How to get the total lines of a file before it's loaded into a buffer?

Question

As title.

• My target is to disable a plugin before it is applied to a buffer that will be loaded with the content of a file.
• I can get the file name before it is loaded.
• The file to be opened is assumed to be very large. (>1MB)
• What's the recommended way to do this? Is there any API for this (considering the performance) or should I use some terminal program to get it?

If any info. is missing please let me know.

Answer 1

How to get the total lines of a file [...]

This can be done in many ways in Neovim and/or Lua (based on the tags you used):

1. You can use the Lua API io.popen(str_term_cmd):read(...) to read the output of the command str_term_cmd.
2. You can use the Neovim port of Vim's built-in API vim.fn.system(). I will prefer this way since:
   1. If {cmd} is a List it runs directly (no 'shell'), from the docs.
   2. Some extra checks are done by Bram: Result is a String, filtered to avoid platform-specific quirks, from the docs.
   3. This way is more "Vim". And you can avoid the need to :close() the file handle.

Remember to turn the output into a number. Here is an example usage:

tonumber(vim.fn.system({ 'wc', '-l', vim.fn.expand('%') }):match('%d+')) > 1000

[...] before it's loaded into a buffer?

If that plugin does provide a callback then you just do the calculation there. Otherwise, you should take a look at BufReadPre, FileReadPre, :help BufReadCmd, :help file-functions, as others have suggested.