Makefile(1) --- .PHONY

.PHONY is a special built-in target name.

.PHONY's prerequisites are considered to be phony targets. When it is time to consider such a target, make will run its recipe unconditionally, regardless of whether a file with that name exists or
what its last-modification time is.

Case 1:

$ ls

Makefile test

$ cat Makefile

test :

echo "target test~"

$ make

make: `test' is up to date.

Case 2:

$ ls

Makefile test

$ cat Makefile

.PHONY : test

test :

echo "target test~"

$ make

echo "target test~"

target test~

用”.PHONY : test“，无论target test是否存在，它的recipes总是无条件执行。