Trying to run task based on existence of a file or not

Help/Discuss
1

I am trying to run a task based on the existence of a file (if it doesn’t exist, I want to run the tasks). I have the following:

task init_file() {
  outputs.file file("init.txt")
  doLast {
    File initFile = new File("init.txt")
  }
}

task test(dependsOn: init_file) {
  println "Test"
}

When I run gradle, it always shows as up-to-date even when the file doesn’t exist. What am I doing wrong?

Thanks,

– Ken

2

A few things…

init_file looks OK. You’re registering an output correctly and you’re doing work in a doLast; however, File initFile = new File("init.txt") isn’t enough to create a file.

So what probably happened, the first time you ran the build, Gradle recorded that the correct state of init.txt after running init_file is to not exist. Since the file doesn’t exist, the task is up-to-date.

You shouldn’t use new File("init.txt") since it will try to create a file relative to the current working directory of the JVM (which might not be in your project directory). You can use file("init.txt") like you used before (it returns a File too).

If you change your task to…

task init_file() {
  outputs.file file("init.txt")
  doLast {
    def initFile = file("init.txt")
    file.text = """
    some content
"""
  }
}

And then run Gradle with --rerun-tasks, that’ll force the task to re-run and it should behave properly after that (deleting init.txt will cause the task to run again).

3

Thanks, that did work. Did not realize new File(“init.txt”) was relative to the JVM working directory.

4

I think, one has to note that “outputs.file” not strictly prevents execution. If properties of the task change, the task will be executed nevertheless.

One or more additional actions for task ‘xyz’ have changed.

As a consequence, for download tasks, it makes sense to do both: “outputs.file” AND “onlyIf … exists()”

Example

val file = layout.buildDirectory.file("resources/main/demo.csv")

outputs.file(file)
onlyIf {!file.get().asFile.exists()}
5

That’s probably not a good idea.
Well, depending on the concrete case.
Of course output file does not prevent the task from running.
If any input or output changed since last run, it is run again. The task implementation logic is also an input, if the implementation changes, the result might differ, so it is not up to date.

6

Another drawback of onlyIf is that the task is NOT (always) executed if the file does NOT exist. But this might correlate with the use of jbang gradle plugin.