For the first time ever in my life i wrote hello world application using TDD (Test driven development). Don’t get me wrong here, i have practiced TDD before for larger applications but never have i done it for an hello world app.

Problem

Write an CLI application that prints out “Hello TDD world!” in STDOUT.

How did i do it?

• I created a project using cargo new hello-tdd-world
• I added a dependency assert_cmd which lets us test out outputs of any binary program
• Created a file tests/cli.rs and I wrote the following test

use assert_cmd::Command;

#[test]
fn test_cli_app_should_print_hello_tdd_world() {
    let mut cmd = Command::cargo_bin("hello-world-tdd").unwrap();
    cmd.assert().success().stdout("Hello TDD world!\n");
}

• Run the test using command cargo test
• The above test will fail saying the following, this happens because the cargo new had generated a src/main.rs file which printed out “Hello world!”

running 1 test
test test_cli_app_should_print_hello_tdd_world ... FAILED

failures:

---- test_cli_app_should_print_hello_tdd_world stdout ----
thread 'test_cli_app_should_print_hello_tdd_world' panicked at 'Unexpected stdout, failed diff original var
├── original: Hello TDD world!
├── diff:
│   --- 	orig
│   +++ 	var
│   @@ -1 +1 @@
│   -Hello TDD world!
│   +Hello world!
└── var as str: Hello world!

• Now that we have a failing test(RED), the next job is to make it green by modifying the contents of the println statement.
• Run cargo test again, The test now passes (Green)

The code for this is available at https://github.com/kishaningithub/hello-world-tdd

Links

• Command line rust OReilly book