Makefile: return first two characters of a string

Given a string in a Makefile, is it possible to ex开发者_开发技巧tract 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

---

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 substr function. 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:

EMPTY:=
SPACE:=$(EMPTY) $(EMPTY)
VAR := UnKnown
TMP:=$(subst a,a ,$(subst b,b ,$(subst c,c ,.........$(VAR)))))
# TMP now has "U n K n o w n"

Next you can use the $(wordlist) function to grab the first two "words" of the intermediate result:

TMP2:=$(wordlist 1,2,$(TMP))
# TMP2 now has "U n"

Finally, you use $(subst) again, now to strip out the space that you injected originally:

VAR_TERSE := $(subst $(SPACE),,$(TMP2))
# VAR_TERSE now has "Un"

---

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.

$(eval FOO := $$$(VAR))
$(eval FOO := $$$(FOO))
VAR_TERSE:= $(VAR:$(FOO)=)

---

Use the shell and sed?

define get_first_two_char
$(shell echo $1 | sed 's/^\(..\).*/\1/' )
endef