Unix in a Nutshell, 4th Edition

Makefile Lines

Instructions in the makefile are interpreted as single lines. If an instruction must span more than one input line, use a backslash (\) at the end of the line so that the next line is considered a continuation. The makefile may contain any of the following types of lines:

Blank lines

Blank lines are ignored.

Comment lines

A number sign (#) can be used at the beginning of a line or anywhere in the middle. make ignores everything after the #.

Dependency lines

One or more target names, a single- or double-colon separator, and zero or more prerequisites:

targets : prerequisitestargets ::prerequisites

In the first form, subsequent commands are executed if the prerequisites are newer than the target. The second form is a variant that lets you specify the same targets on more than one dependency line. (This second form is useful when the way you rebuild the target depends upon which prerequisite is newer.) In both forms, if no prerequisites are supplied, subsequent commands are always executed (whenever any of the targets are specified). For example, the following is invalid, since single-colon rules do not allow targets to repeated:

    # PROBLEM: Single colon rules disallow repeating targets
    whizprog.o: foo.h
            $(CC) -c $(CFLAGS) whizprog.o
            @echo built for foo.h

    whizprog.o: bar.h
            $(CC) -c $(CFLAGS) whizprog.o
            @echo built for bar.h

In such a case, the last set of rules is used and make issues a diagnostic. However, double-colon rules treat the dependencies separately, running each ...