Makefile Cheatsheet

  • References
  • Targets
    • target_name: dependencies
    • A target name can be a filename, multiple filenames (space separated), or just a word.
    • Dependencies work the same way.
    • The indented lines after a target are the commands to run.
    • all: $(wildcard *.json)
        convert_json $@
    • A target can use double colons file:: to allow defining the target multiple times. Otherwise this is an error.
  • Two types of variables
    • Recursively-expanded variables are lazily evaluated, and reevaluated every time the variable is used. These use the = operator to assign. e.v. VARNAME = value
    • Simply-expanded variables use := to assign, and are just evaluated at the time of assignment. e.g. VARNAME := VALUE
    • Variables are referenced using $(VARNAME) or ${VARNAME}
  • Automatic Variables
    • Make provides a bunch of ways to make things easier when working with multiple targets at once.
    • $@ will reference the dependency that the target is running against, one dependency at a time.
    • $< references the target for the current dependency (i.e. the output filename when running on filenames)
    • $? is the list of all dependencies newer than the target.
    • $^ is the list of all prerequisites.
  • Pattern matching
    • The % wildcard operator is used to perform pattern match and replacement.
    • $(filenames): %.html: will match every *.md target and generate a matching .html target for that filename.
    • Then later you can do %.html: with no dependencies to define the commands to run.
    • You can also just do %.html: to set up a dependency from any .md to turn into a .html file.
    • You can still override these targets with a target like index.html: if you want a different behavior for a particular file.
    • But you still need a way to set up the main target, and so something like this usually works well:
      • SRC := $(wildcard *.pptx)
        OUT := $(SRC:.pptx=.json)
        all: $(OUT)
        %.json: %.pptx $< > $@
  • Working with filenames
    • $(wildcard *.json) will execute a file glob and expand to the matching filenames.
  • String substitution
    • This has numerous shorthands
    • $(VARIABLE:%.o=%.c) to change .o to .c for every file in VARIABLE.
    • $(VARIABLE:.o=.c) also works when it's just a suffix.
  • When running at command, prefixing it with @ will suppress printing the command output as it runs.
  • Shell commands can be run using VARNAME := $(shell command args)
  • If you need to recursively call make, use $(MAKE) instead since it will use all the same CLI flags passed to the current call.

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