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; see 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
Sometimes you want to split a long line inside of single quotes, but
you don't want the backslash-newline to appear in the quoted content.
This is often the case when passing scripts to languages such as Perl,
where extraneous backslashes inside the script can change its meaning
or even be a syntax error. One simple way of handling this is to
place the quoted string, or even the entire command, into a
make variable then use the variable in the recipe. In this
situation the newline quoting rules for makefiles will be used, and
the backslash-newline will be removed. If we rewrite our example
above using this method:
HELLO = 'hello \ world' all : ; @echo $(HELLO)
we will get output like this:
If you like, you can also use target-specific variables (see Target-specific Variable Values) to obtain a tighter correspondence between the variable and the recipe that uses it.