. How can I easily redirect the output of program Pi

Answer 1

My solutions uses pipexec (Most of the function implementation comes from your answer):

square.sh

function square() {
  # square numbers

  read j                         # receive first "request"
  while [ "$j" != "" ]; do
    let jj=$j*$j
    echo "square($j) = $jj" >&2  # debug message

    echo $jj                     # send square

    read j                       # receive next "request"
  done
}

square $@

calc.sh

function calc() {
  # calculate sum of squares of numbers 0,..,10

  sum=0
  for ((i=0; i<10; i++)); do
    echo $i                   # "request" the square of i

    read ii                   # read the square of i
    echo "got $ii" >&2          # debug message

    let sum=$sum+$ii
 done

 echo "sum $sum" >&2           # output result to stderr
}

calc $@

The command

pipexec [ CALC /bin/bash calc.sh ] [ SQUARE /bin/bash square.sh ] \
    "{CALC:1>SQUARE:0}" "{SQUARE:1>CALC:0}"

The output (same as in your answer)

square(0) = 0
got 0
square(1) = 1
got 1
square(2) = 4
got 4
square(3) = 9
got 9
square(4) = 16
got 16
square(5) = 25
got 25
square(6) = 36
got 36
square(7) = 49
got 49
square(8) = 64
got 64
square(9) = 81
got 81
sum 285

Comment: pipexec was designed to start processes and build arbitrary pipes in between. Because bash functions cannot be handled as processes, there is the need to have the functions in separate files and use a separate bash.