Check if directory exists in lua?

问题:

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)

回答1:

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.

回答2:

This is a way that works on both Unix and Windows, without any external dependencies:

--- Check if a file or directory exists in this path
function exists(file)
   local ok, err, code = os.rename(file, file)
   if not ok then
      if code == 13 then
         -- Permission denied, but it exists
         return true
      end
   end
   return ok, err
end

--- Check if a directory exists in this path
function isdir(path)
   -- "/" works on both Unix and Windows
   return exists(path.."/")
end

回答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

回答4:

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.

回答5:

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

Then in Lua do:

require 'paths'

if paths.dirp('your_desired_directory') then
    print 'it exists'
else
    print 'it does not exist'
end

回答6:

This is tested for the windows platform. It is quite easy actually:

local function directory_exists( sPath )
  if type( sPath ) ~= "string" then return false end

  local response = os.execute( "cd " .. sPath )
  if response == 0 then
    return true
  end
  return false
end

Obviously, this may not work on other OS's. But for windows users, this can be a solution :)

回答7:

here is a simple way to check if a folder exists WITHOUT ANY EXTERNAL LIBRARY DEPENDENCIES :)

function directory_exists(path)
  local f  = io.popen("cd " .. path)
  local ff = f:read("*all")

  if (ff:find("ItemNotFoundException")) then
    return false
  else
    return true
  end  
end

print(directory_exists("C:\\Users"))
print(directory_exists("C:\\ThisFolder\\IsNotHere"))

If you copy and paste the above into Lua you should see

false
true

good luck :)

回答8:

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. You said you dont want to use lfs. If you dont you cant differentiate between files and directorys 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, because of that you should use lfs. If you don't care about performance (and errorhandling -.-) you can just use it.

Have fun coding!

回答9:

My preferred way of doing this in linux is

if os.execute '[ -e "/home" ]' then
  io.write "it exists"
  if os.execute '[ -d "/home" ]' then
    io.write " and is a directory"
  end
  io.write "\n"
end

or, to put this into a function:

function is_dir(path)
  return os.execute(('[ -d "%s" ]'):format(path))
end -- note that this implementation will return some more values

回答10:

For Linux users:

function dir_exists( path )
if type( path ) ~= 'string' then
    error('input error')
    return false
end
local response = os.execute( 'cd ' .. path )
if response == nil then
    return false
end
return response
end