Makefile-仅当文件不存在时才进行依赖 [英] Makefile - Make dependency only if file doesn't exist

问题描述

就像标题中所说的，我只想在某个文件不存在时进行依赖，而不是每次更新时都进行依赖.

Like the title says, I would like to make a dependency only if a certain file does not exist, NOT every time it updates.

我有一个根目录(一个带有makefile的目录)，在其中有一个名为"example"的子目录.在我的根目录中，有四个.h文件(functions.h，parser.h，node.h和exception.h)，如果这些.h文件不存在，我希望将其复制到"example"子目录中在示例"中.

I have a root directory (the one with the makefile) and in it a sub-directory called "example". In my root directory are four .h files (functions.h, parser.h, node.h, and exception.h) which I would like to copy to the "example" sub-directory if those .h files do not already exist in "examples".

不幸的是，我不能仅仅建立标准的依赖项来检查示例"中的头文件，因为每次我将头文件从根目录复制到示例"时，示例"中的头文件都将被视为已更新并将每次我运行make时，都会触发该依赖项.我想让我的makefile仅在示例"中不存在头文件的情况下才将头文件从根目录复制到示例".

Unfortunately I can not just make a standard dependency to check for the header files in "example" because each time I copy the header files from root to "example", the header files in "example" will be considered updated and will trigger that dependency each time I run make. I would like for a way to have my makefile copy the header files from the root directory to "example" only if they do not exist in "example".

推荐答案

这是仅订购的先决条件/依赖项用于:

但是，有时候，您希望对要调用的规则施加特定的顺序，但如果执行了其中一个规则，则不必强制更新目标.在这种情况下，您要定义仅订购的先决条件.可以通过在先决条件列表中放置管道符号(|)来指定仅订购的先决条件.右侧的所有先决条件仅是订购.

Occasionally, however, you have a situation where you want to impose a specific ordering on the rules to be invoked without forcing the target to be updated if one of those rules is executed. In that case, you want to define order-only prerequisites. Order-only prerequisites can be specified by placing a pipe symbol (|) in the prerequisites list: any prerequisites to the left of the pipe symbol are normal; any prerequisites to the right are order-only.

在您的情况下:

examples/%.h : | %.h
    cp $| $@

另请参见:不显示仅订购的先决条件在$^或$+中.

See also: Order-only prerequisites do not show up in $^ or $+.

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