Rooting Samsung Galaxy S Plus GT I9001 with Android 2.3.6 Gingerbread onboard

1. Connect your phone to the computer with USB cable.
2. Download Root-i9001-Signed.zip file and copy it to the phone memory. Ubuntu 13.04 supports mtp protocol so you can do mentioned operation.
3. Turn off your smartphone but don't disconnect the USB cable.
4. Boot into recovery mode. Press Volume Up + Power + Home keys and hold them down until model/manufacturer text appears on the screen.
5. Wait for "Android out of the box" picture appears. Press once the Menu button. Presented picture should now be a bit pale coloured.
6. Wait for text menu to appear on the screen. You can navigate up/down inside it by pressing Volume Up and Volume Down keys. Choose "apply update from sdcard" option and press Home button.
7. In the next submenu choose Root-i9001-Signed.zip file and press Home key.
8. Now the files should install inside your phone. After that choose "reboot system now" option and press Home key.
9. Now you have rooted phone.

Unrooting is done much the same way. The difference is that you choose UnRoot-i9001-Signed.zip file instead of Root-i9001-Signed.zip one.