Makefile dependencies don't work for phony tar

问题:

Here is a reduced version of my Makefile:

.PHONY: all 

all: src/server.coffee
  mkdir -p bin
  ./node_modules/.bin/coffee -c -o bin src/server.coffee

I want to run make and only have it recompile when src/server.coffee has changed. However, it recompiles every time I run make:

$ make
mkdir -p bin
./node_modules/.bin/coffee -c -o bin src/server.coffee
$ make
mkdir -p bin
./node_modules/.bin/coffee -c -o bin src/server.coffee

If I change my Makefile to not use a phony target, it works as expected. New Makefile:

bin/server.js: src/server.coffee
  mkdir -p bin
  ./node_modules/.bin/coffee -c -o bin src/server.coffee

Result:

$ make
mkdir -p bin
./node_modules/.bin/coffee -c -o bin src/server.coffee
$ make
make: `bin/server.js' is up to date.

Why won't it respect my dependencies with a phony target? The reason I ask is because in reality, I won't just be compiling a single file into a single other file, so I don't want to have to keep track of the names of all the output files to use as targets.

回答1:

Rather than a phony target (which as @cmotley points out, is working exactly as it should) what you might use when you want to avoid extra work is an "empty target":

The empty target is a variant of the phony target; it is used to hold recipes for an action that you request explicitly from time to time. Unlike a phony target, this target file can really exist; but the file's contents do not matter, and usually are empty.

The purpose of the empty target file is to record, with its last-modification time, when the rule's recipe was last executed. It does so because one of the commands in the recipe is a touch command to update the target file.

However, in this case there's really no need to add an extra empty output file — you already have the output of your CoffeeScript compilation! That fits the more typical Makefile pattern, as you already demonstrated in your question. What you could do is refactor to this approach:

.PHONY: all
all: bin/server.js

bin/server.js: src/server.coffee
  mkdir -p bin
  ./node_modules/.bin/coffee -c -o bin src/server.coffee

Now you have two things: a nice conventional "all" target that is correctly phony but won't do extra work. You are also in a better position to make this more generic so you can easily add more files:

.PHONY: all
all: bin/server.js bin/other1.js bin/other2.js

bin/%.js: src/%.coffee
  mkdir -p bin
  ./node_modules/.bin/coffee -c -o bin $<

回答2:

According to the Make documentation:

The prerequisites of the special target .PHONY are considered
to be phony targets. When it is time to consider such a target, 
make will run its recipe unconditionally, regardless of whether 
a file with that name exists or what its last-modification time is.

http://www.gnu.org/software/make/manual/html_node/Special-Targets.html

Make runs the recipe of PHONY targets unconditionally - prerequisites don't matter.

回答3:

There needs to be some target file to compare against the modification time of server.coffee file. Since you don't have a concrete target make cannot know if the output is newer then the dependency or not, so it will always build all.

回答4:

As the others mentioned, make looks at the timestamps of the files to figure out if the dependencies have changed.

If you want to "emulate" a phony target with dependencies, you will have to create a real file with that name and use the touch command (on Unix systems).

I needed a solution to only clean the directory if the makefiles were changed (i.e. the compiler flags were changed, so the object files needed to be recompiled).

Here's what I used (and runs before every compilation) with a file named makefile_clean:

makefile_clean: makefile
    @rm '*.o'
    @sudo touch makefile_clean

The touch command updates the last-modified timestamp to the current time.