# Set a PATH (!!! impure).
export PATH=/bin:/usr/bin:$PATH
mkdir $out
cat > $out/substituter <<EOF
#! /bin/sh -ex
echo \$*
case \$* in
*aaaa*)
exit 1
;;
*)
mkdir \$1
echo Foo \$3 \$4 > \$1/hello
esac
EOF
chmod +x $out/substituter