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I know gradle has a system for not re-running tasks if input files haven't changed, but how do I do that for a custom task? like run "yarn run build" only if files in src/js/ haven't changed?

When you create a custom task you can set what are the inputs of that task

<https://docs.gradle.org/current/userguide/implementing_custom_tasks.html>

Concretely:

<https://docs.gradle.org/current/userguide/implementing_custom_tasks.html#declaring_inputs_and_outputs>

regarding the exec task, what I'm a looking for? <https://docs.gradle.org/current/dsl/org.gradle.api.tasks.Exec.html>

Do you mean you want to implement a custom task that uses Exec?

In that case you can inject `ExecOperations` via constructor

<https://docs.gradle.org/current/kotlin-dsl/gradle/org.gradle.process/-exec-operations/index.html>

do I need a custom task in buildSrc? currently I'm defining it like this
```tasks.register&lt;Exec&gt;("compileOldJs") {
    workingDir = layout.projectDirectory.dir("oldsrc/").asFile
    commandLine = listOf("yarn", "run", "build")
}```

you can use `inputs` and `outputs` inside the definition to configure inputs and outputs.

Another option you can use the Node Gradle plugin

• <https://github.com/node-gradle/gradle-node-plugin>

looks like they're using
```task buildAngularApp(type: NpxTask) {
  dependsOn npmInstall
  command = 'ng'
  args = ['build', '--prod']
  inputs.files('package.json', 'package-lock.json', 'angular.json', 'tsconfig.json', 'tsconfig.app.json')
  inputs.dir('src')
  inputs.dir(fileTree("node_modules").exclude(".cache"))
  outputs.dir('dist')
}```

however this is re-run every time:

```tasks.register&lt;Exec&gt;("compileOldJs") {
    inputs.dir(fileTree("./oldsrc/src/"))
    workingDir = layout.projectDirectory.dir("oldsrc/").asFile
    commandLine = listOf("yarn", "run", "build")
}```

I think you need to declare outputs as well. Run Gradle with —info to find out why a task is being rerun.

```tasks.register&lt;Exec&gt;("compileOldJs") {
    inputs.dir(layout.projectDirectory.dir("./oldsrc/src/"))
    outputs.dir(layout.projectDirectory.dir("./oldsrc/dist/"))
    workingDir = layout.projectDirectory.dir("oldsrc/").asFile
    commandLine = listOf("yarn", "run", "build")
}```