A makefile recipe runs every time even when target is more recent than dependency

Question

Makefile:

node_modules: package.json ## Run npm install
        @npm install

Why does make node_modules excecute the receipe npm install every time I run make node_modules?

I believe the recipe should not get executed if my file package.json is not more recent that the directory node_modules.

Answer 1

The link that @IsmailBadawi provided in comments is the key here.

Make determines if a target is out of date based on its last-modified time (mtime). However, a directory is only considered to be modified when one of its direct children are added/removed/renamed (at least on Unix-like systems).

This is insufficient in your case. Imagine the following sequence:

1. Run make for the first time.

   • Rule is not skipped.
   • mtime(node_modules) > mtime(package.json).

2. Run make again.

   • Rule is skipped.

3. Change a dependency version in package.json.

   • mtime(node_modules) < mtime(package.json).

4. Run make again.

   • Rule is not skipped.
   • Dependency is updated in node_modules.
   • But mtime(node_modules) is unaffected.

5. Run make again.

   • Rule is not skipped.

6. ... and so on forever ...

You can probably fix this by adding touch -m node_modules to your recipe.