How to Test Command-Line Applications in Rust (assert_cmd)

Use the assert_cmd crate to spawn your binary as a subprocess and assert its exit code, stdout, and stderr.

use assert_cmd::Command;

#[test]
fn test_minigrep() {
    let mut cmd = Command::cargo_bin("minigrep").unwrap();
    cmd.arg("searchstring")
       .arg("example-filename.txt")
       .assert()
       .success()
       .stdout(predicates::str::contains("expected output"));
}

Add assert_cmd = "2.0" and predicates = "3.0" to your Cargo.toml dependencies.

Testing command-line applications in Rust with assert_cmd lets you test your tool exactly like a user would, by running it as a separate program. Instead of calling functions directly, you feed it arguments and check the text it prints or the error code it returns. It's like writing a script that runs your app and verifies the result automatically.