• Relies on fork, waitpid, and execvp.
• This first version operates as a read-eval-print loop (keyword: repl), responding to many things we type in by forking off child processes.
  • Each child process is initially a deep clone of the shell.
  • Each child proceeds to replace its own process image with the new one we specify (e.g. ls, cp, our own CS110 search (which we wrote the second day of class), or even emacs.
  • A trailing ampersand (&) (e.g. emacs &) is an instruction to execute in the background, without blocking access to the terminal.
• Implementation of simplesh is presented over the next three slides. Where helper functions don't rely on CS110 concepts, I omit their implementations (but will describe them in lecture).

• Here's the first half (full implementation right here)

int main(int argc, char *argv[]) {
  while (true) {
    char command[kMaxCommandLength + 1];
    readCommand(command, sizeof(command) - 1);
    if (feof(stdin)) break;
    char *arguments[kMaxArgumentCount + 1];
    int count = parseCommandLine(command, arguments, 
                                 sizeof(arguments)/sizeof(arguments[0]));
    if (count == 0) continue;
    bool builtin = handleBuiltin(arguments);
    if (builtin) continue; // it's been handled, move on
    bool isBackgroundProcess = strcmp(arguments[count - 1], "&") == 0;
    if (isBackgroundProcess) arguments[--count] = NULL; // overwrite "&"
    pid_t pid = forkProcess();

• Here's the second half

    if (pid == 0) {
      if (execvp(arguments[0], arguments) < 0) {
        printf("%s: Command not found\n", arguments[0]);
        exit(0);
      }
    }

    if (!isBackgroundProcess) {
      waitForChildProcess(pid);
    } else {
      printf("%d %s\n", pid, command);
    }
  }

  printf("\n");
  return 0;
}

• Here are a few helper routines that do rely on CS110 material:

static bool handleBuiltin(char *arguments[]) {
  if (strcasecmp(arguments[0], "quit") == 0) exit(0);
  return strcmp(arguments[0], "&") == 0;
}

static pid_t forkProcess() {
  pid_t pid = fork();
  exitIf(pid == -1, kForkFailed, stderr, "fork function failed.\n");
  return pid;
}

static void waitForChildProcess(pid_t pid) {
  exitUnless(waitpid(pid, NULL, 0) == pid, kWaitFailed,
             stderr, "Error waiting in foreground for process %d to exit", pid);
}