push

PUSH  -  start running a subshell

Usage:  PUSH

The PUSH command will start up a new shell under the MM process.
  When you leave the PUSHed shell (done by using the "exit" shell
  command), you will be back in MM.  PUSH is an advanced command for
  people familiar with the use of shells.

If you are using MM and want to run some shell command, there are two
  somewhat easier ways:

  o  At an MM prompt, type "!" and the shell command, for example:
        MM>!ls
     "!" does the one command in a subshell (like a PUSH).

  o  Suspend MM using the QUIT or SUSPEND commands, type the shell
     command(s) at the shell prompt, and then type "mm" to re-enter MM.  
     
For more help type "help" and one of these topics:
    quit   suspend   exit   bye   shell