GNU make manual 翻译( 一百二十九)

继续翻译

5.1.1 Splitting Recipe Lines
----------------------------

One of the few ways in which `make' does interpret recipes is checking
for a backslash just before the newline.  As in normal makefile syntax,
a single logical recipe line can be split into multiple physical lines
in the makefile by placing a backslash before each newline.  A sequence
of lines like this is considered a single recipe line, and one instance
of the shell will be invoked to run it.

   However, in contrast to how they are treated in other places in a
makefile, backslash-newline pairs are _not_ removed from the recipe.
Both the backslash and the newline characters are preserved and passed
to the shell.  How the backslash-newline is interpreted depends on your
shell.  If the first character of the next line after the
backslash-newline is the recipe prefix character (a tab by default;
*note Special Variables::), then that character (and only that
character) is removed.  Whitespace is never added to the recipe.

   For example, the recipe for the all target in this makefile:

     all :
             @echo no\
     space
             @echo no\
             space
             @echo one \
             space
             @echo one\
              space

consists of four separate shell commands where the output is:

     nospace
     nospace
     one space
     one space

   As a more complex example, this makefile:

     all : ; @echo 'hello \
             world' ; echo "hello \
         world"

will invoke one shell with a command of:

     echo 'hello \
     world' ; echo "hello \
         world"

which, according to shell quoting rules, will yield the following
output:

     hello \
     world
     hello     world

Notice how the backslash/newline pair was removed inside the string
quoted with double quotes (`"..."'), but not from the string quoted
with single quotes (`'...'').  This is the way the default shell
(`/bin/sh') handles backslash/newline pairs.  If you specify a
different shell in your makefiles it may treat them differently.

5.1.1 分割片段行
----------------------------

make 确实检查片段的少数几种方法之一是 检查新行之前的反斜线，在正常的makefile语法中，一个单独的逻辑片段行可以通过反斜线，被分割成多个物理行。像这样的一系列的行被认为是一个单独的行，并且一个shell的实例会被激活来运行它。

但是，与之相对的，在它们如何被对待的问题上，反斜线-新行 对不会被从片段中移出。反斜线和新行符号都是被保留并且传递给shell。shell 如何解析反斜线要依赖于你的shell。如果下一行的反斜线后的第一个符号是片断的前缀符号(缺省是 tab符号，*note Special Variables::)，那么此符号会被移出，空格键永远也不会加到片段里。

例如，下面这个makefile的 片段:

all :
　　@echo no\
space
　　@echo no\
　　space
　　@echo one \
space

　　@echo one\
　　 space

生成如下的四个输出:

nospace
nospace
one space
one space

 这里我插入下：

第一个是  no 直接跟反斜线，下一行无空格，也无tab符号，直接space,输出 nospace

第二个是 no 直接跟反斜线，下一行，有tab符号，然后跟space，输出 nospace

第三个是 one 后有一个空格，然后是跟反斜线，下一行，有tab符号，然后跟space，输出 one space

第四个是 one 后跟反斜线，下一行，有tab符号，然后跟一个空格，然后跟space，输出 one space

下面的makefile的更复杂的例子：

all : ; @echo 'hello \
world' ; echo "hello \
world"

会激活一个shell，执行下列指令:

echo 'hello \
world' ; echo "hello \
world"

根据shell对 反斜线的解释，会形成下列输出:

hello \
world
hello world

请注意反斜线和新行对视如何在双引号内被移出德。但是在单引号中，却没有移出来，缺省shell /bin/sh的处理方式就是这样，如果你指定了别的shell, 可能处理方式会不一样。 

后文待续