在make中未设置变量时获取默认值 [英] get a default value when variable is unset in make

问题描述

(根据@Michael反馈，问题更准确)

(edit: question more accurate based on @Michael feedback)

在bash中，我经常使用参数扩展:未设置$VARNAME时，以下命令将显示"default value"，否则将显示VARNAME内容.

In bash, I often use parameter expansion: the following commands print "default value" when $VARNAME is unset, otherwise it prints the VARNAME content.

echo ${VARNAME:-default value}  #if VARNAME empty => print "default value" 
echo ${VARNAME-default value}   #if VARNAME empty => print "" (VARNAME string)

我未找到的类似功能GNU make. 我终于在我的Makefile中写道:

I did not find a similar feature on GNU make. I finally wrote in my Makefile:

VARNAME ?= "default value"
all:
        echo ${VARNAME}

但是我对这种解决方案不满意:它总是创建变量VARNAME，这可能会更改某些makefile上的行为.

But I am not happy with this solution: it always creates the variable VARNAME and this may change the behavior on some makefiles.

是否有更简单的方法来获取未设置变量的默认值?

推荐答案

如果您要使用GNU make变量的扩展名(如果它是非空的)和默认值(如果它是空的)，但不设置该变量，您可以执行以下操作:

If you want to use the expansion of a GNU make variable if it is non-empty and a default value if it is empty, but not set the variable, you can do something like this:

all:
        echo $(or $(VARNAME),default value)

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