Inter-process communication without FIFOs

Question

Inside a BASH script we can have multiple processes running in background which intercommunicate using named pipes, FIFOs registered on the filesystem. An example of this could be:

#!/bin/bash
mkfifo FIFO

# BG process 1
while :; do echo x; done & >FIFO

# BG process 2
while :; do read; done & <FIFO

exit

I wonder if it's possible to do the same intercommunication between background processes of a script without using a FIFO on filesystem, maybe with some kind of file-descriptor redirection.

Answer 1

Here's an example that runs two subprocesses implemented as functions of the same shell-script... One subprocess generates numbers 1...5 (sleeps in between prints), the second one reads from a fixed filedescriptor (5, to which STDOUT of the first FD is redirected to), multiplies by 2 and prints again. The main process redirects STDOUT of that second process to another fixed filedescriptor (6) and later on reads from that one in the loop.

It works basically the same as you'd do in C-code with fd pairs created by the pipe(2) system call. To understand what's happening run the script under strace -f!

Bash Version is 4.2.24(1) running on Ubuntu/x86.

[ubuntu /home/chris]
$ bash --version
GNU bash, version 4.2.24(1)-release (i686-pc-linux-gnu)
Copyright (C) 2011 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>

This is free software; you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Output of script:

[ubuntu /home/chris]
$ ./read_from_fd.sh
Got number 2.
Got number 4.
Got number 6.
Got number 8.
Got number 10.

Source code:

#!/bin/bash

# Generate data, output to STDOUT.
generate_five_numbers() {
        for n in `seq 5` ; do
                echo $n
                sleep 2
        done
}

# Read data from FD#5, multiply by two, output to STDOUT.
multiply_number_from_fd5_by_two() {
        while read n <&5 ; do
                echo "$(( $n * 2 ))"
        done
}

# choose your FD number wisely ;-)

# run generator with its output dup'ed to FD #5
exec 5< <( generate_five_numbers )

# run multiplyier (reading from fd 5) with output dup'ed to FD #6
exec 6< <( multiply_number_from_fd5_by_two )

# read numbers from fd 6
while read n <&6 ; do
        echo "Got number $n."
done

Process tree while running:

──read_from_fd.sh(8118)─┬─read_from_fd.sh(8119)───sleep(8123)
                        └─read_from_fd.sh(8120)