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 make, secondary expansion (see Secondary Expansion), which will allow automatic variable values to be used in prerequisite lists.
Here is a table of automatic variables:
$@
The file name of the target of the rule. If the target is an archive member, then `$@' is the name of the archive file. In a pattern rule that has multiple targets (see Introduction to Pattern Rules), `$@' is the name of whichever target caused the rule's commands to be run.
2.
6.3.1 Substitution References
A substitution reference substitutes the value of a variable with alterations that you specify. It has the form `$(var:a=b)' (or `${var:a=b}') and its meaning is to take the value of the variable var, replace every a at the end of a word with b in that value, and substitute the resulting string.
When we say “at the end of a word”, we mean that a must appear either followed by whitespace or at the end of the value in order to be replaced; other occurrences of a in the value are unaltered. For example:
foo := a.o b.o c.o
bar := $(foo:.o=.c)
sets `bar' to `a.c b.c c.c'. See Setting Variables.
A substitution reference is actually an abbreviation for use of the patsubst expansion function (see Functions for String Substitution and Analysis). We provide substitution references as well as patsubst for compatibility with other implementations of make.
Another type of substitution reference lets you use the full power of the patsubst function. It has the same form `$(var:a=b)' described above, except that now a must contain a single `%' character. This case is equivalent to `$(patsubst a,b,$(var))'. See Functions for String Substitution and Analysis, for a description of the patsubst function.
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