I forgot you have to give it write permissions (minified)

halyde/apps/cat.lua

@@ -1,20 +1 @@
-local files = {...}
-local fs = import("filesystem")
-if not files or not files[1] then
-  shell.run("help cat")
-  return
-end
-for _, file in ipairs(files) do 
-  if file:sub(1, 1) ~= "/" then
-    file = fs.concat(shell.workingDirectory, file)
-  end
-  if not fs.exists(file) then
-    print("\27[91mFile does not exist.")
-  end
-  local handle = fs.open(file, "r")
-  local data
-  repeat
-    data = handle:read(math.huge or math.maxinteger)
-    termlib.write(data)
-  until not data
-end
+Error: no such file. (``)