Prevent saving buffer with new name

Question

I am developing a command line application that uses VIM to edit an existing file - think visudo or crontab -e

When I launch VIM, I'm passing it the path of an existing file. I want the user to be able to modify and save the file (e.g. :w or ZZ) but I do not want the user to have the ability to save the file with another name, e.g. :w new-name.txt

Is there any command line option that I can pass to VIM to disable or prevent saving to a new or other file?

Answer 1

I don't believe it's possible to completely prevent the user from doing this.

However, you could prevent the user from doing it inadvertently via the use of autocommands. Here's a rough proof of concept that ignores the new file name when the user attempts :w newname.txt:

augroup MySave
autocmd BufWriteCmd * :w % | set nomodified
augroup END

However, note that there are many ways that a user can save text to a file in Vim. e.g. If you also want to catch :saveas, you're going to also need to look into the BufFilePre/BufFilePost autocommands; if you want to prevent the user writing ranges of the file, you'll also need to set up something for FileWriteCmd etc.

You might also want to start Vim with the -Z argument. See :help -Z.

See :help autocmd.txt for more details, in particular, :help Cmd-event.

A sufficiently knowledgeable user could simply remove these autocommands, (and even if they couldn't, they could just copy the text to the clipboard!) so this will only be a speedbump for a someone determined to save the text elsewhere.

Answer 2

As suggested by the answer from @Rich, you'll have to jump through lots of hoops to make this work well in Vim. Depending on how secure you want it to be, you might be better off looking to the hosting operating system (ACL, file system permissions, chroot/jails etc.) For instance, you might set up a user with very tight restrictions on what they are permitted to do (e.g. they aren't allowed to create files on disk) and run vim as that user. This way you'll have the assurance of being backed by a proven authorization solution.

However you do it take note of :h -Z. This puts Vim in restricted mode which blocks commands that use an external shell...the first thing someone serious about getting around the restrictions would look at. - Aha, I just noticed that @Rich already mentioned this. Well, it's a good idea! :)

Any way you slice it I don't see this being an easy problem to solve.