Make updates a target if it depends on prerequisite files that have been modified since the target was last modified, or if the target does not exist.
Makefile contains a sequence of entries that specify dependencies. The first line of an entry is a blank-separated list of targets, then a colon, then a list of prerequisite files. Text following a semicolon, and all following lines that begin with a tab, are shell commands to be executed to update the target.
Sharp and newline surround comments.
The following makefile says that `pgm' depends on two files `a.o' and `b.o', and that they in turn depend on `.c' files and a common file `incl'.
Makefile entries of the form
are macro definitions. Subsequent appearances of $(string1) are replaced by string2. If string1 is a single character, the parentheses are optional.
Make infers prerequisites for files for which makefile gives no construction commands. For example, a `.c' file may be inferred as prerequisite for a `.o' file and be compiled to produce the `.o' file. Thus the preceding example can be done more briefly:
Prerequisites are inferred according to selected suffixes listed as the `prerequisites' for the special name `.SUFFIXES'; multiple lists accumulate; an empty list clears what came before. Order is significant; the first possible name for which both a file and a rule as described in the next paragraph exist is inferred. The default list is
The rule to create a file with suffix s2 that depends on a similarly named file with suffix s1 is specified as an entry for the `target' s1s2. In such an entry, the special macro $* stands for the target name with suffix deleted, $@ for the full target name, $< for the complete list of prerequisites, and $? for the list of prerequisites that are out of date. For example, a rule for making optimized `.o' files from `.c' files is
Certain macros are used by the default inference rules to communicate optional arguments to any resulting compilations. In particular, `CFLAGS' is used for cc and f77(1) options, `LFLAGS' and `YFLAGS' for lex and yacc(1) options.
Command lines are executed one at a time, each by its own shell. A line is printed when it is executed unless the special target `.SILENT' is in makefile, or the first character of the command is `@'.
Commands returning nonzero status (see intro(1)) cause make to terminate unless the special target `.IGNORE' is in makefile or the command begins with <tab><hyphen>.
Interrupt and quit cause the target to be deleted unless the target depends on the special name `.PRECIOUS'.
Other options: