【已解决】Makefile中判断变量是否为空无效

折腾：

【已解决】Makefile中如何判断一个字符串变量存在另外一个字符串或列表中

期间，对于findstring

GNU make: Text Functions

“$(findstring find,in)

Searches in for an occurrence of find. If it occurs, the value is find; otherwise, the value is empty. You can use this function in a conditional to test for the presence of a specific substring in a given string. Thus, the two examples,

$(findstring a,a b c)

$(findstring a,b c)

produce the values ‘a’ and ‘’ (the empty string), respectively. See Testing Flags, for a practical application of findstring.”

中返回的empty的话，接着去判断是否为空

结果用：

$(info BOOK_NAME=$(BOOK_NAME))
FOUND_BOOK := $(findstring $(BOOK_NAME), $(IGNORE_FILE_CONTENT))
$(info FOUND_BOOK=$(FOUND_BOOK))
ifeq ($(FOUND_BOOK), "")
$(info not found $(BOOK_NAME) in $(IGNORE_FILE_CONTENT))
# else
# $(info is found $(BOOK_NAME) in $(IGNORE_FILE_CONTENT))
endif

结果输出不对：没有输出。

后来自己研究了，发现是：

【总结】

要么不加””

ifeq ($(FOUND_BOOK), )

要么都加””

ifeq ("$(FOUND_BOOK)", "")

不能写成：

ifeq ($(FOUND_BOOK), "")

否则本身是空的empty的，逻辑是是null，但是和一个空字符串””去比较，就不对了。

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