Bash complex pipeline dependencies

One way to make this work for OSX & Linux is to use namedpipes to signal to batch jobs that they can continue. Unfortunately it requires splitting up "Task *.1" & "Task *.2", but it's the best I can come up with for the moment.

#!/bin/bash

function cleanup {
  rm -f .task.a.1.done .task.b.1.done
}
trap cleanup EXIT
cleanup

mkfifo .task.a.1.done
mkfifo .task.b.1.done

{
    echo "Task A.1"
    echo -n "" > .task.a.1.done
} &

{
    echo "Task B.1"
    echo -n "" > .task.b.1.done
} &

{
    echo "Task X"
}

{
    cat < .task.a.1.done
    echo "Task A.2"
} &

{
    cat < .task.b.1.done
    echo "Task B.2"
} &

wait

As the comments to the question point out, the following Makefile is equivalent to your shell script (but doesn?t touch the file system)

.PHONY: a1 a2 b1 b2 x

all: a2 b2 

a1:
    @echo "Task A.1"

b1:
    @echo "Task B.1"

x:
    @sleep 1.5; echo "Task X"

a2: a1 x
    @echo "Task A.2"

b2: b1 x
    @echo "Task B.2"

Try it with make -j5 to allow for five parallel threads. It requires GNU make, which is available for every platform in existence, even if it might not be distributed by default. The initial .PHONY: target ensures that all tasks are executed even if files named a1, a2, ... happen to exist.