Shell: Update shebang handling logic

This bit of code was kept unmodified since it was first implemented,
and I'm not entirely convinced that it ever actually worked :P
This commit updates the code to use "modern" classes and constructs,
and fixes an issue where the shebang would still contain the '#!'
when it was passed to execvp().
Fixes SerenityOS#6774.

Userland/Shell/Shell.cpp

@@ -869,20 +869,24 @@ void Shell::execute_process(Vector<const char*>&& argv)
  _exit(126);
  }
  if (saved_errno == ENOENT) {
- int shebang_fd = open(argv[0], O_RDONLY);
- auto close_argv = ScopeGuard([shebang_fd]() { if (shebang_fd >= 0) close(shebang_fd); });
- char shebang[256] {};
- ssize_t num_read = -1;
- if ((shebang_fd >= 0) && ((num_read = read(shebang_fd, shebang, sizeof(shebang))) >= 2) && (StringView(shebang).starts_with("#!"))) {
- StringView shebang_path_view(&shebang[2], num_read - 2);
- Optional<size_t> newline_pos = shebang_path_view.find_first_of("\n\r");
- shebang[newline_pos.has_value() ? (newline_pos.value() + 2) : num_read] = '\0';
- argv[0] = shebang;
+ do {
+ auto file_result = Core::File::open(argv[0], Core::IODevice::OpenMode::ReadOnly);
+ if (file_result.is_error())
+ break;
+ auto& file = file_result.value();
+ auto line = file->read_line();
+ if (!line.starts_with("#!"))
+ break;
+ GenericLexer shebang_lexer { line.substring_view(2) };
+ auto shebang = shebang_lexer.consume_until(is_any_of("\n\r")).to_string();
+ argv.prepend(shebang.characters());
  int rc = execvp(argv[0], const_cast<char* const*>(argv.data()));
- if (rc < 0)
- fprintf(stderr, "%s: Invalid interpreter \"%s\": %s\n", argv[0], &shebang[2], strerror(errno));
- } else
- fprintf(stderr, "%s: Command not found.\n", argv[0]);
+ if (rc < 0) {
+ fprintf(stderr, "%s: Invalid interpreter \"%s\": %s\n", argv[0], shebang.characters(), strerror(errno));
+ _exit(126);
+ }
+ } while (false);
+ fprintf(stderr, "%s: Command not found.\n", argv[0]);
  } else {
  if (S_ISDIR(st.st_mode)) {
  fprintf(stderr, "Shell: %s: Is a directory\n", argv[0]);