nohup原理

nohup

nohup 执行会忽略信号 SIGHUP，并将 stdout/stderr 重定向到文件 nohup.out。以便shell在关闭或注销后命令可以在后台继续运行 。nohup做的工作就是让 nohup 后的命令不在是当前 shell 的子命令。而是PPID=1的进程（进程的PPID=1）。这种情况下不能被带回到前台。

signal (SIGHUP, SIG_IGN); // 忽略信号SIGHUP

char **cmd = argv + optind;
execvp (*cmd, cmd); // 在执行这个命令，而不是当前shell

对于输出的重定向，对于STDOUT/STDERR会忽略，然后写入到 nohup.out

  ignoring_input = isatty (STDIN_FILENO);
  redirecting_stdout = isatty (STDOUT_FILENO);
  stdout_is_closed = (!redirecting_stdout && errno == EBADF);
  redirecting_stderr = isatty (STDERR_FILENO);

  /* If standard input is a tty, replace it with /dev/null if possible. Note that it is deliberately opened for *writing*, to ensure any read evokes an error. */
  if (ignoring_input)
    {
    
      if (fd_reopen (STDIN_FILENO, "/dev/null", O_WRONLY, 0) < 0)
        error (exit_internal_failure, errno,
               _("failed to render standard input unusable"));
      if (!redirecting_stdout && !redirecting_stderr)
        error (0, 0, _("ignoring input"));
    }

  /* If standard output is a tty, redirect it (appending) to a file. First try nohup.out, then $HOME/nohup.out. If standard error is a tty and standard output is closed, open nohup.out or $HOME/nohup.out without redirecting anything. */
  if (redirecting_stdout || (redirecting_stderr && stdout_is_closed))
    {
    
      char *in_home = NULL;
      char const *file = "nohup.out";
      int flags = O_CREAT | O_WRONLY | O_APPEND;
      mode_t mode = S_IRUSR | S_IWUSR;
      mode_t umask_value = umask (~mode);
      out_fd = (redirecting_stdout
                ? fd_reopen (STDOUT_FILENO, file, flags, mode)
                : open (file, flags, mode));

Referece what is the function of the nohup command