Running fzf natively in Neovim

Recently, I've been experimenting with running fzf natively in Neovim. By natively, I mean without using fzf.vim, fzf-lua, or even fzf's built-in vim helpers. Just a terminal buffer in Neovim with a command piped to fzf. This approach has some cool benefits:

• You can write a working proof of concept in ~50 LOC
• It's performant - fzf opens immediately with results streaming in over time
• It's consistent with fzf in the terminal - options are specified exactly as they are when using fzf in the shell

There's downsides too, of course - aside from what's already included in fzf, you'll get nothing for free. If you like a feature-full picker, you should stick with fzf-lua. Even if you just want a set of barebones fzf functions, you should probably stick with fzf's builtin helpers - 1000 LOC for a robust set of primitives.

But if you're interested in how you could natively integrate fzf with Neovim yourself, read on.

The core of our approach will be as follows:

• Open a terminal buffer in Neovim
• Run a terminal command and output the result to fzf
  • The command will either be a traditional terminal command or a command to execute a standalone lua script. For the latter, we'll execute the lua script in a headless nvim instance that can communicate with the primary nvim instance using RPC.
• Interact with fzf in the terminal buffer to select the result(s)
• Redirect fzf's stdout to a temporary file and read it to access the selected items

We'll start at the top and work our way down.

First, we need to open a terminal buffer in Neovim (which itself is probably running in a terminal 😅). This can be done with vim.fn.jobstart(), which takes in two args: the cmd string to run, and a table of opts.

When opts is passed term = true, the command will be spawned in a terminal buffer connected to the current buffer. To create a dedicated buffer for the terminal, we can use nvim_create_buf and nvim_set_current_buf:

local listed = false
local scratch = true
local term_bufnr = vim.api.nvim_create_buf(listed, scratch)
vim.api.nvim_set_current_buf(term_bufnr)
vim.fn.jobstart("fd", { term = true, })

In this code, fd is executed and its stdout is output in the terminal buffer as it would be in a regular terminal.

Opening the terminal buffer in the current window works fine, but I prefer a dedicated window to create a floating effect. Using nvim_open_win, this isn't too hard.

local listed = false
local scratch = true
local term_bufnr = vim.api.nvim_create_buf(listed, scratch)

local editor_height = vim.o.lines - 1
local border_height = 2
local enter = true
vim.api.nvim_open_win(term_bufnr, enter, {
  relative = "editor",
  row = editor_height,
  col = 0,
  width = vim.o.columns,
  height = math.floor(editor_height / 2 - border_height),
  border = "rounded",
  title = "Fzf term",
})
vim.fn.jobstart("fd", { term = true, })

Looks good! Now that we can run a fd command in a terminal buffer, how can we start fuzzy filtering the output with fzf?

Just like in a regular terminal, we can pipe the fd command to fzf:

-- ... open the window
vim.fn.jobstart("fd | fzf", {
  term = true,
  on_exit = function()
    local force = true
    vim.api.nvim_win_close(term_winnr, force)
  end,
})
vim.cmd "startinsert"

By default, opening a terminal buffer puts us in normal mode where we can navigate the terminal like its any other buffer. In our case though, we want to interact with the fzf process in insert mode

• :startinsert gets us there.

We also added the on_exit callback, which does what it sounds like. Currently, we're using the callback to close the window when the fzf process exits - i.e. when selecting an item. But how do we access the item that was selected? A simple approach is to create a temporary file, pipe the output of fzf to the temp file, and read from the file in on_exit as well:

local tempname = vim.fn.tempname()
-- ... open the window
local cmd = ("fd | fzf > %s"):format(tempname)
vim.fn.jobstart(cmd, {
  term = true,
  on_exit = function()
    local force = true
    vim.api.nvim_win_close(term_winnr, force)
    local fzf_content = vim.fn.readfile(tempname)
    if #fzf_content > 0 then
      vim.cmd("edit " .. fzf_content[1])
    end
    vim.fn.delete(tempname)
  end,
})
vim.cmd "startinsert"

vim.fn.readfile returns a list where each item is a line in the file that's passed to it - very convenient.

