linux – 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

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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