bashpipesubshell

How to populate global variable from function which is in a pipe?

I would like to have function change value of global variable, but it fails when the function is used in a pipe:

#!/bin/bash

declare GVAR="abc"

func1() {
        read GVAR

        echo>&2 "func1: GVAR is $GVAR"
}

func2() {
        echo "xyz"
}

func2 | func1

echo>&2 "main: GVAR is $GVAR"

Output:

func1: GVAR is xyz
main: GVAR is abc

This is obviously because "Each command in a multi-command pipeline, where pipes are created, is executed in its own subshell" as per Bash manual on pipelines.

The best I could achieve is use shopt -s lastpipe, but then this only works when the function is last one (with extra conditions), which makes it basically unusable to 'tee out' something that is passing through a pipe before letting it continue to another command.

If impossible, what is the alternative approach to achieve the same? Can I avoid having to create temporary files?

Solution

  • If you reassign your file descriptors so stdout goes to real stdout but an alternate FD goes to a FIFO that the parent process reads from, then you can insert code anywhere in your pipeline that writes to that alternate fd.

    #!/usr/bin/env bash

gvar_parent=abc

# backup "real" stdout so we can reach it even within a redirection
exec 3>&1

func1() {
   read -r gvar_child
   echo "func1: GVAR is $gvar_child" >&2
   printf '%s\0' "$gvar_child" >&4 # return new GVAR result to parent
}

func2() { echo "xyz"; }

# aside: don't use all-caps variable names
IFS= read -r -d '' gvar_parent < <(exec 4>&1 >&3; func2 | func1)
echo "GVAR is $gvar_parent"

    Some notes: