如果有空，如何有条件地将Makefile变量设置为某些值? [英] How to conditionally set Makefile variable to something if it is empty?

问题描述

我想设置一个变量，如果它是空的.我以这种方式尝试过:

I want to set a variable if it is empty. I tried in this way:

....
TEST := $(something)
...
TEST ?= $(something else)

第一个 $(something)可能返回一个空字符串，但是条件赋值?=仅在未设置前一个变量的情况下有效，而在不为空的情况下不起作用.

The first $(something) may return an empty string, however the conditional assignment ?= works only if the previous variable is not set, not if empty.

是否有任何优雅的解决方案来将变量设置为空?

Any elegant solution to set the variable if empty?

编辑我找到了这个解决方案:

EDIT I found this solution:

....
TEST := $(something)
...
TEST += $(something else)
TEST := $(word 1, $(TEST))

但我认为还会再有一种优雅.

but I think that there will be one more elegant.

推荐答案

是否有任何优雅的解决方案来将变量设置为空?

Any elegant solution to set the variable if empty?

GNU make鲜为人知的优雅解决方案.除非您发现活板门和雷区都很优雅.我只知道完成您想要的两种方法:

GNU make is hardly known for elegant solutions. Unless you find trapdoors and minefields to be elegant. I know only of the two ways to accomplish what you want:

1. 标准 ifeq / endif 解决方案:

ifeq ($(TEST),)
TEST := $(something else)
endif

使用 $(if)函数:

TEST := $(if $(TEST),$(TEST),$(something else))

也可以尝试将该构造打包为一个函数，但这是不可取的.如果该函数包含 (仅存在任性的解决方法)，则该函数将具有偶尔破坏 $(其他东西)的隐患.(诸如 $(if)之类的内置函数不受，错误的影响.)

One can try to package that construct into a function too, but that is inadvisable. The function would have the hidden pitfall of occasionally breaking the $(something else) if it contains the , (for which there are only wayward workarounds). (The built-in functions like $(if) are immune to the , bug.)

优雅测试由您决定.

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