Check if a file exists with Lua

Question

How can I check if a file exists using Lua?

Maculation

@Mitch as in #1340730 ? – Vocalist

@Vocalist - I guess more as in lua.org/pil/21.2.html – Unriddle

@Liutauras that is even close to a real answer. I only did a quick check only on so – Vocalist

Hi, Thx for the quick respond. I am doing: assert(io.input(fileName), "Error opening file") But when i give a dummy filename, i don't get the error message : "Error opening file". The message i get is: "bad argument #1 to 'input' (/pfrm2.0/share/lua/5.1/db/fake.dbdl: No such file or directory)" any thoughts ? – Maculation

Yoni, I understand you just joined SO. Welcome. Few things to mention. 1) Don't answer your own question with a new question. 2) Try to search around (Google is your friend) for more info and only if you are completely stuck ask here. This I believe will make you a better developer. – Unriddle

The reason you get "bad argument#1 to 'input'" is that input in Lua only takes one argument and you are passing 2. Plus you should assert io.open not input. – Unriddle

Hi Liutauras, i will take more notice for next time, sorry for that. – Maculation

Also, i've change to io.open and it works now. Thank you – Maculation

No problem Yoni. Thanks for sorting out your question. It just looks nicer now. Isn't why we love SO. Because it's nice. – Unriddle

Answer 1

Try

function file_exists(name)
   local f=io.open(name,"r")
   if f~=nil then io.close(f) return true else return false end
end

but note that this code only tests whether the file can be opened for reading.

Arielariela

Note: If the file is a directory, this method returns false – Interactive

Answer 2

Using plain Lua, the best you can do is see if a file can be opened for read, as per LHF. This is almost always good enough. But if you want more, load the Lua POSIX library and check if posix.stat(path) returns non-nil.

Gossoon

LuaFileSystem works on Windows too. Use lfs.attributes(path,'mode') – Fourscore

Answer 3

Lua 5.1:

function file_exists(name)
   local f = io.open(name, "r")
   return f ~= nil and io.close(f)
end

Comforter

Answer 4

I will quote myself from here

I use these (but I actually check for the error):

require("lfs")
-- no function checks for errors.
-- you should check for them

function isFile(name)
    if type(name)~="string" then return false end
    if not isDir(name) then
        return os.rename(name,name) and true or false
        -- note that the short evaluation is to
        -- return false instead of a possible nil
    end
    return false
end

function isFileOrDir(name)
    if type(name)~="string" then return false end
    return os.rename(name, name) and true or false
end

function isDir(name)
    if type(name)~="string" then return false end
    local cd = lfs.currentdir()
    local is = lfs.chdir(name) and true or false
    lfs.chdir(cd)
    return is
end

os.rename(name1, name2) will rename name1 to name2. Use the same name and nothing should change (except there is a badass error). If everything worked out good it returns true, else it returns nil and the errormessage. If you dont want to use lfs you cant differentiate between files and directories without trying to open the file (which is a bit slow but ok).

So without LuaFileSystem

-- no require("lfs")

function exists(name)
    if type(name)~="string" then return false end
    return os.rename(name,name) and true or false
end

function isFile(name)
    if type(name)~="string" then return false end
    if not exists(name) then return false end
    local f = io.open(name)
    if f then
        f:close()
        return true
    end
    return false
end

function isDir(name)
    return (exists(name) and not isFile(name))
end

It looks shorter, but takes longer... Also open a file is a it risky

Have fun coding!

Ferraro

How are errors from os.rename handled regarding renaming read-only files? – Balsa

What are the risks in simply opening a file from lua? – Microbiology

@Microbiology If you try to open a locked file and read from it it may result in an error (you still want to know whether its a file or not). Same goes for directories (if diectory lock is supported on the host). – Ferraro

@HenrikErlandsson What do you mean? With 'badass error' i did not mean something you could fix by code. However, AFAIK you can use pcall to capture them. Handling might be complicated and uninformative error messages could be returned. – Ferraro

For the lfs case, use lfs.attributes to query the file/folder without renaming or opening it. – Intreat

Answer 5

If you're using Premake and LUA version 5.3.4:

if os.isfile(path) then
    ...
end

Confucian

this is not official function, it's function of premake – Dyspeptic

@Dyspeptic AH. My mistake. premake is all I use lua for. :( – Confucian

No problem mate – Dyspeptic

Answer 6

If you are willing to use lfs, you can use lfs.attributes. It will return nil in case of error:

require "lfs"

if lfs.attributes("non-existing-file") then
    print("File exists")
else
    print("Could not get attributes")
end

Although it can return nil for other errors other than a non-existing file, if it doesn't return nil, the file certainly exists.

