Support scripts without extension (#49332)

Co-authored-by: Rikki Gibson <[email protected]>
Co-authored-by: Jan Jones <[email protected]>

Author: 3 people

documentation/general/dotnet-run-file.md

@@ -47,7 +47,8 @@ The command takes a path which can be either
 ## Target path
 
 The path passed to `dotnet run ./some/path.cs` is called *the target path*.
-The target path must be a file which has the `.cs` file extension.
+The target path must be a file which either has the `.cs` file extension,
+or a file whose contents start with `#!`.
 *The target directory* is the directory of the target file.
 
 ## Integration into the existing `dotnet run` command
@@ -57,7 +58,7 @@ specifically `file.cs` is passed as the first command-line argument to the targe
 We preserve this behavior to avoid a breaking change.
 The file-based build and run kicks in only when:
 - a project file cannot be found (in the current directory or via the `--project` option), and
-- if the target file exists and has the `.cs` file extension.
+- if the target file exists, and has the `.cs` file extension or contents that start with `#!`.
 
 File-based programs are processed by `dotnet run` equivalently to project-based programs unless specified otherwise in this document.
 For example, the remaining command-line arguments after the first argument (the target path) are passed through to the target app

src/Cli/dotnet/Commands/Run/VirtualProjectBuildingCommand.cs

@@ -874,7 +874,28 @@ public static void RemoveDirectivesFromFile(ImmutableArray<CSharpDirective> dire
 
     public static bool IsValidEntryPointPath(string entryPointFilePath)
     {
-        return entryPointFilePath.EndsWith(".cs", StringComparison.OrdinalIgnoreCase) && File.Exists(entryPointFilePath);
+        if (!File.Exists(entryPointFilePath))
+        {
+            return false;
+        }
+
+        if (entryPointFilePath.EndsWith(".cs", StringComparison.OrdinalIgnoreCase))
+        {
+            return true;
+        }
+
+        // Check if the first two characters are #!
+        try
+        {
+            using var stream = File.OpenRead(entryPointFilePath);
+            int first = stream.ReadByte();
+            int second = stream.ReadByte();
+            return first == '#' && second == '!';
+        }
+        catch
+        {
+            return false;
+        }
     }
 }
 

test/dotnet.Tests/CommandTests/Run/RunFileTests.cs

@@ -246,14 +246,37 @@ Hello from App
     }
 
     /// <summary>
-    /// Only <c>.cs</c> files can be run without a project file,
-    /// others fall back to normal <c>dotnet run</c> behavior.
+    /// When a file is not a .cs file, we probe the first characters of the file for <c>#!</c>, and
+    /// execute as a single file program if we find them.
     /// </summary>
     [Theory]
     [InlineData("Program")]
     [InlineData("Program.csx")]
     [InlineData("Program.vb")]
-    public void NonCsFileExtension(string fileName)
+    public void NonCsFileExtensionWithShebang(string fileName)
+    {
+        var testInstance = _testAssetsManager.CreateTestDirectory();
+        File.WriteAllText(Path.Join(testInstance.Path, fileName), """
+            #!/usr/bin/env dotnet
+            Console.WriteLine("hello world");
+            """);
+
+        new DotnetCommand(Log, "run", fileName)
+            .WithWorkingDirectory(testInstance.Path)
+            .Execute()
+            .Should().Pass()
+            .And.HaveStdOutContaining("hello world");
+    }
+
+    /// <summary>
+    /// When a file is not a .cs file, we probe the first characters of the file for <c>#!</c>, and
+    /// fall back to normal <c>dotnet run</c> behavior if we don't find them.
+    /// </summary>
+    [Theory]
+    [InlineData("Program")]
+    [InlineData("Program.csx")]
+    [InlineData("Program.vb")]
+    public void NonCsFileExtensionWithNoShebang(string fileName)
     {
         var testInstance = _testAssetsManager.CreateTestDirectory();
         File.WriteAllText(Path.Join(testInstance.Path, fileName), s_program);