How Make Decides What to Do

Scope: the timestamp rule — the three questions Make asks about every target, and the three answers you'll see.

The three questions

For each target, Make asks:

1. Does the target file exist? If not, run the recipe.
2. Is any prerequisite newer than the target? If so, the target is out of date — run the recipe.
3. Otherwise, skip it.

That's it. Make knows nothing about what your commands do; it only compares file modification times. From the C build:

$ make
gcc -Wall -Wextra -g -c main.c -o main.o
gcc -Wall -Wextra -g -c greet.c -o greet.o
gcc -Wall -Wextra -g -o hello main.o greet.o
$ make
make: 'hello' is up to date.

The second run does nothing: hello exists and is newer than both .o files. Now touch the header — which both .c files include:

$ touch greet.h
$ make
gcc -Wall -Wextra -g -c main.c -o main.o
gcc -Wall -Wextra -g -c greet.c -o greet.o
gcc -Wall -Wextra -g -o hello main.o greet.o

Everything rebuilds, because greet.h is newer than the .o files, which makes them newer than hello. One touch, and Make traced the whole chain.

The three answers

You'll see these messages constantly:

Message	Meaning
(commands run)	Target was missing or out of date; Make rebuilt it.
make: 'hello' is up to date.	Target exists and is newer than everything it needs. Nothing to do.
make: Nothing to be done for 'backup'.	The target's prerequisites are all satisfied; typically seen for .PHONY task targets.

The limitation

Timestamps are all Make sees. If a recipe's output depends on something invisible to Make — an environment variable, a database, a file the recipe reads but isn't listed as a prerequisite — Make won't rebuild when it should. The fix is honesty: list every real input as a prerequisite, or mark the target .PHONY so it always runs.

Check: after any build, run make again immediately. If it rebuilds things you didn't touch, a prerequisite is missing or a recipe isn't writing the file it claims to — both are worth fixing before the Makefile grows.