To apply options to our fzf process, we can simply pass them along to the terminal command:

--- ... open the window
local fzf_opts = {
  "--cycle",
  [[--bind='ctrl-d:preview-page-down']],
  [[--bind='ctrl-u:preview-page-up']],
  [[--preview='bat --style=numbers --color=always {1}']],
}
local formatted_opts = table.concat(fzf_opts, " ")
local cmd = ("fd | fzf %s > %s"):format(formatted_opts, tempname)
vim.fn.jobstart(cmd, {
  term = true,
  on_exit = function()
    local force = true
    vim.api.nvim_win_close(term_winnr, force)
    local fzf_content = vim.fn.readfile(tempname)
    if #fzf_content > 0 then
      vim.cmd("edit " .. fzf_content[1])
    end
    vim.fn.delete(tempname)
  end,
})
vim.cmd "startinsert"

This right here is the core of our functionality. Let's make it a bit nicer:

--- @class FzfOpts
--- @field source string
--- @field options? string[]
--- @field sink? fun(entry: string)
--- @field sinklist? fun(entry:string[])

--- @param opts FzfOpts
local function fzf(opts)
  opts.options = opts.options or {}
  local tempname = vim.fn.tempname()

  local editor_height = vim.o.lines - 1
  local border_height = 2

  local listed = false
  local scratch = true
  local term_bufnr = vim.api.nvim_create_buf(listed, scratch)
  local term_winnr = vim.api.nvim_open_win(term_bufnr, true, {
    relative = "editor",
    row = editor_height,
    col = 0,
    width = vim.o.columns,
    height = math.floor(editor_height / 2 - border_height),
    border = "rounded",
    title = "Fzf term",
  })

  local cmd = ("%s | fzf %s > %s"):format(opts.source, table.concat(opts.options, " "), tempname)
  vim.fn.jobstart(cmd, {
    term = true,
    on_exit = function()
      vim.api.nvim_win_close(term_winnr, true)
      local temp_content = vim.fn.readfile(tempname)
      if #temp_content > 0 then
        if opts.sink then
          opts.sink(temp_content[1])
        elseif opts.sinklist then
          opts.sinklist(temp_content)
        end
      end

      vim.fn.delete(tempname)
    end,
  })
  vim.cmd "startinsert"
end

fzf {
  source = "fd",
  options = {
    "--cycle",
    [[--bind='ctrl-d:preview-page-down']],
    [[--bind='ctrl-u:preview-page-up']],
    [[--preview='bat --style=numbers --color=always {1}']],
  },
  sink = function(entry)
    vim.cmd("edit " .. entry)
  end,
}

The function we have works great for simple sources like fd, but how can we populate fzf with something Neovim-specific? Personally, I like use global marks in my workflow - let's use that as an example. How can we write a script that outputs the current global marks so it can be piped to fzf? One approach is to run a lua script in a headless nvim instance which communicates with the main nvim instance using RPC.

Let's start small: to execute a lua script with Neovim, we can use the nvim command as follows:

nvim --headless -l path/to/lua_script.lua

An exercise for the reader: look into --clean, -u NONE, and --noplugin options

--headless does what it sounds like, and -l executes the next argument as a lua script.

Like our fd command, we want the nvim --headless -l path/to/lua_script.lua command to print to stdout so it can be piped to fzf. This can be achieved with io.write:

-- path/to/lua_script.lua
for _, item in pairs { "one", "two", "three", } do
  io.write(item .. "\n")
end

-- main nvim config
local source = table.concat({ "nvim", "--clean", "--headless", "-l", path_to_lua_script, }, " ")
fzf {
  source = source,
  sink = function(entry)
    vim.cmd("edit " .. entry)
  end,
}

This is a great foundation for using lua to generate and pipe items to fzf. One benefit to this approach is performance: since we're piping a terminal command to fzf, fzf will open immediately and the items can filter in over time as they become available - no coroutines necessary. Try this out:

-- path/to/lua_script.lua
for _, item in pairs { "one", "two", "three", } do
  vim.cmd "sleep"
  io.write(item .. "\n")
end

fzf even adds a little loading spinner as the results stream in.

