<p>How can I permanently enable line numbers in IntelliJ IDEA?</p>

IntelliJ 14.X Onwards
From version 14.0 onwards, the path to the setting dialog is slightly different, a
General
submenu has been added between
Editor
and
Appearance
as shown below
IntelliJ 8.1.2 - 13.X
From IntelliJ 8.1.2 onwards, this option is in
File | Settings
1
. Within the
IDE Settings
section of that dialog, you'll find it under
Editor | Appearance.
On a Mac, these are named
IntelliJ IDEA | Preferences...