Phony

PHONY refers to a category of targets used in build systems, most prominently GNU Make, that do not correspond to actual files. A phony target is treated as always out of date, so its recipe is executed whenever the target is requested, regardless of whether a file with the same name exists or what its timestamp is.

In Makefiles, phony targets are declared with the special target .PHONY followed by one or more target names. Declaring a target as phony helps ensure that Make runs its recipe instead of trying to decide whether a corresponding file is up to date. For example:

.PHONY: clean

clean:

rm -f *.o

rm -f core

Common phony targets include build, test, install, all, and clean. These are typically used to perform actions rather than to produce or update a file with the same name. By declaring them as phony, make avoids confusion if a file with the same name appears in the directory.

The behavior of phony targets interacts with prerequisites in the same way as ordinary targets. If a phony target has prerequisites, Make will build those prerequisites as needed before executing the phony target’s recipe. However, the phony target itself is not gated by the existence or timestamp of a file named after it; its recipe will be executed when invoked.

PHONY usage is supported by GNU Make and is widely adopted in other make implementations. It helps prevent accidental clashes with real files and clarifies the intended actions of the build process.