One of the few ways in which make does interpret command lines
is checking for a backslash just before the newline. As in normal
makefile syntax, a single command can be split into multiple lines in
the makefile by placing a backslash before each newline. A sequence
of lines like this is considered a single command, 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 command. 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 a tab, then that tab (and only that tab) is removed. Whitespace is never added to the command.
For example, 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 run one shell with a command script 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.
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 command. 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:
hello world
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 command that uses it.