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tomjlw is translating
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Rediscovering make: the power behind rules
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======
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I used to think makefiles were just a convenient way to list groups of shell commands; over time I've learned how powerful, flexible, and full-featured they are. This post brings to light over some of those features related to rules.
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### Rules
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Rules are instructions that indicate `make` how and when a file called the target should be built. The target can depend on other files called prerequisites.
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You instruct `make` how to build the target in the recipe, which is no more than a set of shell commands to be executed, one at a time, in the order they appear. The syntax looks like this:
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```
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target_name : prerequisites
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recipe
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```
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Once you have defined a rule, you can build the target from the command line by executing:
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```
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$ make target_name
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```
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Once the target is built, `make` is smart enough to not run the recipe ever again unless at least one of the prerequisites has changed.
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### More on prerequisites
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Prerequisites indicate two things:
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* When the target should be built: if a prerequisite is newer than the target, `make` assumes that the target should be built.
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* An order of execution: since prerequisites can, in turn, be built by another rule on the makefile, they also implicitly set an order on which rules are executed.
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If you want to define an order, but you don't want to rebuild the target if the prerequisite changes, you can use a special kind of prerequisite called order only, which can be placed after the normal prerequisites, separated by a pipe (`|`)
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### Patterns
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For convenience, `make` accepts patterns for targets and prerequisites. A pattern is defined by including the `%` character, a wildcard that matches any number of literal characters or an empty string. Here are some examples:
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* `%`: match any file
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* `%.md`: match all files with the `.md` extension
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* `prefix%.go`: match all files that start with `prefix` that have the `.go` extension
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### Special targets
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There's a set of target names that have special meaning for `make` called special targets.
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You can find the full list of special targets in the [documentation][1]. As a rule of thumb, special targets start with a dot followed by uppercase letters.
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Here are a few useful ones:
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**.PHONY** : Indicates `make` that the prerequisites of this target are considered to be phony targets, which means that `make` will always run it's recipe regardless of whether a file with that name exists or what its last-modification time is.
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**.DEFAULT** : Used for any target for which no rules are found.
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**.IGNORE** : If you specify prerequisites for `.IGNORE`, `make` will ignore errors in execution of their recipes.
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### Substitutions
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Substitutions are useful when you need to modify the value of a variable with alterations that you specify.
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A substitution has the form `$(var:a=b)` and its meaning is to take the value of the variable `var`, replace every `a` at the end of a word with `b` in that value, and substitute the resulting string. For example:
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```
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foo := a.o
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bar : = $(foo:.o=.c) # sets bar to a.c
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```
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note: special thanks to [Luis Lavena][2] for letting me know about the existence of substitutions.
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### Archive Files
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Archive files are used to collect multiple data files together into a single file (same concept as a zip file), they are built with the `ar` Unix utility. `ar` can be used to create archives for any purpose, but has been largely replaced by `tar` for any other purposes than [static libraries][3].
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In `make`, you can use an individual member of an archive file as a target or prerequisite as follows:
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```
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archive(member) : prerequisite
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recipe
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```
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### Final Thoughts
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There's a lot more to discover about make, but at least this counts as a start, I strongly encourage you to check the [documentation][4], create a dumb makefile, and just play with it.
