Going back to a machine home every start up

You can not use DC motors as encoders, they are not the same thing, not even close. You can use a DC motor as a tacho for biger motors and that would be velocity mode control so you would nees apropriate drives, plus encoders if you want position mode control.
Tell us more anout your existing machine and what you want to do with it.
If you just need to know XYZ positions after a shut down, just add a file named last_position to your config directory, and add the entry to ini file.
That way the machine will know the position it was shut down at.
If you need to have repeatable 0 position, you need home switches added to the machine and wired, that way no matter what position machine was on, it will find the same starting position all by itself.

Wolfmansbrother wrote:

I want to write this position to a file, and when linuxcnc starts up, it looks at these positions and "subtracts" that distance in movement so the machine starts in the same place each time.

Sounds like you want the [TRAJ] POSITION_FILE option.

linuxcnc.org/docs/2.7/html/config/ini-config.html#_traj_section