What is the difference between `filetype plugin indent on` and `filetype indent on`?

Question

I see one of these two lines in .vimrc files. This one seems to be most common:

filetype plugin indent on

But how does it differ from this line:

filetype indent on

I've found documentation for the latter, but I'm still confused.

Answer 1

filetype plugin indent on is like a combination of these commands:

filetype on
filetype plugin on
filetype indent on

It turns on "detection", "plugin" and "indent" at once. You can check for yourself by reading :help :filetype-overview.

Detection

What does filetype "detection" do? From the docs:

Each time a new or existing file is edited, Vim will try to recognize the type of the file and set the 'filetype' option. This will trigger the FileType event, which can be used to set the syntax highlighting, set options, etc.

This is less confusing if you realise that the filetype command is distinct from the filetype option. (The command :filetype..., the option: :set filetype...)

Plugin

What does the "plugin" part do? From the docs:

This actually loads the file "ftplugin.vim" in 'runtimepath'.

The result is that when a file is edited its plugin file is loaded (if there is one for the detected filetype).

The file being loaded is not necessarily named ftplugin.vim, it could be ftplugin/html_example.vim for instance.

Indent

What does the "indent" part do? From the docs:

This actually loads the file "indent.vim" in 'runtimepath'.

The result is that when a file is edited its indent file is loaded (if there is one for the detected filetype). indent-expression

Again, the file may not be named indent.vim, it could be named indent/html_example.vim for instance.