Anatomy of a rule #
report.pdf: report.md style.css
pandoc report.md --css style.css -o report.pdf
The target comes before the colon and prerequisites after it. Each recipe line runs in its own shell.
Describe how files are rebuilt from their prerequisites and let GNU Make run only the recipes that are out of date.
Version 3.81 Updated
make compares file modification times and reruns a rule's recipe when its target is missing or older than a prerequisite. This sheet sticks to features present in GNU Make 3.81; newer releases add more.
Rules
Recipe lines need a tab #
Warning
clean:
rm -f *.o app
Recipe lines must begin with a real tab character. Spaces cause a "missing separator" error in 3.81.
Phony targets #
.PHONY: all test clean
all: app
test: app
./run-tests.sh
clean:
$(RM) app *.o
Mark targets that are not files as phony, so a stray file named test never makes them look up to date.
Recipe prefixes #
| Prefix | Effect |
|---|---|
@echo done | Run without echoing the command |
-rm stale.lock | Ignore a failing exit status |
$$HOME | Pass a literal $ to the shell |
Default and order-only #
.DEFAULT_GOAL := all
build/app.o: app.c | build
$(CC) -c app.c -o $@
build:
mkdir -p build
Without .DEFAULT_GOAL, make builds the first target in the file. Prerequisites after | must exist but never trigger a rebuild by being newer.
Variables
Assignment flavors #
| Form | Meaning |
|---|---|
SRC = $(wildcard *.c) | Recursive: expanded each time it is used |
NOW := $(shell date +%s) | Simple: expanded once, at definition |
CC ?= cc | Set only if not already defined |
CFLAGS += -Wall | Append with a space |
make CFLAGS=-O2 | Command-line value overrides the makefile |
Automatic variables #
| Variable | Value inside a recipe |
|---|---|
$@ | The target being built |
$< | The first prerequisite |
$^ | All prerequisites, duplicates removed |
$+ | All prerequisites, duplicates kept |
$? | Prerequisites newer than the target |
$* | The stem matched by % |
$(@D), $(@F) | Directory and file part of $@ |
Built-in variables #
| Variable | Typical built-in default |
|---|---|
CC | cc |
RM | rm -f |
SHELL | /bin/sh |
MAKE | The running make, for recursion |
CURDIR | The working directory |
Print the full built-in database with make -p -f /dev/null.
Per-target values and export #
debug: CFLAGS += -g -O0
debug: app
export LANG := C
A target-specific value applies while that target and its prerequisites are built. export passes a variable to recipe shells.
Patterns and functions
Pattern rules #
OBJS := main.o parse.o util.o
app: $(OBJS)
$(CC) $(LDFLAGS) $^ -o $@
%.o: %.c defs.h
$(CC) $(CFLAGS) -c $< -o $@
% matches the same stem on both sides. A built-in %.o: %.c rule already exists; make -r disables built-in rules.
Text functions #
| Call | Result |
|---|---|
$(wildcard src/*.c) | Existing files matching a glob |
$(patsubst %.c,%.o,$(SRC)) | Pattern replacement per word |
$(SRC:.c=.o) | Suffix substitution shorthand |
$(addprefix build/,$(OBJS)) | Prefix every word |
$(filter %.c,$(FILES)) | Keep matching words |
$(filter-out test_%,$(SRC)) | Drop matching words |
$(notdir src/a.c), $(dir src/a.c) | a.c and src/ |
Loops and shell output #
DIRS := lib cli docs
STAMPS := $(foreach d,$(DIRS),$(d)/.stamp)
REV := $(shell git rev-parse --short HEAD)
$(shell ...) runs while the makefile is read; pair it with := so it runs once. Commit IDs are covered on the git sheet.
Structure and conditionals
Conditionals #
ifeq ($(OS),Windows_NT)
EXE := .exe
else
EXE :=
endif
ifndef PREFIX
PREFIX := /usr/local
endif
ifneq and ifdef complete the set. Conditionals are evaluated while the makefile is read, not when recipes run.
Include and fail early #
include config.mk
-include $(OBJS:.o=.d)
ifeq ($(strip $(VERSION)),)
$(error VERSION is not set)
endif
-include ignores missing files. $(error ...) stops make with a message.
Recursive make #
SUBDIRS := lib cli
all:
for d in $(SUBDIRS); do $(MAKE) -C $$d || exit 1; done
Use $(MAKE) rather than make so options such as -n and -j reach the child.
Running make
Common options #
| Option | Effect |
|---|---|
-n | Print the recipes without running them |
-j 4 | Run up to four jobs at once |
-k | Keep building other targets after an error |
-B | Rebuild every target unconditionally |
-C dir | Change to dir before reading makefiles |
-f build.mk | Use another makefile |
-s | Do not echo recipes |
-q | Run nothing; exit status tells if up to date |
Debugging a build #
make -n install
make --debug=b app
make -W parse.c -n
make --warn-undefined-variables
-W file pretends a file just changed; with -n it previews the rebuild.
Exit status #
Note
| Status | Meaning |
|---|---|
0 | All makefiles parsed and targets built |
1 | With -q, something needs rebuilding |
2 | An error occurred |
Without -f, make looks for GNUmakefile, makefile, then Makefile.