How can I permanently enable line numbers in IntelliJ?

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 | Settings¹. Within the IDE Settings section of that dialog, you'll find it under Editor | Appearance.

1. On a Mac, these are named IntelliJ IDEA | Preferences...

I add this response for IntelliJ IDEA 2018.2 - Ultimate.

Using menu

IntelliJ Idea > Preferences > Editor > General > Appearance > Show Line Numbers

Using Shortcuts - First way

For Windows : Ctrl+Shift+a
For Mac : Cmd+shift+a

Using Shortcuts - Seconde way

Touch Shift twice

These three methods exist since the last 4 versions of Intellij and I think they remain valid for a long time.

IntelliJ 2019 community edition has line number by default. If you want to show or hide line numbers, go to the following settings to change the appearance.

go to → File → Setting → Editor → General → Appearance → [Check] Show line numbers

For IntelliJ 20.1 or above, on Mac OSX:

IntelliJ IDEA -> Editor -> General -> Appearance -> Show line numbers

Point to be noted: Always look for Editor

For shortcut:

⌘ + ⇧ + A (command + shift + A)

type

and click on the pop up to turn on Show line numbers and you are good to go.