Modeline

Modeline is a feature in Vi-compatible editors, most notably Vim, that allows editors to read and apply per-file settings embedded directly in the text. By placing special lines near the top or bottom of a file, a user can customize options for that specific file or buffer without altering global configuration. This can help ensure consistent formatting and behavior when working on projects with particular editing requirements.

In Vim, modelines are lines that contain ex commands starting with variants of set or setlocal and are placed within a limited portion of the file, typically the first and last few lines. A common form uses a token such as vim: or vi:, for example: /* vim: set ts=4 sw=4 et: */. When modelines are enabled, the editor parses these lines and applies the indicated options, which can include tab stops, indentation behavior, text width, wrapping, and other buffer-local settings. The number of lines scanned is controlled by the modelines setting (often 5), and there is also a modelineexpr option that can evaluate expressions in a modeline, with associated security considerations.

Security concerns are a notable aspect: because modelines can alter editor behavior or execute expressions, they can be a vector for unintended actions in untrusted files. Many environments recommend keeping modelines disabled or restricting their use in shared or untrusted sources, or disabling modelineexpr to mitigate risks.

See also: mode line in Emacs, which serves as the editor’s status line, and the general concept of a status or modeline in other editors.