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--------------------------------------------------------------------------------
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via: https://monades.roperzh.com/rediscovering-make-power-behind-rules/
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作者：[Roberto Dip][a]
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译者：[tomjlw](https://github.com/tomjlw)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]:https://monades.roperzh.com
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[1]:https://www.gnu.org/software/make/manual/make.html#Special-Targets
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[2]:https://twitter.com/luislavena/
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[3]:http://tldp.org/HOWTO/Program-Library-HOWTO/static-libraries.html
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[4]:https://www.gnu.org/software/make/manual/make.html
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@ -0,0 +1,101 @@
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重新发现 make： 规则背后的力量
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======
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我过去认为 makefiles 只是一种将一组组的 shell 命令列出来的简便方法；过了一段时间我了解到它们是有多么的强大、灵活以及功能齐全。这篇文章带你领略其中一些有关规则的特性。
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### 规则
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规则是表明 `make` 如何并且何时搭建一个被称作为目标（target）的文件的指令。目标文件可以依赖于其它被称作为前提（prerequisite）的文件。
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你指明 `make` 如何在配方（recipe）中搭建目标，这个过程不过是一套按照出现顺序一次执行一个的 shell 命令。像这样的句法：
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```
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target_name : prerequisites
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recipe
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```
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一但你定义好了规则，你就可以通过从命令行执行以下命令搭建目标：
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```
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$ make target_name
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```
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目标一经搭建，除非前提改变，否则 `make` 会足够聪明地不再去运行配方。
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### 关于前提的更多信息
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前提表明了两件事情：
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* 当目标应当被搭建时：如果其中一个前提比目标更新，`make` 假定目的应当被搭建。
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* 执行的顺序：鉴于前提可以反过来在 makefile 中被另一套规则所搭建，它们同样暗中定下了一个执行规则的顺序。
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如果你想要定义一个顺序但是你不想在前提改变的时候重新搭建目标，你可以使用一种特别的叫做“唯顺序”的前提。这种前提可以被放在普通的前提之后，用管道符（`|`）进行分隔。
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### 样式
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为了便利，`make` 接受目标和前提的样式。通过包含 `%` 符号可以定义一种式样。这个符号是一个可以匹配任何长度的文字符号或者空隔的通配符。以下有一些示例：
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* `%`：匹配任何文件
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* `%.md`：匹配所有 `.md` 结尾的文件
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* `prefix%.go`：匹配所有以 `prefix` 开头以 `go` 结尾的文件
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### 特殊目标
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有一系列目标名字，它们对于 `make` 来说有特殊的意义，被称作特殊目标。
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你可以在这个[文档][1]发现全套特殊目标。作为一种经验法则，特殊目标以点开始后面跟着大写字母。
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以下是几个有用的特殊目标：
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**.PHONY**：向 `make` 表明此目标的前提被当成伪目标。这意味着 `make` 将总是运行无论有那个名字的文件是否存在或者上次被修改的时间是什么。
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**.DEFAULT**：被用于任何没有规则的目标。
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**.IGNORE**：如果你指定 **.IGNORE** 为前提，`make` 将忽略执行配方过程中的错误。
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### 替代（substitution）
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当你需要以你指定的改动方式改变一个变量的值，替代就十分有用了。
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一个替代有着 `$(var:a=b)` 的形式，它的意思是获取变量 `var` 的值，用值里面的 `b` 替代词末尾的每个 `a` 以代替最终的字符串。例如：
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```
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foo := a.o
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bar : = $(foo:.o=.c) # sets bar to a.c
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```
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注意：特别感谢 [Luis Lavena][2] 让我们知道替代的存在。
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### 档案文件
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档案文件是用来一起将多个数据文档（和压缩文件同样的概念）收集成一个文件。它们由 `ar` Unix 设施搭建。`ar` 可以被用来为任何目的创建档案，但除了在[静态库][3]方面已经被 `tar` 大量替代。
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在 `make` 中，你可以使用一个档案文件中的单独一个成员作为目标或者前提，就像这样：
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```
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archive(member) : prerequisite
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recipe
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```
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### 最后的想法
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关于 make 还有更多可探索的，但是至少这是一个起点，我强烈鼓励你去查看[文档][4]，创建一个笨拙的 makefile 然后就可以探索它了。
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--------------------------------------------------------------------------------
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via: https://monades.roperzh.com/rediscovering-make-power-behind-rules/
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作者：[Roberto Dip][a]
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译者：[tomjlw](https://github.com/tomjlw)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]:https://monades.roperzh.com
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[1]:https://www.gnu.org/software/make/manual/make.html#Special-Targets
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[2]:https://twitter.com/luislavena/
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[3]:http://tldp.org/HOWTO/Program-Library-HOWTO/static-libraries.html
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[4]:https://www.gnu.org/software/make/manual/make.html
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