Tool cheatsheet

GNU Make 4 Cheatsheet

Write Makefiles with targets, variables, pattern rules and phony targets, and the options for running make.

Rules and targets

Basic rule — A target, its prerequisites and a recipe. Recipe lines must begin with a tab, not spaces.
Declare phony targets — Mark targets that are commands, not files, so a file with the same name doesn't stop them running.
Set the default target — Run this target when you type make with no arguments.
Write a clean target — Remove build outputs. The leading minus ignores errors, such as files that don't exist.
Compile every source file — Build any .o file from the matching .c file. $< is the first prerequisite and $@ is the target.
Hide commands from the output — Start a recipe line with @ so make doesn't print it.
Run commands in one shell — Each recipe line runs in its own shell, so join commands with && or use a line continuation.
Create a folder before building — An order-only prerequisite (after |) must exist but doesn't force a rebuild.

Define variables — = is evaluated when used, := is evaluated once when defined, and ?= sets a default.
Automatic variables — $@ is the target, $< the first prerequisite, $^ all prerequisites.
List files and change extensions — Find source files, then derive object file names from them.
Run a command and capture its output — Store the output of a shell command in a variable.
Use conditionals — Choose settings based on a variable.
Include another makefile — Include shared rules. The leading minus ignores a missing file.
Pass variables to commands — Export a variable to the environment of recipe commands.
Self-documenting help — Print each target's ## comment. Run it with make help.

Run a target — Run one specific target from the Makefile.
Preview the commands — Print what would run without running it.
Build in parallel — Run up to four jobs at once. Use $(nproc) for one per CPU core.
Set a variable on the command line — Command-line variables override those in the Makefile.
Force a rebuild — Rebuild everything even if files look up to date.
Use another Makefile — Read a different file, or run in another directory.
Continue after errors — Keep building independent targets after a failure.
Print why targets are rebuilt — Show make's decisions, useful when a target rebuilds unexpectedly.

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