How do I set Vim options on a per-file basis without changing my vimrc?

Explanation

A modeline is a special comment embedded in a file that Vim reads to apply file-specific settings automatically. When Vim opens a file, it scans the first and last few lines for a modeline pattern and applies any options it finds. This lets you enforce consistent formatting (tabs, indentation, textwidth, etc.) for individual files regardless of each contributor's vimrc.

How it works

• Add a comment near the top or bottom of your file in the format: vim: set {options} :
• Vim checks the first and last modelines lines of the file (default: 5 lines)
• Any recognized Vim option in the modeline is applied as a local setting for that buffer
• The modeline must appear inside a valid comment for the file's language to avoid syntax errors

Modeline formats

Vim recognizes two modeline styles:

// vim: set ts=4 sw=4 expandtab :

or the shorter form:

// vim: ts=4 sw=4 expandtab

The first form uses set and ends with a colon. The second form omits set and ends at the end of the line.

Example

A Python file with 2-space indentation enforced by modeline:

# mymodule.py
# vim: set ts=2 sw=2 expandtab :

def greet(name):
  print(f"Hello, {name}")

A JavaScript file at the bottom:

function main() {
  return 42;
}
// vim: ts=2 sw=2 expandtab

A Makefile that must use real tabs:

# vim: set noexpandtab ts=8 sw=8 :
all:
	echo "building"

Tips

• Modelines must be within the first or last N lines, controlled by :set modelines=5 (default is 5)
• Modelines must be enabled: :set modeline (it is on by default in most Vim builds, but disabled for root)
• Not all options can be set via modeline for security reasons — options like shell, makeprg, and grepprg are blocked unless you :set secure is off
• Common modeline options: ts (tabstop), sw (shiftwidth), sts (softtabstop), et (expandtab), tw (textwidth), ft (filetype), ff (fileformat), fenc (fileencoding)
• Use ft=python in a modeline to force Vim's filetype detection when the extension is nonstandard or missing
• Many open-source projects use modelines to ensure consistent formatting across contributors without requiring everyone to adjust their vimrc
• EditorConfig is a modern cross-editor alternative, but modelines are Vim-native and require zero plugins