Port command (#5849)

* implement port command

* better comment

* fmt code

* fix example description

* fix usage

* fix tests

crates/nu-command/tests/commands/mod.rs

@@ -36,6 +36,7 @@ mod math;
 mod merge;
 mod mkdir;
 mod move_;
+mod network;
 mod open;
 mod parse;
 mod path;

crates/nu-command/tests/commands/network/mod.rs

@@ -0,0 +1 @@
+mod port;

crates/nu-command/tests/commands/network/port.rs

@@ -0,0 +1,52 @@
+use nu_test_support::{nu, pipeline};
+use std::net::TcpListener;
+use std::sync::mpsc;
+
+#[test]
+fn port_with_invalid_range() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+        port 4000 3999
+        "#
+    ));
+
+    assert!(actual.err.contains("Invalid range"))
+}
+
+#[test]
+fn port_with_already_usage() {
+    let (tx, rx) = mpsc::sync_channel(0);
+
+    // let system pick a free port for us.
+    let free_port = {
+        let listener = TcpListener::bind("127.0.0.1:0").expect("failed to pick a port");
+        listener.local_addr().unwrap().port()
+    };
+    let handler = std::thread::spawn(move || {
+        let _listener = TcpListener::bind(format!("127.0.0.1:{free_port}"));
+        let _ = rx.recv();
+    });
+    let actual = nu!(
+        cwd: ".", pipeline(&format!("port {free_port} {free_port}"))
+    );
+    let _ = tx.send(true);
+    // make sure that the thread is closed and we release the port.
+    handler.join().unwrap();
+
+    // check for error kind str.
+    assert!(actual.err.contains("AddrInUse"))
+}
+
+#[test]
+fn port_from_system_given() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+        port
+        "#
+    ));
+
+    // check that we can get an integer port from system.
+    assert!(actual.out.parse::<u16>().unwrap() > 0)
+}