#!/bin/sh -eu
# A target must be rebuilt when a dependency changes.
>all.do printf 'redo-ifchange a\n'
>a.do printf 'redo-ifchange b\ndate +%s\n'
>b printf '1'

redo
<a read -r timestamp_a1

sleep 1
>b printf '2'

redo
<a read -r timestamp_a2

test ${timestamp_a2} -gt ${timestamp_a1}