linux C bash / Makefile中双符号的意思是什么？

在Makefile中插入一个 shell脚本时,我们有(？)使用一个双重美元符号($$)来引用变量.为什么？ for number in 1 2 3 4 ; do echo $$number ; done 解决方法 根据 gnu make official doc：

Variable and function references in recipes have identical syntax and
semantics to references elsewhere in the makefile. They also have the
same quoting rules: if you want a dollar sign to appear in your
recipe,you must double it (‘$$’). For shells like the default shell,
that use dollar signs to introduce variables,it’s important to keep
clear in your mind whether the variable you want to reference is a
make variable (use a single dollar sign) or a shell variable (use two
dollar signs).

简而言之：

> makefile variable =>使用一个美元符号> shell variable =>使用两个美元的标志

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