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6.1 Basics of Variable References=================================To substitute a variable's value, write a dollar sign followed by thename of the variable in parentheses or braces: either `$(foo)' or`${foo}' is a valid reference to the variable `foo'. This specialsignificance of `$' is why you must write `$$' to have the effect of asingle dollar sign in a file name or recipe. Variable references can be used in any context: targets,prerequisites, recipes, most directives, and new variable values. Hereis an example of a common case, where a variable holds the names of allthe object files in a program: objects = program.o foo.o utils.o program : $(objects) cc -o program $(objects) $(objects) : defs.h Variable references work by strict textual substitution. Thus, therule foo = c prog.o : prog.$(foo) $(foo)$(foo) -$(foo) prog.$(foo)could be used to compile a C program `prog.c'. Since spaces before thevariable value are ignored in variable assignments, the value of `foo'is precisely `c'. (Don't actually write your makefiles this way!) A dollar sign followed by a character other than a dollar sign,open-parenthesis or open-brace treats that single character as thevariable name. Thus, you could reference the variable `x' with `$x'.However, this practice is strongly discouraged, except in the case ofthe automatic variables (*note Automatic Variables::). 6.1 变量参照的基本
=================================为了替换一个变量的值，要写一个美元符号后面跟着括号或者大括号，里面是变量名成：或者$(foo) 或者 ${foo}是一个对变量 foo的有效参照。这个$的特殊重要性就是为什么你必须写 $$来在文件名或者片段中来代表$符号本身。
变量参照可以用在任何上下文中：目的，前提条件，片段，大多数的指令，还有几个变量值中。这里有一个一般情况的例子，此处变量持有在一个程序中的所有的目标文件名:
objects = program.o foo.o utils.o
program : $(objects) cc -o program $(objects)$(objects) : defs.h
变量参照采用严格的文本替换因此，下列规则
foo = c
prog.o : prog.$(foo) $(foo)$(foo) -$(foo) prog.$(foo)可以被用来编译一个C 程序 prog.c， 因为变量前的空格会被在变量赋值的时候被忽略，foo的值就是精确的c(当然，实际上不要像这样来写你的makefile)。
一个非美元符号，非开放括号或者非开放打括号前面有一个美元符号的，被当作一个变量对待。因此你你可以用$X来参照x，但是这个实践是强烈不推荐的。除了在自动变量的场合(*note Automatic Variables::)。
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