Gradle task that runs detekt only over changed files

detekt.gradle

configurations { detekt }
dependencies { detekt "io.gitlab.arturbosch.detekt:detekt-cli:$detektVersion" }

task detektCi(type: JavaExec, group: "verification") {
  description = "Run Kotlin static analysis on changed files."
  group = "CI"
  main = "io.gitlab.arturbosch.detekt.cli.Main"
  classpath = configurations.detekt

  doFirst {
    def changedFilesList = getDiffedFilesFromBranch("main")

    if (changedFilesList.isEmpty()) {
      println("No kotlin files changed! Skipping task...")
      // forces detekt to ignore all files
      changedFilesList = "$rootDir/gradle"
    } else {
      println("Running detekt on the changed files:")
      println(changedFilesList)
    }

    def reportDir = "${buildDir}/reports/detekt"
    def params = [
      "--input", "$changedFilesList",
      "--excludes", "!**/src/**/*Test.kt, **/spotless.kt",
      "--config", "$rootDir/ci-cd/detekt-config.yml",
      "--report", "xml:$reportDir/detekt-checkstyle.xml",
      "--report", "html:$reportDir/report.html"
    ]
    args(params)
  }
}

private static String getDiffedFilesFromBranch(String branch) {
  def outputStream = new ByteArrayOutputStream()
  def cmd = "git diff --diff-filter=d --name-only origin/$branch --relative | grep '\\.kt\\?\$'"
  execute(cmd, outputStream)

  if (outputStream.toString().isEmpty()) return ""
  // get comma separated string of files
  return outputStream.toString()
    .trim()
    .replace("\n", ",")

}

private static def execute(cmd, output) {
  ['sh', '-c', cmd].execute().waitForProcessOutput(output, System.err)
}

Thank you for this nice little task! :) Just wanted to mention that some CIs only shallow clone and therefore git diff doesn't work. The result looks like:

fatal: ambiguous argument 'origin/main': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'

What helped was to fetch the needed branch before:

private static String getDiffedFilesFromBranch(String branch) {
    def outputStream = new ByteArrayOutputStream()
    def fetchCommand = "git fetch origin $branch"
    def diffCommand = "git diff --diff-filter=d --name-only origin/$branch --relative | grep '\\.kt\\?\$'"

    execute(fetchCommand)
    println("Fetched base branch to calculate diff for Detekt")

    execute(diffCommand, outputStream)

    if (outputStream.toString().isEmpty()) return ""
    // get comma separated string of files
    return outputStream.toString()
            .trim()
            .replace("\n", ",")
}

Maybe you like to add it if others stumble upon this problem too.