How to use files with Luart
By Samir Tine, modified on December 2021
Creating a File value
At this point, it's important to understand that a File object represents an abstraction of a file that may *or may not* physically exist. For example, when creating a new file, only the call to the File:open() method will physically create the file on the computer storage:
local newfile = sys.File("hello.txt") -- newfile holds a File value print(newfile.exists) -- outputs false newfile:open("write") -- creates the file physically on the disk newfile:close() -- closes the opened File print(newfile.exists) -- now outputs true
A File object can be used to perform file operations:
local file = sys.File("test.txt") file:copy("test_copy.txt") -- copies file file:move("C:\") -- moves file file:remove() -- deletes the file
Opening and closing a File
A File can be opened to read or write its content. This is done by calling the File:open() method. By default, the file is open in "read" mode. The file encoding can be specified as a second argument or autodetected using Byte Mark Order :
local file = File("test.bin") -- opens the file "test.bin" in read mode (encoding is autodetected) file:open() -- closes the file file:close() -- open the file in write mode (erasing its previous content), using "binary" encoding(ie raw binary) file:open("write", "binary")
Do not forget to close a file after the transaction on this one ended. This is done automatically when the File object is garbage collected, but that cannot be predicted.
Reading and writing FilesOnce the File object has been created, and the File:open() method called, you can write and/or read on the File (depending on the open mode used). These methods return Buffer objects in "binary" mode (containing in memory binary data) or strings in "utf8" or "unicode" mode.
Reading file contentThere are two methods for reading the content of a file: File:read() and File:readln()
The File:read() method reads a certain number of characters (when using "utf8" or "unicode" mode) or bytes (in "binary" mode) specified as argument, or all the content of the file if no argument is provided.
local file = File("README.TXT"):open() -- opens the File for reading print(file:read()) -- reads and prints all the file content
The File:readln() method reads a line, until end of line characters "\r\n" or "\n" are encountered). You can iterate through lines of the file with the File.lines iterator.
Writing to filesThere are two methods for writing data to a file: File:write() and File:writeln().
Each method take one argument, converted to a string to be written to the file. In "binary" mode, Buffer objects can be used.
File:writeln() writes a line to the file, appending "\r\n" (the End Of Line marker on Windows platform).
Using File positionThe File.position property get or set the current file position, as a number. File position always starts at 1 (as for all indexed/positioned values in Lua).
File position interpretation depends on the file encoding :
- In "binary" encoding : the position is expressed in bytes.
- In "utf8" or "unicode" encoding : the position is expressed in characters.
Using File timestampsThe File.accessed, File.modified and File.created properties permits to set or get File timestamps using a Datetime object.
Managing File attributesThe File.hidden, File.temporary and File.readonly permits to set or get the corresponding File attributes.
Managing File path and namesThe File.fullpath, File.name, File.extension, File.path properties ease filename parsing. Please note that you can use "/" or "\" for path separators.
As you can see, File objects provide easy access to all of the functionality related to creating, reading, writing and manipulating files in Luart. These features go beyond the capabilities offered by the standard Lua library on the Windows platform.