Set the arg table within shell.run

This makes us a little more compatible with Lua

src/main/resources/assets/computercraft/lua/rom/programs/shell.lua

@@ -130,8 +130,12 @@ local function run( _sCommand, ... )
             end
             multishell.setTitle( multishell.getCurrent(), sTitle )
         end
+
         local sDir = fs.getDir( sPath )
-        local result = os.run( createShellEnv( sDir ), sPath, ... )
+        local env = createShellEnv( sDir )
+        env[ "arg" ] = { [0] = _sCommand, ... }
+        local result = os.run( env, sPath, ... )
+
         tProgramStack[#tProgramStack] = nil
         if multishell then
             if #tProgramStack > 0 then

src/test/resources/test-rom/spec/programs/shell_spec.lua

@@ -0,0 +1,17 @@
+describe("The shell", function()
+    describe("shell.run", function()
+        it("sets the arguments", function()
+            local handle = fs.open("test-files/out.txt", "w")
+            handle.writeLine("_G.__arg = arg")
+            handle.close()
+
+            shell.run("/test-files/out.txt", "arg1", "arg2")
+            fs.delete("test-files/out.txt")
+
+            local args = _G.__arg
+            _G.__arg = nil
+
+            expect(args):same { [0] = "/test-files/out.txt", "arg1", "arg2" }
+        end)
+    end)
+end)