This change extends command parsing and execution to recognize append-style stdout and stderr redirection, covering both builtins and external commands. The command runner was refactored into a reusable execute_command function, and file handling now switches between truncate and append modes based on the redirect operator. A substantial set of tests was added for parsing, builtin behavior, external command behavior, and realistic shell scenarios, which makes the new redirection semantics much safer to evolve. Practically, the shell can now accumulate output in files the way developers expect.