linux - Makefile 中的 $${HOME} 或 ${HOME}？

Question

$ cat Makefile 
all:
    echo VAR is ${HOME}
    echo VAR is $${HOME}

给予

$ make
echo VAR is /home/abc
VAR is /home/abc
echo VAR is ${HOME}
VAR is /home/abc

为什么 echo VAR is ${HOME} 语法在 Makefile 中有效？我想，要使用 shell 变量，您必须使用 $${HOME}}

Answer 1

是也不是。最好使用$$ 来明确。然而，there is a special rule for environment variables :

Variables in make can come from the environment in which make is run. Every environment variable that make sees when it starts up is transformed into a make variable with the same name and value. But an explicit assignment in the makefile, or with a command argument, overrides the environment. (If the `-e' flag is specified, then values from the environment override assignments in the makefile. See section Summary of Options. But this is not recommended practice.)