TL;DR:使用误差函数:

ifndef MY_FLAG
$(error MY_FLAG is not set)
endif

注意，行不能缩进。更准确地说，这些行之前不能有制表符。

---

通用解决方案

如果你要测试很多变量，为它定义一个辅助函数是值得的:

# Check that given variables are set and all have non-empty values,
# die with an error otherwise.
#
# Params:
#   1. Variable name(s) to test.
#   2. (optional) Error message to print.
check_defined = \
    $(strip $(foreach 1,$1, \
        $(call __check_defined,$1,$(strip $(value 2)))))
__check_defined = \
    $(if $(value $1),, \
      $(error Undefined $1$(if $2, ($2))))

下面是如何使用它:

$(call check_defined, MY_FLAG)

$(call check_defined, OUT_DIR, build directory)
$(call check_defined, BIN_DIR, where to put binary artifacts)
$(call check_defined, \
            LIB_INCLUDE_DIR \
            LIB_SOURCE_DIR, \
        library path)

这将输出如下错误:

Makefile:17: *** Undefined OUT_DIR (build directory).  Stop.

注:

真正的检查是这样的:

$(if $(value $1),,$(error ...))

这反映了ifndef条件的行为，因此定义为空值的变量也被认为是“未定义的”。但这只适用于简单变量和显式空递归变量:

# ifndef and check_defined consider these UNDEFINED:
explicitly_empty =
simple_empty := $(explicitly_empty)

# ifndef and check_defined consider it OK (defined):
recursive_empty = $(explicitly_empty)

正如@VictorSergienko在评论中所建议的，可能需要稍微不同的行为:

$(if $(value $1)测试值是否为非空。如果变量定义为空值，有时是可以的。我将使用$(if $(过滤器未定义，$(起源$1))…

And:

此外，如果它是一个目录，并且在运行检查时它必须存在，我将使用$(if $(通配符$1))。而是另一个函数。

有针对性的检查

还可以扩展解决方案，以便只有在调用某个目标时才需要变量。

$(调用check_defined，…

只需将支票移动到食谱中:

foo :
    @:$(call check_defined, BAR, baz value)

前导@符号关闭命令回显，:是实际的命令，一个shell no-op存根。

显示目标名称

check_defined函数也可以改进为输出目标名称(通过$@变量提供):

check_defined = \
    $(strip $(foreach 1,$1, \
        $(call __check_defined,$1,$(strip $(value 2)))))
__check_defined = \
    $(if $(value $1),, \
        $(error Undefined $1$(if $2, ($2))$(if $(value @), \
                required by target `$@')))

因此，现在一个失败的检查会产生一个格式化的输出:

Makefile:7: *** Undefined BAR (baz value) required by target `foo'.  Stop.

check-defined-MY_FLAG特殊目标

就我个人而言，我会使用上面简单直接的解决方案。然而，例如，这个答案建议使用一个特殊的目标来执行实际的检查。我们可以尝试将其概括，并将目标定义为隐式模式规则:

# Check that a variable specified through the stem is defined and has
# a non-empty value, die with an error otherwise.
#
#   %: The name of the variable to test.
#   
check-defined-% : __check_defined_FORCE
    @:$(call check_defined, $*, target-specific)

# Since pattern rules can't be listed as prerequisites of .PHONY,
# we use the old-school and hackish FORCE workaround.
# You could go without this, but otherwise a check can be missed
# in case a file named like `check-defined-...` exists in the root 
# directory, e.g. left by an accidental `make -t` invocation.
.PHONY : __check_defined_FORCE
__check_defined_FORCE :

用法:

foo :|check-defined-BAR

注意，检查定义的bar被列为仅限订单(|…)的先决条件。

优点:

(可以说)一个更干净的语法

缺点:

不能指定自定义错误消息 运行make -t(参见Instead of execution Recipes)将会用大量check-defined-…文件。这是模式规则不能被声明为. phony的一个遗憾的缺点。

我相信，这些限制可以通过一些计算魔法和二次扩展技巧来克服，尽管我不确定这是否值得。

使用shell函数测试:

foo:
    test $(something)

用法:

$ make foo
test 
Makefile:2: recipe for target 'foo' failed
make: *** [foo] Error 1
$ make foo something=x
test x

使用shell错误处理未设置变量(注意双$):

$ cat Makefile
foo:
        echo "something is set to $${something:?}"

$ make foo
echo "something is set to ${something:?}"
/bin/sh: something: parameter null or not set
make: *** [foo] Error 127


$ make foo something=x
echo "something is set to ${something:?}"
something is set to x

如果需要自定义错误消息，请将其添加在?:

$ cat Makefile
hello:
        echo "hello $${name:?please tell me who you are via \$$name}"

$ make hello
echo "hello ${name:?please tell me who you are via \$name}"
/bin/sh: name: please tell me who you are via $name
make: *** [hello] Error 127

$ make hello name=jesus
echo "hello ${name:?please tell me who you are via \$name}"
hello jesus

你可以使用IF来测试:

check:
        @[ "${var}" ] || ( echo ">> var is not set"; exit 1 )

结果:

$ make check
>> var is not set
Makefile:2: recipe for target 'check' failed
make: *** [check] Error 1

为了简单和简洁:

$ cat Makefile
check-%:
        @: $(if $(value $*),,$(error $* is undefined))

bar:| check-foo
        echo "foo is $$foo"

输出:

$ make bar
Makefile:2: *** foo is undefined. Stop.
$ make bar foo="something"
echo "foo is $$foo"
foo is something

另一个选择:

MY_FLAG = $(error Please set this flag)

尝试在任何地方使用此变量都会导致错误，除非从命令行覆盖它。

要接受环境变量，使用?=:

MY_FLAG ?= $(error Please set this flag)