TopolearnEvery Makefile, from a two-line task runner to a large C build, is made of this one construct.
A rule has three parts:
target: prerequisites
recipe line
recipe line- The target is usually a file the rule produces, such as
appormain.o. - The prerequisites are files (or other targets) it is built from.
- The recipe is one or more shell commands. Each recipe line must start with a tab character, not spaces. That is the most famous make error:
*** missing separator. Stop.
Run make target to build a target, or plain make to build the default goal, which is the first target in the file (by convention named all).
To build a target, make first makes sure each prerequisite is up to date, recursively applying their rules, then runs the target's recipe if needed. So rules form a dependency graph, and make walks it from the bottom up.
Before running each recipe line make prints it (the echo can be silenced, as shown later). If a command fails (non-zero exit status), make stops immediately with an error, so later steps never run on broken inputs.
A Makefile is normally called Makefile in the project root; make -f other.mk uses another file.
Example
app: main.o util.o
cc -o app main.o util.o
main.o: main.c util.h
cc -c main.c
util.o: util.c util.h
cc -c util.c
# $ make
# cc -c main.c
# cc -c util.c
# cc -o app main.o util.o
# $ make
# make: 'app' is up to date.Remember
- target: prerequisites, then tab-indented recipe lines
- Plain make builds the first target in the file
- make stops at the first failing command
Watch out for
- Indenting recipes with spaces (often an editor setting), which gives 'missing separator'.