You can run UNIX commands and see their output without leaving vi. You can also insert the output of a UNIX command into the file you that are editing.
To run a single UNIX command use the command:
:!UNIX_command
You can start a shell from within vi and use it as you would your usual UNIX environment, then exit the shell and return to vi
To start up a shell enter the command:
:sh
The type of shell that is started is determined by the $SHELL variable. You can specify that some other shell is to be started by setting the vi shell option
Return to using vi by entering the command exit or Ctrl-D
To insert the output from a UNIX command into a file, immediately after the cursor:
:r!command
For example, this facility would be very useful if you were using vi to document a UNIX command and you wanted to include examples of the output from this command.