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Fetcher plugins retrieve file metadata asynchronously and in batches. They’re perfect for tasks like detecting MIME types, reading Git status, or extracting custom file properties.

​
What Are Fetchers?

Fetchers run in the background to collect file metadata without blocking the UI. Common uses:
  • MIME type detection - Identify file types
  • Git status - Show version control info
  • Image metadata - Extract dimensions, format, EXIF
  • Audio tags - Read ID3 tags from music files
  • Custom properties - Any file-specific data
Yazi’s built-in MIME fetchers are essential examples.

​
Fetcher Structure

local M = {}

function M:fetch(job)
  -- job.files: Array of files to process
  -- Returns: State array (true/false for each file)
  
  for i, file in ipairs(job.files) do
    -- Process file
    -- Update metadata with ya.emit("update_mimes", ...)
  end
  
  return state  -- Array of booleans
end

return M

​
The Job Object

function M:fetch(job)
  job.files     -- Array of File objects
  job.args      -- Additional arguments
  
  -- Each file has:
  -- file.url    -- File URL
  -- file.path   -- File path
  -- file.cha    -- File characteristics (size, mode, etc.)
  -- file.cache  -- Cache path (if applicable)
end

​
Example: MIME Type Fetcher

Yazi’s local MIME fetcher using the file command: 
local M = {}

function M:fetch(job)
  local urls, paths = {}, {}
  
  -- Build path arrays
  for i, file in ipairs(job.files) do
    if file.cache then
      urls[i], paths[i] = tostring(file.url), tostring(file.cache)
    else
      paths[i] = tostring(file.path)
    end
  end
  
  -- Run file command
  local child, err = M.spawn_file1(paths)
  if not child then
    M.placeholder(err, urls, paths)
    return true, err
  end
  
  -- Process output
  local updates, last = {}, ya.time()
  local flush = function(force)
    if not force and ya.time() - last < 0.3 then
      return  -- Throttle updates
    end
    if next(updates) then
      ya.emit("update_mimes", { updates = updates })
      updates, last = {}, ya.time()
    end
  end
  
  local i, state = 1, {}
  repeat
    local line, event = child:read_line_with { timeout = 300 }
    if event == 3 then
      flush(true)
      goto continue
    elseif event ~= 0 then
      break
    end
    
    -- Parse MIME type from output
    local match = M.match_mimetype(line)
    if match then
      updates[urls[i] or paths[i]], state[i], i = match, true, i + 1
      flush(false)
    else
      state[i], i = false, i + 1
    end
    
    ::continue::
  until i > #paths
  
  flush(true)
  return state
end

function M.match_mimetype(line)
  local patterns = {
    "text", "image", "video", "application",
    "audio", "font", "inode"
  }
  
  for _, pat in ipairs(patterns) do
    local typ, sub = line:match(string.format("(%s/)([+-.a-zA-Z0-9]+)%%s+$", pat))
    if sub and line:find(typ .. sub, 1, true) == 1 then
      return typ:gsub("^x%-", "", 1) .. sub:gsub("^x%-", "", 1)
    end
  end
end

function M.spawn_file1(paths)
  local bin = os.getenv("YAZI_FILE_ONE") or "file"
  local windows = ya.target_family() == "windows"
  
  local cmd = Command(bin)
    :arg({ "-bL", "--mime-type" })
    :stdout(Command.PIPED)
  
  if windows then
    cmd:arg({ "-f", "-" }):stdin(Command.PIPED)
  else
    cmd:arg("--"):arg(paths)
  end
  
  local child, err = cmd:spawn()
  if not child then
    return nil, Error.fs {
      kind = err.kind or "Other",
      message = string.format("Failed to start '%s': %s", bin, err),
    }
  elseif windows then
    child:write_all(table.concat(paths, "\n"))
    child:flush()
    ya.drop(child:take_stdin())
  end
  
  return child
end

function M.placeholder(err, urls, paths)
  if err.kind ~= "NotFound" then
    return
  end
  
  -- Set placeholder MIME when tool not found
  local updates = {}
  for i = 1, #paths do
    updates[urls[i] or paths[i]] = "null/file1-not-found"
  end
  
  ya.emit("update_mimes", { updates = updates })
end

return M
Configure in yazi.toml: 
[[plugin.fetchers]]
id = "mime"
url = "local://*"
run = "mime.local"
prio = "high"

​
Example: Directory MIME Fetcher

Mark directories with a special MIME type: 
local M = {}

function M:fetch(job)
  local updates = {}
  
  for _, file in ipairs(job.files) do
    if file.cha.is_dir then
      updates[tostring(file.url)] = "inode/directory"
    end
  end
  
  if next(updates) then
    ya.emit("update_mimes", { updates = updates })
  end
  
  return true
end

return M

[[plugin.fetchers]]
id = "mime"
url = "*/"
run = "mime.dir"
prio = "high"

​
Example: Remote MIME Fetcher

Detect MIME types for remote files: 
local M = {}

function M:fetch(job)
  local updates = {}
  
  for _, file in ipairs(job.files) do
    local url = file.url
    
    -- Use HEAD request to get Content-Type
    local output, err = Command("curl")
      :arg({ "-sI", "-L", tostring(url) })
      :stdout(Command.PIPED)
      :output()
    
    if output and output.status.success then
      local mime = output.stdout:match("Content%-Type:%s*([^%s;]+)")
      if mime then
        updates[tostring(url)] = mime
      end
    end
  end
  
  if next(updates) then
    ya.emit("update_mimes", { updates = updates })
  end
  
  return true
end

return M

[[plugin.fetchers]]
id = "mime"
url = "remote://*"
run = "mime.remote"
prio = "normal"

