blog: Make Pattern Rules

content/blog/2024/make-pattern-rules/Makefile

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+# A Makefile that explores pattern and grouped rules. This Makefile expects the
+# GNU version of Make.
+
+# Relevant docs:
+# https://www.gnu.org/software/make/manual/html_node/Automatic-Variables.html
+# https://www.gnu.org/software/make/manual/html_node/Pattern-Match.html
+# https://www.gnu.org/software/make/manual/html_node/Pattern-Intro.html
+# https://www.gnu.org/software/make/manual/html_node/Multiple-Targets.html
+
+
+NAMES=zip zap
+
+.PHONY: all clean
+
+all: $(NAMES:%=%.x) $(NAMES:%=%.y)
+
+
+# Here's a simple pattern rule that generates a .txt file from a name. In this
+# rule % represents the name (the 'stem' in Makefile lingo) in the targets and
+# prerequisites, and $* represents the stem in the recipe.
+
+%.txt:
+	@echo "=> Making $*.txt"
+	python -c 'print("$*!".title())' > "$@"
+
+
+# Here are a couple explicit rules that generate a single file given some input
+# files. Note the input files (prerequisites) will be generated with the pattern
+# rule above.
+
+zip.a : florp.txt
+	@echo "=> Making $@ with $<"
+	cat "$<" > "$@"
+
+zap.a : bloop.txt
+	@echo "=> Making $@ with $<"
+	cat "$<" > "$@"
+
+
+# This is a Grouped rule, marked with &:. It indicates that the two target files
+# are produced together in one invocation of the rule, rather than two separate
+# files produced by the same rule.
+
+zip.x zip.y &: zip.a
+	@echo "=> Making XY files from $< with explicit rule"
+	echo "[X]" > zip.x
+	cat "$<" >> zip.x
+	echo "[Y]" > zip.y
+	cat "$<" >> zip.y
+
+
+# Pattern rules with multiple targets are always Grouped, so you don't need the
+# &: separator.
+
+%.x %.y : %.a
+	@echo "=> Making XY files from $< with pattern rule"
+	echo "[X]" > $*.x
+	cat "$<" >> $*.x
+	echo "[Y]" > $*.y
+	cat "$<" >> $*.y
+
+
+clean:
+	rm -f bloop.txt florp.txt
+	rm -f $(NAMES:%=%.a)
+	rm -f $(NAMES:%=%.x)
+	rm -f $(NAMES:%=%.y)