📄 make-06.html
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通过这个例子会发现变量的展开过程和</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">c</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">语言中的宏展开的过程相同，是一个严格的文本替换过程。上例中变量“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">foo</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">”被展开的过程中，变量值中的前导空格会忽略。举这个例子的目的是为了让我们更清楚地了解变量的展开过程，而不是建议大家按照这样的方式来书写</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">Makefile</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">。在实际书写时，最好不要这么干。否则将会给你带来很多不必要的麻烦。</span></p>
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注意：</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">Makefile</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">中在对一些简单变量的引用，我们也可以不使用“（）”和“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">{}</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">”来标记变量名，而直接使用“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">$x</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">”的格式来实现，此种用法仅限于变量名为单字符的情况。另外自动化变量也使用这种格式。对于一般多字符变量的引用必须使用括号了标记，否则</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">make</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">将把变量名的首字母作为作为变量而不是整个字符串（“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">$PATH</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">”在</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">Makefile</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">中实际上是“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">$(P)ATH</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">”）。这一点和</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">shell</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">中变量的引用方式不同。</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">shell</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">中变量的引用可以是“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">${xx}</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">”或者“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">$xx</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">”格式。但在</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">Makefile</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">中多字符变量名的引用只能是“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">$(xx)</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">”或者“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">${xx}</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">”格式。</span></b></p>
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</span></b></p>
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<span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: blue">
一般在我们书写</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: blue">Makefile</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: blue">时，各部分变量引用的格式我们建议如下：</span></p>
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<span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: blue">
1.<span style="font:7.0pt "Times New Roman""> </span>make</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: blue">变量（</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: blue">Makefile</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: blue">中定义的或者是</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: blue">make</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: blue">的环境变量）的引用使用“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: blue">$(VAR)</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: blue">”格式，无论“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: blue">VAR</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: blue">”是单字符变量名还是多字符变量名。</span></p>
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<span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: blue">
2.<span style="font:7.0pt "Times New Roman""> </span></span>
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出现在规则命令行中</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: blue">shell</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: blue">变量（一般为执行命令过程中的临时变量，它不属于</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: blue">Makefile</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: blue">变量，而是一个</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: blue">shell</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: blue">变量）引用使用</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: blue">shell</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: blue">的“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: blue">$tmp</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: blue">”格式。</span></p>
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3.<span style="font:7.0pt "Times New Roman""> </span></span>
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对出现在命令行中的</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: blue">make</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: blue">变量我们同样使用“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: blue">$(CMDVAR)</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: blue">”</span><span style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: blue">
</span>
<span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: blue">
格式来引用。</span></p>
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<span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: blue">
</span></p>
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<span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: blue">
例如：</span></p>
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<b><i>
<span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: blue">
# sample Makefile</span></i></b></p>
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<span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: blue">
……</span></i></b></p>
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<span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: blue">
SUBDIRS := src foo</span></i></b></p>
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</span></i></b></p>
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<span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: blue">
.PHONY : subdir</span></i></b></p>
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<span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: blue">
Subdir :</span></i></b></p>
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<span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: blue">
@for dir in $(SUBDIRS); do \</span></i></b></p>
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<span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: blue">
$(MAKE) –C $$dir || exit 1; \</span></i></b></p>
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<span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: blue">
done</span></i></b></p>
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<b><i>
<span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: blue">
……</span></i></b></p>
<h2><a name="_gnu_make_6.1"><span style="font-size: 14pt" lang="en-us"><b>6</b></span><b><span lang="EN-US" style="font-size: 14.0pt">.2<span style="font-style: normal; font-variant: normal; font-weight: normal; font-size: 7.0pt; font-family: Times New Roman">
</span></span></b><span style="font-size: 14.0pt; font-family: 楷体_GB2312">
两种变量定义（赋值）</span></a></h2>
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<span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">
在</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">GNU
make</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">中，变量的定义有两种方式（或者称为风格）。我们把使用这两种方式定义的变量可以看作变量的两种不同风格。变量的这两种不同的风格的区别在于：</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">1.
</span>
<span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">
定义方式；</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">2.
</span>
<span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">
展开时机。下边我们分别对这两种不同的风格进行详细地讨论。</span></p>
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