GNU Makefile Conventions Cheat Sheet (Based on Section 7.2 of GNU Coding Standards)

General Structure

all: Compile the entire program.
install: Compile and copy executables, libraries, etc., to their installation directories.
install-html, install-dvi, install-pdf, install-ps: Alternative installations for documentation.
uninstall: Delete all installed files.
clean: Delete files created during compilation.
distclean: Delete all files not in the source distribution.
mostlyclean: Like clean, but keep files you may want to keep around.
maintainer-clean: Delete almost everything that can be rebuilt.
TAGS: Update all tags files.
check or test: Run test suites.
installcheck: Check the installed programs or libraries.
dist: Create a source distribution archive.

stage1, stage2, etc.: Partially build the system, useful for bootstrapping or rebuilding.
info, dvi, pdf, ps, html: Create documentation in alternative formats.

prefix: Prefix for installation directories, usually /usr/local.
exec_prefix: Prefix for executable-specific files, derived from prefix.
bindir: Location for executables, typically ${exec_prefix}/bin.
sbindir: Location for system executables, typically ${exec_prefix}/sbin.
libexecdir: Location for program executables not meant for user interaction, typically ${exec_prefix}/libexec.
datarootdir: Location for read-only data, typically ${prefix}/share.
datadir: Location for data files that can be shared among different architecture platforms, typically ${datarootdir}.
sysconfdir: Location for configuration files, typically ${prefix}/etc.
sharedstatedir: Location for data that can be shared by different versions of a program, usually ${prefix}/com.
localstatedir: Location for data that changes as the program runs, usually ${prefix}/var.
includedir: Header file directory, typically ${prefix}/include.
oldincludedir: Directory for storing old header files, typically /usr/include.
docdir: Location for documentation files, typically ${datarootdir}/doc/${PACKAGE}.
infodir, htmldir, dvidir, pdfdir, psdir: Specialized directories for different types of documentation, typically under ${datarootdir}.

DESTDIR: Use this variable for staged installs, which allows you to install files into a temporary location (DESTDIR) rather than their final location.
- Commonly used in package building.
- Example: make install DESTDIR=/tmp/package-root
- Note: DESTDIR is prepended to the file paths but should not change the code's behavior.

ifeq ($(CC),gcc)
CFLAGS += -Wall
endif

Use .PHONY to declare phony targets, ensuring they always execute.

.PHONY: all install uninstall clean distclean mostlyclean maintainer-clean TAGS check test installcheck dist info dvi pdf ps html

Suppress command echoing with @:

@echo "Building..."

Continue despite errors in certain rules with -:

-rm some-file

=: Recursive expansion. Value is expanded each time the variable is used.
```
VAR = value
```
:=: Simple expansion. Value is expanded at the time of assignment.
```
VAR := value
```
?=: Conditional variable assignment. Assigns value only if the variable is not yet defined.
```
VAR ?= value
```
+=: Append to the variable value. Adds the given value to the existing variable value.
```
VAR += value
```

Define implicit rules for transforming file types.
```
%.o: %.c
    $(CC) $(CFLAGS) -c $< -o $@
```
Implicit rules can simplify your Makefile by applying general rules for file transformations.