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4.4.1 Wildcard Examples ----------------------- Wildcards can be used in the recipe of a rule, where they are expanded by the shell. For example, here is a rule to delete all the object files: clean: rm -f *.o Wildcards are also useful in the prerequisites of a rule. With the following rule in the makefile, `make print' will print all the `.c' files that have changed since the last time you printed them: print: *.c lpr -p $? touch print This rule uses `print' as an empty target file; see *note Empty Target Files to Record Events: Empty Targets. (The automatic variable `$?' is used to print only those files that have changed; see *note Automatic Variables::.) Wildcard expansion does not happen when you define a variable. Thus, if you write this: objects = *.o then the value of the variable `objects' is the actual string `*.o'.However, if you use the value of `objects' in a target or prerequisite, wildcard expansion will take place there. If you use the value of `objects' in a recipe, the shell may perform wildcard expansion when the recipe runs. To set `objects' to the expansion, instead use: objects := $(wildcard *.o) *Note Wildcard Function::.
4.4.1 通配符例子
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通配符可以被使用在规则的片段中，在此时它们被shell所解释/展开。例如，这里有一个删除所有目标文件的规则:
clean:
rm -f *.o
通配符在一个规则的前提条件中也是很有用的。通过makefile的下列例子，
make print 会打印出所有自从上次打印后改变的.c 文件:
print: *.c
lpr -p $?
touch print
这个规则 使用 print 作为空的目的文件；参见 *note Empty Target Files to Record Events: Empty Targets (自动变量 $ 用来只打印出那些发生了改变的文件；参见 *note Automatic Variables::)
在你定义一个变量的时候，通配符扩展不会发生,所以，如果你这么写：
objects = *.o
那么变量 objects 的值会是真的字符串 '*.o'。但是如果你是在目的或者前提条件中使用 objects的值，通配符扩展还是会发生。如果你在片段里使用 objects 的值，当片段运行时，通配符扩展会发生。
如果想要把 objects 设置在扩展里，应当使用：
objects := $(wildcard *.o)
*Note Wildcard Function::.
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