What do @, – and + do as prefixes to recipe lines in Make?

They control the behaviour of make for the tagged command lines:

• @ suppresses the normal ‘echo’ of the command that is executed.

• - means ignore the exit status of the command that is executed (normally, a non-zero exit status would stop that part of the build).

• + means ‘execute this command under make -n‘ (or ‘make -t’ or ‘make -q’) when commands are not normally executed. See also the POSIX specification for make and also §9.3 of the GNU Make manual.

The + notation is a (POSIX-standardized) generalization of the de facto (non-standardized) mechanism whereby a command line containing ${MAKE} or $(MAKE) is executed under make -n.

(@ is discussed in §5.2 of the GNU Make manual; - is described in §5.5; and §5.7.1 mentions the use of +.)