Describe how to create temporary directories, and how to execute the CLI

test with a specific current working directory (CWD).

This expands on the basic examples and ensures the common needs for CLI
app testing are covered. Hopefully, this will make testing more
accessible to people reading the book.

Author: Greenheart

src/tutorial/testing.md

@@ -472,7 +472,7 @@ at the end of the function,
 the actual temporary file will automatically get deleted.
 
 ```rust,ignore
-{{#include testing/tests/cli.rs:17:32}}
+{{#include testing/tests/cli.rs:17:30}}
 ```
 
 <aside class="exercise">
@@ -483,6 +483,31 @@ Adjust the program as needed.
 
 </aside>
 
+## Executing tests in a temporary directory
+
+To fully control the behavior of your CLI app, you can use
+`assert_fs::TempDir::new()` to create a temporary directory.
+This is useful when you need to read and write
+multiple directories or files. You can also use it as the
+current working directory (CWD) when executing your CLI app,
+ensuring test results are consistent and don't interfere with
+other, unrelated files.
+
+Temporary directories allow you to create multiple child
+directories and files within them, enabling more advanced
+test cases. Just like for single temporary files,
+all nested files and directories within temporary directories
+are also cleaned up after the test is completed.
+
+```rust,ignore
+{{#include testing/tests/cli.rs:32:56}}
+```
+
+You now know the core features of the crates `assert_cmd`,
+`assert_fs` and `predicates` and are ready to start testing
+your own CLI apps. Feel free to check out their documentation
+to find more useful features.
+
 ## What to test?
 
 While it can certainly be fun to write integration tests,

src/tutorial/testing/tests/cli.rs

@@ -28,3 +28,29 @@ fn find_content_in_file() -> Result<(), Box<dyn std::error::Error>> {
 
     Ok(())
 }
+
+#[test]
+fn find_content_in_file_of_tmp_dir() -> Result<(), Box<dyn std::error::Error>> {
+    let cwd = assert_fs::TempDir::new()?;
+
+    let child_dir = cwd.child("nested/child_dir");
+    let child_file = child_dir.child("sample.txt");
+
+    child_file.write_str("The first\ntest file.\nLast line of first file.")?;
+
+    // Files can be nested several levels within the temporary directory
+    assert!(child_file.path().ends_with("nested/child_dir/sample.txt"));
+
+    cargo_bin_cmd!("grrs")
+        // Execute in the temporary directory
+        .current_dir(cwd.path())
+        .arg("first")
+        .arg(child_file.path())
+        .assert()
+        .success()
+        .stdout(predicate::str::contains(
+            "The first\nLast line of first file.",
+        ));
+
+    Ok(())
+}