gui of ubantu

If your purpose is to temporarily disable GUI - you can stop the lightdm service ( or whichever greeter you're using ) with 

sudo service lightdm stop (14.04 and earlier)

or sudo systemctl stop lightdm (since 15.04) from tty1,

as shown by others already.

 

If your purpose to boot into command line for extended period of time between reboots, you can open 

/etc/default/grub with the choice of your text editor. I personally use nano, so for me the command would be sudo nano /etc/default/grub.

Now, find GRUB_CMDLINE_LINUX="" to GRUB_CMDLINE_LINUX="text".

Or you can create two lines, one commented , one uncommented like so:

#GRUB_CMDLINE_LINUX="text"
GRUB_CMDLINE_LINUX=""

Once necessary, just uncomment the line for which mode you wanna enter.

Upon completing editing the /etc/default/grub file, save it, and

run sudo update-grub to make the configurations loaded into the actual grub.cfg file.

Reboot, and you should enter the command line directly.

In the same way, if you miss the GUI of Ubantu, just edit the variable to GRUB_CMDLINE_LINUX="",update configure and reboot your system.