Compile and launch android app from the command line.

build.gradle

task deployDebug(type: Exec, dependsOn: 'app:installDebug') {
  def rootDir = project.rootDir
  def localProperties = new File(rootDir, "local.properties")
  if (localProperties.exists()) {
    Properties properties = new Properties()
    localProperties.withInputStream {
      inputStream -> properties.load(inputStream)
    }
    def sdkDir = properties.getProperty('sdk.dir')
    def adb = "$sdkDir/platform-tools/adb"
    commandLine "$adb", 'shell', 'am', 'start', '-n', 'com.package/com.package.YourMainActivity'
  }
}

You can also create a Gradle run configuration on Android Studio/Intellij.

Instead of doing all this $adb parsing, you can write
commandLine android.getAdbExe(), 'shell', 'am', 'start', '-n', "${android.defaultConfig.applicationId}/.MainActivity"

@Whoww the problem is that if you have a project with multiple modules.
My solution is for the main build.gradle file where you do not have any android plugin get the adb location. 😄

task start(type: Exec) {
    dependsOn 'installDebug'
    commandLine 'adb', 'shell', 'am', 'start', '-n', "$android.defaultConfig.applicationId/.MainActivity"
}

@jlobos thanks!!