xv6 Shell

xv6 shell's source code is a good starting point for understanding xv6 and Unix's syscall interface design.

sh.c - mit-pdos/xv6-riscv

Shell is essentially an interpreter. It takes user input, parses it, and use relevant system calls to finish the job. It behaves just like any other normal user program.

We begin from main() function.

int
main(void)
{
  static char buf[100];
  int fd;

  // Ensure that three file descriptors are open.
  while ((fd = open("console", O_RDWR)) >= 0) {
    if (fd >= 3) {
      close(fd);
      break;
    }
  }
  ...
}

The while loop is to ensure that the first three file descriptors (stdin, stdout, stderr) are open. The operating system assigns fd by increasing order. When fd 0, 1, 2 are accidentally closed, what the shell opens next might be assigned to fd 0, 1, or 2, leading to unexpected write/read behavior, which could be a security issue.

Then it reads a line from stdin, stores it in buf like [xxx\n\0], and strips the leading blank characters. If the command is cd, it calls chdir(), otherwise it does a fork syscall.

int
main(void)
{
  ...
  // Read and run input commands.
  while (getcmd(buf, sizeof(buf)) >= 0) {
    char *cmd = buf;
    while (*cmd == ' ' || *cmd == '\t')
      cmd++;
    if (*cmd == '\n') // is a blank command
      continue;
    if (cmd[0] == 'c' && cmd[1] == 'd' && cmd[2] == ' ') {
      // Chdir must be called by the parent, not the child.
      cmd[strlen(cmd) - 1] = 0; // chop \n
      if (chdir(cmd + 3) < 0)
        fprintf(2, "cannot cd %s\n", cmd + 3);
    } else {
      if (fork1() == 0)
        runcmd(parsecmd(cmd));
      wait(0);
    }
  }
  exit(0);
}

In the child process, fork returns 0, and it calls runcmd(parsecmd(cmd)) to parse the command and execute it. In the parent process, fork returns the child's pid, and it calls wait(0) to wait for the child process to finish.

So far, cmd is a string, parsecmd(cmd) turns it into a struct cmd * by parsing the string.