How do I duplicate every non-blank line in a buffer with one Ex command?

Explanation

The :global command can apply an Ex action to every line that matches a pattern. Combining it with :copy (:t) gives you a compact way to duplicate matched lines without recording a macro or touching Visual mode. This is especially useful when you need a quick expansion pass, such as duplicating checklist items, log statements, or template lines while keeping blank-line structure intact.

How it works

:g/./t.

• :g/./ selects every line matching . (any non-empty line)
• t. copies each matched line to . (the current line), which means directly below itself
• Because :global computes matches first, each original non-blank line is duplicated once

Example

Before:

alpha

beta
gamma

Run:

:g/./t.

After:

alpha
alpha

beta
beta
gamma
gamma

Tips

• Restrict to a range: :10,40g/./t.
• Use a tighter pattern, e.g. :g/^\s*TODO/t. to duplicate only TODO lines
• Use :g/pattern/m$ instead when you want to gather matches at the end instead of duplicating in place