Build all files with a given extension

The most basic use of Make (other a simple task runner) is reading some files, compiling them somehow and outputting other files.

Here’s a minimal example that takes an existing file with extension .in (some/dir/anything.in) and writes “Created from filename.in” to a sibling file with the extension .out:

%.out: %.in
	echo "Created from $<" > $@

$< and $@ are two special sigils that are replaced with the actual filenames of the target and the first dependency, respectively. So if make were invoked with make some/dir/anything.out, the expanded command would read echo "Created from /some/dir/anything.in" > /some/dir/anything.out.

Note that the target needs to correspond to a specific input file that exists on the filesystem; if it doesn’t, make will say something like make: *** No rule to make target `nonexistentfile.out'. Stop. In order to build all files with a given extension, we need to use wildcards. Makefile tutorial has a short summary of the different types, and it’s not entirely clear to me what the difference is, but here’s my best understanding of how to use them:

INPUTS := $(wildcard *.in)
OUTPUTS := $(patsubst %.in,%.out,$(INPUTS))

%.out: %.in
	echo "Created from $<" > $@

build: $(OUTPUTS)
	echo "Built!"

This creates two variables, INPUTS and OUTPUTS. For INPUTS, $(wildcard *.in) will be expanded to all files with the extension .in. The list can be filtered further: $(wildcard src/**/*.in) will only show files in descendent directories of src. For OUTPUTS, $(patsubst %.in,%.out,$(INPUTS)) changes the extension of every .in file name in INPUTS to .out.

The task build: $(OUTPUTS) will build all the input files. Let’s say there are two files adjacent to the makefile, one.in and two.in. It’s easiest to think of this in a few steps:

Run make build from the command line.
Make sees that build has a dependency on $(OUTPUTS).
OUTPUTS expands to one.out two.out (since it’s all the files in INPUTS with their extension replaced).
Make runs the dependent tasks make one.out and make two.out, which both correspond to the %.out: %.in task. You can think of this as running that task multiple times with different arguments.
The %.out: %.in task builds one.out and two.out.
Now that the dependencies have been satisfied, make runs the build task.

The benefit to using make is that if we run through the steps again, it will only re-run the %.out: %.in task for .out files that are older than the corresponding .in files.