Once it’s back online, you would need to manually delete languages.csv before make would run the recipe again. If the server is unavailable, languages.csv will be invalid. It avoids the need to manually clean invalid targets while not forcing you to maintain your scripts as prerequisites.įinally, imagine that extract_languages.py makes an HTTP request to a web server. DELETE_ON_ERROR is a practical alternative. However, maintaining script dependencies can be challenging (extract_languages.py may depend on many other python files). Doing so protects you in this situation while also ensuring that make rebuilds your targets if your scripts are updated. The ideal solution is to include your scripts as prerequisites. Therefore, once you do you’ll need to manually delete languages.csv to have make build it again. Since that isn’t a prerequisite, make won’t recognize that languages.csv is out of date even if you fix the bug. Now imagine there is a bug in extract_languages.py. Problem 2: defects in missing dependencies 🔗 DELETE_ON_ERROR prevents this from occurring. If an empty languages.csv didn’t cause problems downstream, make would successfully complete, possibly without the developer realizing the build targets are invalid. It’s easy to imagine a developer-especially one not familiar with the project-rerunning make without fixing the error. However, imagine this rule is part of a complex makefile which was run in parallel. Thus languages.csv will be empty, although make considers it up to date.įixing the syntax error in projects.json increments its last modified time and informs make that it needs to rebuild languages.csv. You run our example makefile and extract_languages.py has a non-zero exit status-but not before the shell’s output redirection truncates language.csv and updates its last modified time. Imagine that you introduce a syntax error while updating projects.json. Problem 1: rerunning Make with invalid targets 🔗 DELETE_ON_ERROR special target, then make deletes the target file if a recipe has an error. This behavior is often confusing and may lead to malformed build artifacts. Thus, next time make runs it won’t update the target file. If the target file was altered prior to exiting, then its last modified time will indicate that it’s up to date-even though it’s likely corrupt or incomplete. Make executes each recipe line in a new shell, one by one, and quits if an invocation exits with a non-zero status. If a prerequisite is newer than the target (or doesn’t exist), then make runs the recipe. Make processes a rule when it compares the last modification time of the target to that of its prerequisites. DELETE_ON_ERROR is the target of a rule, even though it has no prerequisites and no recipe.
Python extract_languages.py projects.json > languages.csvĪ trailing colon signals that. DELETE_ON_ERROR : languages.csv : projects.json
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