Define an Android app's versionName and versionCode from git tags. http://orip.org/2013/10/versioning-android-apps-with-git-tags.html

build.gradle

android {
  def gitTag = {
    def tagMatchOptions
    try {
      tagMatchOptions = "--match ${tagToBuildFrom}"
    } catch (MissingPropertyException) {
      tagMatchOptions = ""
    }
    "git describe --exact HEAD ${tagMatchOptions}".execute().text.trim()
  }()
  def gitHash = "git rev-parse --short HEAD".execute().text.trim()

  def hasModifiedDeletedOrOtherFiles = !"git ls-files -mdo --exclude-standard".execute().text.trim().isEmpty()
  def hasStagedFiles = !"git diff-index --no-ext-diff --name-only --cached HEAD".execute().text.trim().isEmpty()
  def dirtyWorkingCopy = hasModifiedDeletedOrOtherFiles || hasStagedFiles
  def gitDescription = dirtyWorkingCopy ? "${gitHash}-dirty" : gitHash

  // Version name: ignore leading non-digits and everything after the first underscore
  // Version code: take digits after the first underscore
  // e.g. tag xxx1.2.3.foo-bar_15_baz will yield versionName 1.2.3.foo-bar and versionCode 15
  def versionMatcher = gitTag =~ /^[^0-9]*([^_]*)(_([0-9]*))?/

  // TODO: die if building a public release with no tag instead of having defaults
  def versionNameFromTag = versionMatcher[0][1]
  def versionCodeFromTag = versionMatcher[0][3]?.toInteger()

  defaultConfig {
    minSdkVersion 10
    targetSdkVersion 18

    // TODO: die if building a public release with no tag instead of having defaults
    versionName versionNameFromTag ?: "dev"
    versionCode versionCodeFromTag ?: 1

    buildConfig += "public static final String GIT_DESCRIPTION = \"${gitDescription}\";"
    // Show this in the 'about' dialog
    buildConfig += "public static final String VERSION = \"${versionName} ${gitDescription}\";"
  }

  applicationVariants.all { variant ->
    def apkSuffix = "-${variant.mergedFlavor.versionName}-${gitDescription}"
    def originalApkFile = variant.outputFile
    variant.outputFile = file(originalApkFile.name.replace(".apk", "${apkSuffix}.apk"))

    // print the APK path after assemble is done
    variant.assemble.doLast {
      println variant.outputFile
    }

    // print the APK path as the last step before install, at which point the apk is complete
    variant.install?.doFirst { // the install task can be null
      println variant.outputFile
    }
  }
}