How does "make" app know default target to build if no target is specified?

By default, it begins by processing the first target that does not begin with a . aka the default goal; to do that, it may have to process other targets - specifically, ones the first target depends on.

The GNU Make Manual covers all this stuff, and is a surprisingly easy and informative read.

To save others a few seconds, and to save them from having to read the manual, here's the short answer. Add this to the top of your make file:

.DEFAULT_GOAL := mytarget

mytarget will now be the target that is run if "make" is executed and no target is specified.

If you have an older version of make (<= 3.80), this won't work. If this is the case, then you can do what anon mentions, simply add this to the top of your make file:

.PHONY: default
default: mytarget ;

References: https://www.gnu.org/software/make/manual/html_node/How-Make-Works.html

GNU Make also allows you to specify the default make target using a special variable called .DEFAULT_GOAL. You can even unset this variable in the middle of the Makefile, causing the next target in the file to become the default target.

Ref: The Gnu Make manual - Special Variables

bmake's equivalent of GNU Make's .DEFAULT_GOAL is .MAIN:

$ cat Makefile
.MAIN: foo

all:
    @echo all

foo:
    @echo foo
$ bmake
foo

See the bmake(1) manual page.