It is sometimes useful to define commands which do nothing. This is done simply by giving a command that consists of nothing but whitespace. For example:
target: ;
defines an empty command string for target. You could also use a line beginning with a tab character to define an empty command string, but this would be confusing because such a line looks empty.
You may be wondering why you would want to define a command string that
does nothing. The only reason this is useful is to prevent a target
from getting implicit commands (from implicit rules or the
.DEFAULT special target; see Implicit Rules and
see Defining Last-Resort Default Rules).
You may be inclined to define empty command strings for targets that are not actual files, but only exist so that their prerequisites can be remade. However, this is not the best way to do that, because the prerequisites may not be remade properly if the target file actually does exist. See Phony Targets, for a better way to do this.