GNU make manual 翻译(八十四)

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4.4.1 Wildcard Examples                        

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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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