How do I display line numbers in Emacs (not in the mode line)?

M-x linum-mode (or global-linum-mode), and put (global-linum-mode t) in your ~/.emacs (or ~/.emacs.d/init.el) to enable it at startup.

This page:

• https://www.emacswiki.org/emacs/LineNumbers

lists a few different options.

monotux's answer is currently the first option listed there. An alternative (also listed) is NLinum:

• http://elpa.gnu.org/packages/nlinum.html

You should be able to install from ELPA:

M-x package-install RET nlinum RET

and then use it e.g. like this:

(use-package nlinum
  :config
  (global-nlinum-mode))

• linum-mode → old, hack, slow. Emacs 23 (released in 2009).
• global-display-line-numbers-mode → Emacs 26 (beta as of 2018-03-30)

Put this in your emacs init file:

(when (version<= "26.0.50" emacs-version )
  (global-display-line-numbers-mode))

http://ergoemacs.org/emacs/emacs_line_number_mode.html