问题
In a makefile, escaping a new-line with \ allows to split a single-line long string content across multiple source lines. However, the new-line is replaced with a space. Is there a transparent line break in the source that does not affect the string content?
VAR=w\
o\
r\
d
all:
echo $(VAR)
The desired output is 'word', but the actual output is 'w o r d'.
回答1:
The simplest solution is to use $\<newline> to split the
line (at least if you are using GNU Make):
VAR = w$\
o$\
r$\
d
all:
echo $(VAR)
The output will be "word" with no spaces. This is because GNU
Make will replace backslash-newline-whitespace with a single
space, making the assignment to VAR be equivalent to:
VAR = w$ o$ r$ d
From
https://www.gnu.org/software/make/manual/html_node/Reference.html#Reference:
"A dollar sign followed by a character other than a dollar sign,
open-parenthesis or open-brace treats that single character as
the variable name." So the $<space> pairs are expansions of
the variable whose name is a single space character. Since this
variable is not defined by default, it will expand to the empty
string.
Note that the variable VAR will still contain the
$<space> pairs until it is expanded. Most of the time, this
doesn't matter, but if your makefile depends on using
$(value VAR) to process the underlying (unexpanded) value,
the above technique may provide surprising results.
For other ideas, see my answer to a similar question here: How can I break a variable definition across multiple lines in a Makefile without spaces?
A longer treatment of line continuation options can be found in my article "GNU Make line continuations": http://drmikehenry.com/gnu-make-line-continuations/
回答2:
This was just asked yesterday: How can I break a variable definition across multiple lines in a Makefile without spaces?
The short answer is no, there's no way to do that. This behavior is required by the POSIX standard for make.
All you can do is try postprocessing the string to remove the whitespaces using $(subst ...) or similar.
来源：https://stackoverflow.com/questions/21246165/how-to-break-a-string-across-lines-in-a-makefile-without-spaces