Coprocesses expose a way to run a subshell asychronously, while maintaining full communication through pipes. In a typical script, you can run commands in the background (asychronously) with the & operator. This allows tasks to be worked in parallel.

#!/bin/env bash

for url in Linux Linux_kernel Free_software; do
  wget"$url" &

echo "All done!"

The problem here is that once the child processes fork, you don't have any way to communicate with them. Why would we want to do this? Maybe you don't want wget to write to files, but pass the results back up to the parent process so you can search for links; maybe you just wanted to see if those pages are available.

We could do this with FIFOs, but it would be a bit of a mess, and we're still missing out on sending information to the child processes.

#!/bin/env bash

mkfifo child2parent

for url in Linux Linux_kernel Free_software; do
  wget"$url" >> child2parent &

cat child2parent

echo "All done!"


coproc subproc {
  python -u -c '
import time

while True:
        line = raw_input()
    except EOFError:

py-run() {
  echo "$@" >&${subproc[1]}

py-read() {
  IFS= read -ru ${subproc[0]} $1

What's going on here? We're creating a Python interpreter in the background, but maintaining full communication with it. The interpreter's stdin is linked to ${subproc[1]}, which allows py-run() to work. We send a line to the child process, it reads it from it's stdin, executes it, and waits for another command.

We get results out of the child process through it's stdout ${subproc[1]}. py-read takes a line of output from the interpreter and reads it into a variable you provide. Here's some example usage:

py-print() {
  local x; py-read x; echo $x

py-run "x = 'hello'"
py-run "print x[::-1]"
py-read result; echo $result

py-run "y = { 'apple' : '$result', '$(whoami)' : '$(uname)', } "
py-run "print y.items()"


With coproc and a snippet of Python, we can interleave Bash and Python code that persists between calls. Between calls to py-run and py-read, the Python interpreter remains running, so all of the variables we've defined and the changes already made persist.

$ bash                                               
[('apple', 'olleh'), ('chronos', 'Linux')]