Makefile：返回字符串的前两个字符
给定 Makefile 中的字符串，是否可以使用 Makefile 语法（不使用 shell 调用）提取未知字符串的前两个字符？
例如，
VAR := UnKnown
VAR_TERSE = $(call get_first_two_chars, $(VAR))
define get_first_two_char
...
endef
Given a string in a Makefile, is it possible to extract the first two characters of an unknown string using Makefile syntax (without using shell calls)?
for instance,
VAR := UnKnown
VAR_TERSE = $(call get_first_two_chars, $(VAR))
define get_first_two_char
...
endef
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嗯，这是可以做到的，但是手工实现起来相当混乱。最简单的方法是获取 GNU Make 标准库，它有一个内置的
substr< /代码> 函数。如果这太过分了，您可以从库中提取该函数，但正如我所说，它非常混乱。本质上，您对字符串进行一系列替换，以在每个字符后插入一个空格：
接下来，您可以使用
$(wordlist)函数来获取中间结果的前两个“单词”：最后，您再次使用
$(subst)，现在删除您最初注入的空格：Well, it can be done, but it's pretty messy to implement by hand. The easiest thing to do is to get the GNU Make Standard Library, which has a built-in
substrfunction. If that's overkill, you can extract just that function from the library, but like I said, it's surprisingly messy.Essentially you do a series of substitutions on the string to insert a space after each character:
Next you can use the
$(wordlist)function to grab the first two "words" of the intermediate result:Finally, you use
$(subst)again, now to strip out the space that you injected originally:我真的不赞成强迫Make做它显然不想做的事情，但是……我无法抗拒一个好的谜题。
I really don't approve of forcing Make to do things it clearly doesn't want to do, but... I can't resist a good puzzle.
使用 shell 和 sed 吗？
Use the shell and sed?