Temporarily

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If your purpose is to temporarily disable GUI - you can stop the lightdm service with 【sudo service lightdm stop(14.04 and earlier)】

or

【sudo systemctl stop lightdm(since 15.04)】

from tty1.

Permanent

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For Ubuntu 14.04 and earlier

1. Open【/etc/default/grub】with the choice of your text editor.

2. Now, you will find the line :

GRUB_CMDLINE_LINUX=" quiet splash"

then,annotate it.

3. Add a new line 【GRUB_CMDLINE_LINUX="text "】

before the line 【GRUB_CMDLINE_LINUX="  "】

4. 【sudo update-grub】

5. Reboot the system.

For Ubuntu 15.04 and newer

choice-one

【sudo systemctl disable lightdm.service】

then reboot the system.

if you want to restart the GUI,

【sudo systemctl start lightdm.service】

 Reboot, and you should enter the command line directly.