A LUA script for Blackmagic Design Fusion that will both check the current platform, and provide a file system related "dirname" function to trim a file path down to the base directory.

blackmagic_design_fusion_dirname.lua

-- Find out the current operating system platform. The platform local variable should be set to either "Windows", "Mac", or "Linux".
local platform = ""
if string.find(fusion:MapPath("Fusion:\\"), "Program Files", 1) then
  -- Check if the OS is Windows by searching for the Program Files folder
  platform = "Windows"
elseif string.find(fusion:MapPath("Fusion:\\"), "PROGRA~1", 1) then
  -- Check if the OS is Windows by searching for the Program Files folder
  platform = "Windows"
elseif string.find(fusion:MapPath("Fusion:\\"), "Applications", 1) then
  -- Check if the OS is Mac by searching for the Applications folder
  platform = "Mac"
else
  platform = "Linux"
end


-- Find out the current directory from a file path
-- Example: print(dirname("/Users/Shared/file.txt"))
function dirname(mediaFileName)
-- LUA dirname command inspired by Stackoverflow code example:
-- http://stackoverflow.com/questions/9102126/lua-return-directory-path-from-path
  sep = ''
  
  if platform == "Windows" then
    sep = "\\"
  elseif platform == "Mac" then
    sep = "/"
  else
    -- Linux
    sep = "/"
  end
  
  return mediaFileName:match("(.*"..sep..")")
end