​
Example: Git Status Fetcher

Show Git status for files: 
local M = {}

function M:fetch(job)
  -- Get repository root
  local output, err = Command("git")
    :arg({ "rev-parse", "--show-toplevel" })
    :cwd(tostring(job.files[1].url.parent))
    :stdout(Command.PIPED)
    :output()
  
  if not output or not output.status.success then
    return true  -- Not a git repo
  end
  
  local root = output.stdout:gsub("\n$", "")
  
  -- Get status for all files
  output, err = Command("git")
    :arg({ "status", "--porcelain", "-z" })
    :cwd(root)
    :stdout(Command.PIPED)
    :output()
  
  if not output or not output.status.success then
    return true
  end
  
  -- Parse status output
  local statuses = {}
  for status, path in output.stdout:gmatch("(%S+) ([^%z]+)%z") do
    statuses[path] = status
  end
  
  -- Update file metadata
  local updates = {}
  for _, file in ipairs(job.files) do
    local rel = tostring(file.path):sub(#root + 2)
    if statuses[rel] then
      updates[tostring(file.url)] = {
        git_status = statuses[rel]
      }
    end
  end
  
  if next(updates) then
    ya.emit("update_metadata", { updates = updates })
  end
  
  return true
end

return M

​
Updating Metadata

​
Update MIME Types

local updates = {
  ["file:///path/to/file1"] = "image/png",
  ["file:///path/to/file2"] = "text/plain",
}

ya.emit("update_mimes", { updates = updates })

​
Update Custom Metadata

local updates = {
  ["file:///path/to/image.jpg"] = {
    dimensions = "1920x1080",
    format = "JPEG",
    size_kb = 512,
  },
}

ya.emit("update_metadata", { updates = updates })

​
Batch Processing

Process files efficiently in batches: 
function M:fetch(job)
  local batch_size = 50
  local state = {}
  
  for i = 1, #job.files, batch_size do
    local batch = {}
    for j = i, math.min(i + batch_size - 1, #job.files) do
      batch[#batch + 1] = job.files[j]
    end
    
    -- Process batch
    local updates = self:process_batch(batch)
    
    if next(updates) then
      ya.emit("update_mimes", { updates = updates })
    end
    
    for j = i, math.min(i + batch_size - 1, #job.files) do
      state[j] = true
    end
  end
  
  return state
end

​
Throttling Updates

Avoid overwhelming the UI: 
function M:fetch(job)
  local updates, last = {}, ya.time()
  
  local flush = function(force)
    local now = ya.time()
    if not force and now - last < 0.3 then
      return  -- Wait at least 300ms between updates
    end
    
    if next(updates) then
      ya.emit("update_mimes", { updates = updates })
      updates, last = {}, now
    end
  end
  
  for _, file in ipairs(job.files) do
    -- Process file
    updates[tostring(file.url)] = mime
    flush(false)  -- Throttled
  end
  
  flush(true)  -- Force final update
  return true
end

​
Streaming Output

Read command output line by line: 
function M:fetch(job)
  local child = Command("tool")
    :arg(paths)
    :stdout(Command.PIPED)
    :spawn()
  
  if not child then
    return true
  end
  
  local i = 1
  while true do
    local line, event = child:read_line_with { timeout = 300 }
    
    if event == 3 then
      -- Timeout, continue waiting
      goto continue
    elseif event ~= 0 then
      -- EOF or error
      break
    end
    
    -- Process line
    -- Update metadata for job.files[i]
    i = i + 1
    
    ::continue::
  end
  
  child:start_kill()
  return true
end

​
Error Handling

function M:fetch(job)
  local output, err = Command("tool"):output()
  
  if not output then
    ya.err("Fetcher failed: " .. tostring(err))
    return true  -- Don't retry
  end
  
  if not output.status.success then
    ya.err("Tool exited with code: " .. output.status.code)
    
    -- Set error state
    local updates = {}
    for _, file in ipairs(job.files) do
      updates[tostring(file.url)] = "null/error"
    end
    ya.emit("update_mimes", { updates = updates })
  end
  
  return true
end

​
Configuration

Configure fetchers in yazi.toml: 
[plugin]
fetchers = [
  # High priority for essential MIME detection
  { id = "mime", url = "*/", run = "mime.dir", prio = "high" },
  { id = "mime", url = "local://*", run = "mime.local", prio = "high" },
  { id = "mime", url = "remote://*", run = "mime.remote", prio = "normal" },
  
  # Custom fetchers
  { id = "git", url = "local://*", run = "git-status", prio = "low" },
  { id = "exif", url = "*.{jpg,jpeg,png}", run = "exif-reader", prio = "low" },
]

​
Priority Levels

  • high - Essential, run first
  • normal - Standard priority
  • low - Nice-to-have, run last

​
Best Practices

Call external tools once for multiple files:
-- Good: Single call for all files
Command("file"):arg(all_paths):spawn()

-- Bad: One call per file
for _, file in ipairs(job.files) do
  Command("file"):arg(tostring(file.path)):spawn()
end
Don’t emit events too frequently:
local last_update = ya.time()
if ya.time() - last_update >= 0.3 then
  ya.emit("update_mimes", { updates = updates })
  last_update = ya.time()
end
Set placeholder values when tools are unavailable:
if not child then
  local updates = {}
  for _, file in ipairs(job.files) do
    updates[tostring(file.url)] = "null/tool-not-found"
  end
  ya.emit("update_mimes", { updates = updates })
  return true
end
Don’t block indefinitely:
local line, event = child:read_line_with { timeout = 300 }
if event == 3 then
  -- Timeout, handle gracefully
end
Always kill spawned processes:
local child = Command("tool"):spawn()
-- ... use child ...
child:start_kill()  -- Always clean up

​
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