Android Studio: how to generate signed apk using Gradle?

There are three ways to generate your build as per the buildType. (In your case, it's release but it can be named anything you want.)

1. Go to Gradle Task in right panel of Android Studio and search for assembleRelease or assemble(#your_defined_buildtype) under Module Tasks

2. Go to Build Variant in Left Panel and select the build from drop down

3. Go to project root directory in File Explore and open cmd/terminal and run:

   Linux: ./gradlew assembleRelease or assemble(#your_defined_buildtype)

   Windows: gradlew assembleRelease or assemble(#your_defined_buildtype)

If you want to do a release build (only), you can use Build > Generate Signed apk. For other build types, only the above three options are available.

You can find the generated APK in your module/build directory having the build type name in it.

You can use this code

android {
   ...
    signingConfigs {
        release {
            storeFile file("../your_key_store_file.jks")
            storePassword "some_password"
            keyAlias "alias_name"
            keyPassword "key_password"
        }
    }

    buildTypes {

        release {
            signingConfig signingConfigs.release
        }
    }
   ...
}

then from your terminal run

./gradlew assembleRelease

you will get the apk at

your-android-app/build/outputs/apk/your-android-app-release.apk

It is possible to take any existing Android Studio gradle project and build/sign it from the command line without editing any files. This makes it very nice for storing your project in version control while keeping your keys and passwords separate:

./gradlew assembleRelease -Pandroid.injected.signing.store.file=$KEYFILE -Pandroid.injected.signing.store.password=$STORE_PASSWORD -Pandroid.injected.signing.key.alias=$KEY_ALIAS -Pandroid.injected.signing.key.password=$KEY_PASSWORD