Question

What does the following do in a Makefile?

rule: $(deps)
    @:

I can’t seem to find this in the make manual.

Answer 1

It means “don’t echo this command on the output.” So this rule is saying “execute the shell command : and don’t echo the output.

Of course the shell command : is a no-op, so this is saying “do nothing, and don’t tell.”

Why?

The trick here is that you’ve got an obscure combination of two different syntaxes. The make(1) syntax is the use of an action starting with @, which is simply not to echo the command. So a rule like

always:
       @echo this always happens

won’t emit

   echo this always happens
   this always happens

Now, the action part of a rule can be any shell command, including :. Bash help explains this as well as anywhere:

$ help :
:: :
    Null command.

    No effect; the command does nothing.

    Exit Status:
    Always succeeds.