How do I make a buffer completely read-only and unmodifiable?

Explanation

While :set readonly prevents accidental writes, nomodifiable goes further by preventing any changes to the buffer contents entirely. Even :s, dd, or insert mode will fail. This is useful for reference buffers, output displays, or preventing accidental edits.

How it works

• :setlocal nomodifiable — prevents any changes to the buffer
• :setlocal modifiable — re-enables editing
• Different from readonly: readonly only prevents :w, while nomodifiable prevents all edits
• Attempting to edit shows: E21: Cannot make changes, 'modifiable' is off

Example

" Make a reference file read-only
:edit reference.txt
:setlocal nomodifiable

" Create a read-only output buffer
:enew | setlocal nomodifiable buftype=nofile

Attempting dd or i on a nomodifiable buffer:
E21: Cannot make changes, 'modifiable' is off

Tips

• Use in autocmds: autocmd BufRead *.log setlocal nomodifiable
• Temporarily enable edits: :setl ma (short for modifiable), edit, then :setl noma
• Combine with noswapfile for true read-only buffers
• buftype=nofile plus nomodifiable creates a display-only buffer