Suppose you are writing a pattern rule to compile a ‘.c’ file into a ‘.o’ file: how do you write the ‘cc’ command so that it operates on the right source file name? You cannot write the name in the command, because the name is different each time the implicit rule is applied.
What you do is use a special feature of
make, the automatic
variables. These variables have values computed afresh for each rule that
is executed, based on the target and prerequisites of the rule. In this
example, you would use ‘$@’ for the object file name and ‘$<’
for the source file name.
It's very important that you recognize the limited scope in which
automatic variable values are available: they only have values within
the command script. In particular, you cannot use them anywhere
within the target list of a rule; they have no value there and will
expand to the empty string. Also, they cannot be accessed directly
within the prerequisite list of a rule. A common mistake is
attempting to use
$@ within the prerequisites list; this will
not work. However, there is a special feature of GNU
secondary expansion (see Secondary Expansion), which will allow
automatic variable values to be used in prerequisite lists.
Here is a table of automatic variables:
$^contains just one copy of the name. This list does not contain any of the order-only prerequisites; for those see the ‘$|’ variable, below.
In an explicit rule, there is no stem; so ‘$*’ cannot be determined
in that way. Instead, if the target name ends with a recognized suffix
(see Old-Fashioned Suffix Rules), ‘$*’ is set to
the target name minus the suffix. For example, if the target name is
‘foo.c’, then ‘$*’ is set to ‘foo’, since ‘.c’ is a
make does this bizarre thing only for compatibility
with other implementations of
make. You should generally avoid
using ‘$*’ except in implicit rules or static pattern rules.
If the target name in an explicit rule does not end with a recognized suffix, ‘$*’ is set to the empty string for that rule.
‘$?’ is useful even in explicit rules when you wish to operate on only the prerequisites that have changed. For example, suppose that an archive named lib is supposed to contain copies of several object files. This rule copies just the changed object files into the archive:
lib: foo.o bar.o lose.o win.o ar r lib $?
Of the variables listed above, four have values that are single file
names, and three have values that are lists of file names. These seven
have variants that get just the file's directory name or just the file
name within the directory. The variant variables' names are formed by
appending ‘D’ or ‘F’, respectively. These variants are
semi-obsolete in GNU
make since the functions
notdir can be used to get a similar effect (see Functions for File Names). Note, however, that the
‘D’ variants all omit the trailing slash which always appears in
the output of the
dir function. Here is a table of the variants:
Note that we use a special stylistic convention when we talk about these
automatic variables; we write “the value of ‘$<’”, rather than
<” as we would write for ordinary variables
CFLAGS. We think this convention
looks more natural in this special case. Please do not assume it has a
deep significance; ‘$<’ refers to the variable named
as ‘$(CFLAGS)’ refers to the variable named
You could just as well use ‘$(<)’ in place of ‘$<’.