How can I create a keystore?

based on @EliuX answer, Latest tools compatible with app bundle

echo y | keytool -genkey -keystore ./android.jks -dname "n=Mark Jones, ou=JavaSoft, o=Sun, c=US" -alias android -keypass android  -storepass android -keyalg RSA -keysize 2048 -validity 2000

For your build.gradle

    signingConfigs {
        debug {
            storeFile file('android.jks')
            keyAlias 'android'
            keyPassword 'android'
            storePassword 'android'
        }
    }

I'd like to suggest automatic way with gradle only

** Define also at least one additional param for keystore in last command e.g. country '-dname', 'c=RU' **

apply plugin: 'com.android.application'

// define here sign properties
def sPassword = 'storePassword_here'
def kAlias = 'keyAlias_here'
def kPassword = 'keyPassword_here'

android {
    ...
    signingConfigs {
        release {
            storeFile file("keystore/release.jks")
            storePassword sPassword
            keyAlias kAlias
            keyPassword kPassword
        }
    }
    buildTypes {
        debug {
            signingConfig signingConfigs.release
        }
        release {
            shrinkResources true
            minifyEnabled true
            useProguard true
            signingConfig signingConfigs.release
            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
        }
    }
    ...
}

...

task generateKeystore() {
    exec {
        workingDir projectDir
        commandLine 'mkdir', '-p', 'keystore'
    }
    exec {
        workingDir projectDir
        commandLine 'rm', '-f', 'keystore/release.jks'
    }
    exec {
        workingDir projectDir
        commandLine 'keytool', '-genkey', '-noprompt', '-keystore', 'keystore/release.jks',
            '-alias', kAlias, '-storepass', sPassword, '-keypass', kPassword, '-dname', 'c=RU',
            '-keyalg', 'RSA', '-keysize', '2048', '-validity', '10000'
    }
}

project.afterEvaluate {
    preBuild.dependsOn generateKeystore
}

This will generate keystore on project sync and build

> Task :app:generateKeystore UP-TO-DATE
> Task :app:preBuild UP-TO-DATE

If you don't want to or can't use Android Studio, you can use the create-android-keystore NPM tool:

$ create-android-keystore quick

Which results in a newly generated keystore in the current directory.

More info: https://www.npmjs.com/package/create-android-keystore