Check if directory exists in lua?

Question

How do I check if a directory exists in lua, preferably without using the LuaFileSystem module if possible?

Trying to do something like this python line:

os.path.isdir(path)

Answer 1

Well, the 5.1 reference manual doesn't have anything in the os table, but if you use Nixstaller, you get os.fileexists for precisely what you've explained.

If you can afford to fiddle around a bit, or if you know what OS you'll be running on, you might get away with the standard os library's os.execute with some system call that will identify if the file exists.

Even better than os.execute might be os.rename:

os.rename(oldname, newname)

Renames file named oldname to newname. If this function fails, it returns nil, plus a string describing the error.

You could try setting oldname and newname the same -- you might not have write permissions, though, so it might fail because you can't write, even though you can read. In that event, you'd have to parse the returned error string and deduce whether you could write, or you'd have to just try executing your function that needs an existing file, and wrap it in a pcall.

Answer 2

The problem is that the stock Lua distribution (nearly) only includes features that are specified in standard C. Standard C makes no presumptions about there actually being a file system of any specific sort out there (or even an operating system, for that matter), so the os and io modules don't provide access information not available from the standard C library.

If you were attempting to code in pure standard C, you would have the same issue.

There is a chance that you can learn whether the folder exists implicitly from an attempt to use it. If you expect it to exist and be writable to you, then create a temporary file there and if the that succeeds, the folder exists. If it fails, you might not be able to distinguish a non-existent folder from insufficient permissions, of course.

By far the lightest-weight answer to getting a specific answer would be a thin binding to just those OS-specific function calls that provide the information you need. If you can accept the lua alien module, then you can like do the binding in otherwise pure Lua.

Simpler, but slightly heavier, is to accept Lua File System. It provides a portable module that supports most things one might want to learn about files and the file system.

Answer 3

If you're specifically interested in avoiding the LFS library, the Lua Posix library has an interface to stat().

require 'posix'
function isdir(fn)
    return (posix.stat(fn).type == 'directory')
end