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+---
+language: make
+contributors:
+    - ["Robert Steed", "https://github.com/robochat"]
+filename: Makefile
+---
+
+A Makefile defines a graph of rules for creating a target (or targets).
+Its purpose is to do the minimum amount of work needed to update a
+target to the most recent version of the source. Famously written over a
+weekend by Stuart Feldman in 1976, it is still widely used (particularly
+on Unix) despite many competitors and criticisms.
+
+There are many varieties of make in existance, this article assumes that
+we are using GNU make which is the standard on Linux.
+
+```make
+
+# Comments can be written like this.
+
+# Files should be named Makefile and then be can run as `make <target>`.
+# Otherwise we use `make -f "filename" <target>`.
+
+# Warning - only use TABS to indent in Makefiles, never spaces!
+
+#-----------------------------------------------------------------------
+# Basics
+#-----------------------------------------------------------------------
+
+# A rule - this rule will only run if file0.txt doesn't exist.
+file0.txt:
+	echo "foo" > file0.txt
+	# Even comments in these 'recipe' sections get passed to the shell.
+	# Try `make file0.txt` or simply `make` - first rule is the default. 
+
+
+# This rule will only run if file0.txt is newer than file1.txt.
+file1.txt: file0.txt
+	cat file0.txt > file1.txt
+	# use the same quoting rules as in the shell.
+	@cat file0.txt >> file1.txt
+	# @ stops the command from being echoed to stdout.
+	-@echo 'hello'
+	# - means that make will keep going in the case of an error.
+	# Try `make file1.txt` on the commandline.
+
+# A rule can have multiple targets and multiple prerequisites
+file2.txt file3.txt: file0.txt file1.txt
+	touch file2.txt
+	touch file3.txt
+
+# Make will complain about multiple recipes for the same rule. Empty 
+# recipes don't count though and can be used to add new dependencies.
+
+#-----------------------------------------------------------------------
+# Phony Targets
+#-----------------------------------------------------------------------
+
+# A phony target. Any target that isn't a file.
+# It will never be up to date so make will always try to run it.
+all: maker process
+
+# We can declare things out of order.
+maker:
+	touch ex0.txt ex1.txt
+
+# Can avoid phony rules breaking when a real file has the same name by
+.PHONY: all maker process
+# This is a special target. There are several others.
+
+# A rule with a dependency on a phony target will always run
+ex0.txt ex1.txt: maker
+
+# Common phony targets are: all make clean install ...
+
+#-----------------------------------------------------------------------
+# Automatic Variables & Wildcards
+#-----------------------------------------------------------------------
+
+process: file*.txt	#using a wildcard to match filenames
+	@echo $^	# $^ is a variable containing the list of prerequisites
+	@echo $@	# prints the target name
+	#(for multiple target rules, $@ is whichever caused the rule to run)
+	@echo $<	# the first prerequisite listed
+	@echo $?	# only the dependencies that are out of date
+	@echo $+	# all dependencies including duplicates (unlike normal)
+	#@echo $|	# all of the 'order only' prerequisites
+
+# Even if we split up the rule dependency definitions, $^ will find them
+process: ex1.txt file0.txt
+# ex1.txt will be found but file0.txt will be deduplicated.
+
+#-----------------------------------------------------------------------
+# Patterns
+#-----------------------------------------------------------------------
+
+# Can teach make how to convert certain files into other files.
+
+%.png: %.svg
+	inkscape --export-png $^
+
+# Pattern rules will only do anything if make decides to create the \
+target.
+
+# Directory paths are normally ignored when matching pattern rules. But
+# make will try to use the most appropriate rule available.
+small/%.png: %.svg
+	inkscape --export-png --export-dpi 30 $^
+
+# make will use the last version for a pattern rule that it finds.
+%.png: %.svg
+	@echo this rule is chosen
+
+# However make will use the first pattern rule that can make the target
+%.png: %.ps
+	@echo this rule is not chosen if *.svg and *.ps are both present
+
+# make already has some pattern rules built-in. For instance, it knows 
+# how to turn *.c files into *.o files.
+
+# Older makefiles might use suffix rules instead of pattern rules
+.png.ps:
+	@echo this rule is similar to a pattern rule.
+
+# Tell make about the suffix rule
+.SUFFIXES: .png
+
+#-----------------------------------------------------------------------
+# Variables
+#-----------------------------------------------------------------------
+# aka. macros
+
+# Variables are basically all string types
+
+name = Ted
+name2="Sarah"
+
+echo:
+	@echo $(name)
+	@echo ${name2}
+	@echo $name    # This won't work, treated as $(n)ame.
+	@echo $(name3) # Unknown variables are treated as empty strings.
+
+# There are 4 places to set variables.
+# In order of priority from highest to lowest:
+# 1: commandline arguments
+# 2: Makefile
+# 3: shell enviroment variables - make imports these automatically.
+# 4: make has some predefined variables
+
+name4 ?= Jean
+# Only set the variable if enviroment variable is not already defined.
+
+override name5 = David
+# Stops commandline arguments from changing this variable.
+
+name4 +=grey
+# Append values to variable (includes a space).
+
+# Pattern-specific variable values (GNU extension).
+echo: name2 = Sara # True within the matching rule
+	# and also within its remade recursive dependencies
+	# (except it can break when your graph gets too complicated!)
+
+# Some variables defined automatically by make.
+echo_inbuilt:
+	echo $(CC)
+	echo ${CXX)}
+	echo $(FC)
+	echo ${CFLAGS)}
+	echo $(CPPFLAGS)
+	echo ${CXXFLAGS}
+	echo $(LDFLAGS)
+	echo ${LDLIBS}
+
+#-----------------------------------------------------------------------
+# Variables 2
+#-----------------------------------------------------------------------
+
+# The first type of variables are evaluated each time they are used.
+# This can be expensive, so a second type of variable exists which is
+# only evaluated once. (This is a GNU make extension)
+
+var := hello
+var2 ::=  $(var) hello
+#:= and ::= are equivalent.
+
+# These variables are evaluated procedurely (in the order that they 
+# appear), thus breaking with the rest of the language !
+
+# This doesn't work
+var3 ::= $(var4) and good luck
+var4 ::= good night
+
+#-----------------------------------------------------------------------
+# Functions
+#-----------------------------------------------------------------------
+
+# make has lots of functions available.
+
+sourcefiles = $(wildcard *.c */*.c)
+objectfiles = $(patsubst %.c,%.o,$(sourcefiles))
+
+# Format is $(func arg0,arg1,arg2...)
+
+# Some examples
+ls:	* src/*
+	@echo $(filter %.txt, $^)
+	@echo $(notdir $^)
+	@echo $(join $(dir $^),$(notdir $^))
+
+#-----------------------------------------------------------------------
+# Directives
+#-----------------------------------------------------------------------
+
+# Include other makefiles, useful for platform specific code
+include foo.mk
+
+sport = tennis
+# Conditional compilation
+report:
+ifeq ($(sport),tennis)
+	@echo 'game, set, match'
+else
+	@echo "They think it's all over; it is now"
+endif
+
+# There are also ifneq, ifdef, ifndef
+
+foo = true
+
+ifdef $(foo)
+bar = 'hello'
+endif
+```
+
+
+### More Resources
+
++ [gnu make documentation](https://www.gnu.org/software/make/manual/)
++ [software carpentry tutorial](http://swcarpentry.github.io/make-novice/)
++ learn C the hard way [ex2](http://c.learncodethehardway.org/book/ex2.html) [ex28](http://c.learncodethehardway.org/book/ex28.html)
+