autocmd

An autocmd, short for auto-command, is a feature of Vim and Neovim that runs a specified command automatically when a defined event occurs, such as opening a file, entering a buffer, or writing a buffer. Autocommands let users tailor editor behavior without manual invocation.

Autocommands are defined with the syntax: autocmd {event} {pattern} {command}. Events include BufRead, BufNewFile, BufEnter, BufWritePost, FileType, VimEnter, WinEnter, and many others. The pattern matches file names, buffer names, or other context; wildcards like * can restrict where the command applies. Additional qualifiers can refine behavior, such as FileType to react to a certain language.

To organize and manage them, autocommands are typically placed inside augroups. Use augroup Name to start a block, autocmd! to clear existing commands in that group, add individual autocmd lines, and end with augroup END. The ++once modifier can make a specific autocmd run only once for its event.

Common examples:

- autocmd BufReadPost *.md setlocal textwidth=80

- autocmd FileType python setlocal expandtab shiftwidth=4

- autocmd BufNewFile,BufReadPost *.log setlocal nospell

Viewing and learning more: :autocmd lists active autocommands, and :help autocmd opens detailed documentation. Good practice is to group autocommands with augroup and avoid unnecessary work during startup to minimize performance impact.