Gradle, just like other build tools, is primarily run from a command line. That's why it is worth spending some time to get familiar with its command-line interface. Typically, a gradle command is issued from the root of a project directory with some tasks to be executed. Let's say we are in the hello-gradle directory, which is currently empty.

Gradle provides a very simple command-line interface (CLI), which takes the following form:

gradle [options…] [tasks…]

As we can see, apart from the gradle command itself, everything else is optional. The options tweak the execution of the Gradle whereas tasks, which we will see in detail later, are the basic units of work. Options are common across all projects and are specific to Gradle but tasks may vary depending on the project in which the gradle command is being run.

There are some tasks that are available on all projects. One such task is help:

$ gradle help
:help

Welcome to Gradle 2.9.

To run a build, run gradle <task> ...

To see a list of available tasks, run gradle tasks

To see a list of command-line options, run gradle --help

To see more detail about a task, run gradle help --task <task>

BUILD SUCCESSFUL

Total time: 0.639 secs

Gradle is helping us out by telling us how to find all the available tasks and list all command-line options. Let's first check what other tasks are currently available on our project. Remember we are still in the empty directory hello-gradle:

$ gradle tasks
:tasks

------------------------------------------------------------
All tasks runnable from root project
------------------------------------------------------------

Build Setup tasks
-----------------
init - Initializes a new Gradle build. [incubating]
wrapper - Generates Gradle wrapper files. [incubating]

Help tasks
----------
components - Displays the components produced by root project 'hello-gradle'. [incubating]
dependencies - Displays all dependencies declared in root project 'hello-gradle'.
dependencyInsight - Displays the insight into a specific dependency in root project 'hello-gradle'.
help - Displays a help message.
model - Displays the configuration model of root project 'hello-gradle'. [incubating]
projects - Displays the sub-projects of root project 'hello-gradle'.
properties - Displays the properties of root project 'hello-gradle'.
tasks - Displays the tasks runnable from root project 'hello-gradle'.

To see all tasks and more detail, run gradle tasks --all

To see more detail about a task, run gradle help --task <task>

BUILD SUCCESSFUL

Total time: 0.652 secs

This shows us some generic tasks that are available even without us adding any task to our project. We can try running all these tasks and see the output. We will see these tasks in details in the upcoming chapters.

The other useful command gradle help suggested us to check all the available options with the --help option.

When we run the gradle --help command, we get the following output:

$ gradle --help

USAGE: gradle [option...] [task...]

-?, -h, --help          Shows this help message.
-a, --no-rebuild        Do not rebuild project dependencies.
-b, --build-file        Specifies the build file.
…..

(The output is truncated for brevity.)

The option has a long form such as --help and may have a short from such as -h. We have already used one option before, that is --version or -v, which prints information about the Gradle version. The following are some commonly used options; there are many more options, which can be seen using the gradle --help command:

So we are now ready to get our feet wet and see our first Gradle script in action. Let's create a file called build.gradle in the hello-gradle directory. Unless the build file path is provided using the --build-file option, Gradle treats the current directory as a project root and tries to find the build.gradle file there. If we have used Ant or Maven earlier, we can relate this file with build.xml or pom.xml, respectively.

Now, open the build.gradle file and let's declare a task by adding the following line:

task helloWorld

We should be able to see this task on the command line as follows:

$ gradle tasks
...
Other tasks
-----------
helloWorld
...

Here, we have successfully created a task object called helloWorld. Tasks are first-class objects in Gradle, which means they have properties and methods on them. This gives us tremendous flexibility in terms of customizability and programmability of build.

However, this task actually does not do anything yet. So let's add some meaningful action to this task:

task helloWorld << {
  println "Hello, World!"
}

Now from the command line, we can execute this task by issuing the following command:

$ gradle -q helloWorld
Hello, World!

Notice that we used the –q flag to reduce the verbosity in the output. When this task is run, we see the output that our task generates but nothing from Gradle unless it's an error.

Now, let's try to briefly understand the build.gradle file. The first line declares the tasks and starts the body of a code block that will be executed at the end. The left shift operator (<<) might feel oddly placed, but it is very important in this context. We will see in the later chapters what it exactly means. The second line is a Groovy statement that prints the given string to the console. Also, the third line ends the code block.

$ gradle helloWorld --daemon
Starting a new Gradle Daemon for this build (subsequent builds will be faster).
:helloWorld
Hello, World!

BUILD SUCCESSFUL

Total time: 2.899 secs


$ gradle helloWorld
:helloWorld
Hello, World!

BUILD SUCCESSFUL

Total time: 0.6 secs

$ gradle wrapper 

$ gradle wrapper --gradle-version 2.9

$ ./gradlew taskName

$ gradlew taskName

In this chapter, we started with a brief introduction to Gradle. Then, we looked at manual installation and also installation via package managers. We also learned about Gradle's command-line interface. Also, finally, we wrote our first Gradle build script.

If you have followed the chapter until this point, you are all set to check out any Gradle-based project on your machine and execute builds. Also, you are equipped with the knowledge to write a very basic Gradle build script. Going forward, we will look at building Java-based projects with Gradle.