Access Shell From Inside A Program

I would like to be able to call a shell up in my program… ive seen Operating systems on the forum do this before, how would i and if possible is it possible for lets say if i type "exit()" in it it will return to the program?

Not sure if this is what you mean but:

shell.run("shell")

This will run a second shell that will return to your program when it's exited.
However, keep in mind that shell.run will pause your program (unless you're using coroutines) until the other program is finished and all of the changes the program made to the screen will stay even when your program has control again. So if you've got a GUI, you should re-render it after shell.run.

Under ComputerCraft 1.6 onwards, advanced systems have also got the option of shell.openTab().