Makefile中的$（shell ...）命令未正确执行，但在bash / sh中有效

Question

我想计算Makefile中graphviz文件中的节点数，以使用它为每个节点启动进程。 我跑的时候

grep -- -\> graph.gv | while read line; do for w in $line; do echo $w; done; done | grep [Aa-Zz] | sort | uniq | wc -l

在shell中，它按预期打印节点数。 但是，当我在Makefile中使用它时

NODES := $(shell grep -- -\> graph.gv | while read line; do for w in $line; do echo $w; done; done | grep [Aa-Zz] | sort | uniq | wc -l)

${NODES}始终为0.

Answer 1

你需要逃避$标志。说：

NODES := $(shell grep -- -\> graph.gv | while read line; do for w in $$line; do echo $$w; done; done | grep [Aa-Zz] | sort | uniq | wc -l)