Back to our goal of populating fzf with the current global marks. In order for the headless nvim instance to access state from the currently-open nvim instance, we need to use RPC. In short, RPC allows the headless nvim to run a function in the context of the main nvim instance and receive its return value. For example:

-- path/to/get_marks.lua
local servername = 'TODO'
local chan = vim.fn.sockconnect("pipe", servername, { rpc = true, })
local mark_list = vim.rpcrequest(chan, "nvim_call_function", "getmarklist", {})
vim.fn.chanclose(chan)

This code, run in a headless nvim instance, sets mark_list to the result of calling vim.fn.getmarklist() in the main nvim instance.

In order to connect to the main nvim instance's RPC server, we need the name of the server to use. We can simply pass this along as an argument to the lua script:

-- main config
local source = table.concat({ "nvim", "--clean", "--headless", "-l", path_to_lua_script, vim.v.servername, }, " ")

Using this info, we can write our get_marks.lua script:

-- path/to/get_marks.lua
assert(arg[1], "Missing arg1: `servername`")
local servername = arg[1]

local chan = vim.fn.sockconnect("pipe", servername, { rpc = true, })

--- @class MarkListEntry
--- @field file string
--- @field mark string

--- @type MarkListEntry[] | nil
local mark_list = vim.rpcrequest(chan, "nvim_call_function", "getmarklist", {})
vim.fn.chanclose(chan)
if mark_list == nil then return end

for _, mark_entry in pairs(mark_list) do
  -- marks are prefixed with '
  local name = mark_entry.mark:sub(2, 2)
  if not name:match('[A-Z]') then goto continue end

  local formatted_mark = ("%s|%s"):format(name, vim.fs.normalize(mark_entry.file))
  io.write(formatted_mark .. "\n")

  ::continue::
end

-- main config
local source = table.concat({ "nvim", "--headless", "-l", path_to_lua_script, vim.v.servername, }, " ")
fzf {
  source = source,
  options = {
    "--delimiter='|'",
    [[--bind='ctrl-d:preview-page-down']],
    [[--bind='ctrl-u:preview-page-up']],
    [[--preview='bat --style=numbers --color=always {2}']],
  },
  sink = function(entry)
    local filename = vim.split(entry, "|")[2]
    vim.cmd("edit " .. filename)
  end,
}

Note that our lua script formats the mark entry as A|path/to/file, so we need to update some fzf flags as well: --delimiter='|' and --preview to use {2} instead of {1}. With --delimiter set, fzf will split a result by the passed delimiter, meaning {2} - the arg passed to bat - will be the filename.

We can use a similar RPC approach for fzf actions as well, such as a keybinding to delete a mark:

-- path/to/delete_mark.lua
assert(arg[1], "Missing arg1: `servername`")
assert(arg[2], "Missing arg2: `selected_mark`")

local servername = arg[1]
local selected_mark = arg[2]

local chan = vim.fn.sockconnect("pipe", servername, { rpc = true, })
vim.rpcrequest(chan, "nvim_del_mark", selected_mark)
vim.fn.chanclose(chan)

-- main config
local get_marks_source = table.concat({
    "nvim",
    "--clean",
    "--headless",
    "-l",
    path_to_get_marks_script,
    vim.v.servername,
}, " ")
local delete_mark_source = table.concat({
  "nvim",
  "--clean",
  "--headless",
  "-l",
  path_to_delete_mark_script,
  vim.v.servername,
}, " ")

fzf {
  source = get_marks_source,
  options = {
    "--delimiter='|'",
    "--cycle",
    [[--bind='ctrl-d:preview-page-down']],
    [[--bind='ctrl-u:preview-page-up']],
    [[--preview='bat --style=numbers --color=always {2}']],
    ([[--bind='ctrl-x:execute(%s {1})+reload(%s)']]):format(delete_mark_source, get_marks_source),
  },
  sink = function(entry)
    vim.cmd("edit " .. entry)
  end,
}

If you've used fzf's builtin vim functions (i.e. fzf#run), the arguments to our homegrown fzf lua function might look familiar. That's no accident - this article came about because I was experimenting with fzf#run, and I was curious how it worked internally. I was pleasantly surprised by how seamless it is to connect fzf to Neovim with a terminal buffer - even with as little code as I have in this article.

The RPC pattern in this article is pretty flexible - using this approach, you can likely build out all the classic pickers with standalone lua scripts. I'll leave that as an exercise for the reader. Thanks for reading.