#!/bin/sh -eu
# A target must be rebuilt when a non-existence dependency exists.

>all.do printf 'redo-ifchange a\n'
>a.do printf 'test -e b || redo-ifcreate b\ndate +%s\n'

redo
<a read -r timestamp_a1

>b :
sleep 1

redo
<a read -r timestamp_a2

test ${timestamp_a2} -gt ${timestamp_a1}