Open and read file line by line

lib/day1/command.dart

@@ -1,3 +1,6 @@
+import "dart:convert";
+import "dart:io";
+
 import "package:args/command_runner.dart";
 
 class Day1Command extends Command {
@@ -19,6 +22,31 @@ class Day1Command extends Command {
   void run() {
     // [argResults] is set before [run()] is called and contains the flags/options
     // passed to this command.
-    print(argResults?.rest);
+    print(argResults!.rest);
+
+    if (argResults!.rest.length != 1) {
+      print(
+        "Expected 1 positional arguments, found ${argResults!.rest.length}",
+      );
+    }
+    var fileName = argResults!.rest[0];
+
+    var inputFile = File(fileName);
+    var openFile = inputFile.openRead();
+
+    openFile
+        .transform(utf8.decoder)
+        .transform(LineSplitter())
+        .listen(
+          (String line) {
+            print('$line: ${line.length} bytes');
+          },
+          onDone: () {
+            print('File is now closed.');
+          },
+          onError: (e) {
+            print(e.toString());
+          },
+        );
   }
 }