Mesitylene

To only check for files (not directories) you can use lfs.attributes(path, "mode") == "file" – Intreat

Answer 7

For sake of completeness: You may also just try your luck with path.exists(filename). I'm not sure which Lua distributions actually have this path namespace (update: Penlight), but at least it is included in Torch:

$ th

  ______             __   |  Torch7
 /_  __/__  ________/ /   |  Scientific computing for Lua.
  / / / _ \/ __/ __/ _ \  |  Type ? for help
 /_/  \___/_/  \__/_//_/  |  https://github.com/torch
                          |  http://torch.ch

th> path.exists(".gitignore")
.gitignore  

th> path.exists("non-existing")
false   

debug.getinfo(path.exists) tells me that its source is in torch/install/share/lua/5.1/pl/path.lua and it is implemented as follows:

--- does a path exist?.
-- @string P A file path
-- @return the file path if it exists, nil otherwise
function path.exists(P)
    assert_string(1,P)
    return attrib(P,'mode') ~= nil and P
end

Whatsoever

That would be Penlight, and it uses LuaFileSystem behind the scenes. – Burgrave

Answer 8

An answer which is windows only checks for files and folders, and also requires no additional packages. It returns true or false.

io.popen("if exist "..PathToFileOrFolder.." (echo 1)"):read'*l'=='1'

io.popen(...):read'*l' - executes a command in the command prompt and reads the result from the CMD stdout

if exist - CMD command to check if an object exists

(echo 1) - prints 1 to stdout of the command prompt

Yanina

This opens a briefly visible console window, so I'd advise against this idea. – Wigley

Answer 9

An answer which is windows only checks for files and folders, and also requires no additional packages. It returns true or false.

Venetis

Answer 10

You can also use the 'paths' package. Here's the link to the package

Then in Lua do:

require 'paths'

if paths.filep('your_desired_file_path') then
    print 'it exists'
else
    print 'it does not exist'
end

Blessington

Answer 11

Lua 5.4:

function file_exists(name)
   local f <close> = io.open(name, "r")
   return f ~= nil
end

Shuttering

Answer 12

Not necessarily the most ideal as I do not know your specific purpose for this or if you have a desired implementation in mind, but you can simply open the file to check for its existence.

local function file_exists(filename)
    local file = io.open(filename, "r")
    if (file) then
        -- Obviously close the file if it did successfully open.
        file:close()
        return true
    end
    return false
end

io.open returns nil if it fails to open the file. As a side note, this is why it is often used with assert to produce a helpful error message if it is unable to open the given file. For instance:

local file = assert(io.open("hello.txt"))

If the file hello.txt does not exist, you should get an error similar to stdin:1: hello.txt: No such file or directory.

Stauder

Answer 13

For library solution, you can use either paths or path.

From the official document of paths:

paths.filep(path)

Return a boolean indicating whether path refers to an existing file.

paths.dirp(path)

Return a boolean indicating whether path refers to an existing directory.

Although the names are a little bit strange, you can certainly use paths.filep() to check whether a path exists and it is a file. Use paths.dirp() to check whether it exists and it is a directory. Very convenient.

If you prefer path rather than paths, you can use path.exists() with assert() to check the existence of a path, getting its value at the same time. Useful when you are building up path from pieces.

prefix = 'some dir'

filename = assert(path.exist(path.join(prefix, 'data.csv')), 'data.csv does not exist!')

If you just want to check the boolean result, use path.isdir() and path.isfile(). Their purposes are well-understood from their names.

Seale

Answer 14

How about doing something like this?

function exist(file)
  local isExist = io.popen(
    '[[ -e '.. tostring(file) ..' ]] && { echo "true"; }')
  local isIt = isExist:read("*a")
  isExist:close()
  isIt = string.gsub(isIt, '^%s*(.-)%s*$', '%1')
  if isIt == "true" then
    return true
  end
  return false
end

if exist("myfile") then
  print("hi, file exists")
else
  print("bye, file does not exist")
end

Petuu

I get '[[' is not recognized as an internal or external command, operable program or batch file. – Tumbrel

Its working on my unix machine. What OS are you using ? type [[ should say [[ is a shell keyword – Petuu

@RakibFiha he using windows based on the error message and it work on my Linux machine – Blearyeyed

Answer 15

If you use LOVE, you can use the function love.filesystem.exists('NameOfFile'), replacing NameOfFile with the file name. This returns a Boolean value.

Filigree

Answer 16

if you are like me exclusively on Linux, BSD any UNIX type you can use this:

function isDir(name)
    if type(name)~="string" then return false end
    return os.execute('if [ -d "'..name..'" ]; then exit 0; else exit 1; fi')
end

function isFile(name)
    if type(name)~="string" then return false end
    return os.execute('if [ -f "'..name..'" ]; then exit 0; else exit 1; fi')
end

Actually this looks even better:

function isDir(name)
    if type(name)~="string" then return false end
    return os.execute('test -d '..name)
end

function isFile(name)
    if type(name)~="string" then return false end
    return os.execute('test -f '..name)
end

function exists(name)
    if type(name)~="string" then return false end
    return os.execute('test -e '..name)
end

Swagerty

Answer 17

IsFile = function(path)
print(io.open(path or '','r')~=nil and 'File exists' or 'No file exists on this path: '..(path=='' and 'empty path entered!' or (path or 'arg "path" wasn\'t define to function call!')))
end

IsFile()
IsFile('')
IsFIle('C:/Users/testuser/testfile.txt')

Looks good for testing your way. :)

Kohn