The zsh/clone Module (zsh)

### 22.4 The zsh/clone Module ¶

The `zsh/clone` module makes available one builtin command:

`clone` tty ¶

  Creates a forked instance of the current shell, attached to the specified tty. In the new shell, the `PID`, `PPID` and `TTY` special parameters are changed appropriately. `$!` is set to zero in the new shell, and to the new shell’s PID in the original shell.

  The return status of the builtin is zero in both shells if successful, and non-zero on error.

  The target of `clone` should be an unused terminal, such as an unused virtual console or a virtual terminal created by

      xterm -e sh -c 'trap : INT QUIT TSTP; tty;
              while :; do sleep 100000000; done'

  Some words of explanation are warranted about this long xterm command line: when doing clone on a pseudo-terminal, some other session ("session" meant as a unix session group, or SID) is already owning the terminal. Hence the cloned zsh cannot acquire the pseudo-terminal as a controlling tty. That means two things:

  - the job control signals will go to the sh-started-by-xterm process group (that’s why we disable INT QUIT and TSTP with trap; otherwise the while loop could get suspended or killed)
  - the cloned shell will have job control disabled, and the job control keys (control-C, control-\ and control-Z) will not work.

  This does not apply when cloning to an unused vc.

  Cloning to a used (and unprepared) terminal will result in two processes reading simultaneously from the same terminal, with input bytes going randomly to either process.

  `clone` is mostly useful as a shell built-in replacement for openvt.