How can I check file permissions of a Linux file using Haskell?

Question

How can I determine whether a file has write / read / executable permissions for the owner of a file?

I've found FileMode http://hackage.haskell.org/package/base-4.10.0.0/docs/System-Posix-Types.html#t:FileMode, but I can't seem to find a simple way to determine the permissions.

Answer 1

Well FileMode is a type alias for CMode with is a 32-bit number. The package further contains constants like ownerReadMode, ownerWriteMode, ownerExecuteMode; groupReadMode, groupWriteMode, groupExecuteMode; otherReadMode, otherWriteMode and otherExecuteMode.

You can use intersectFileModes :: FileMode -> FileMode -> FileMode to determine the intersection. If the intersection with one of the constants is the same as that constant, then the file has that mode.

For example:

import System.Posix.Files(getFileStatus,fileMode,ownerReadMode)
import System.Posix.Files.ByteString(intersectFileModes)

main = do
    fs <- getFileStatus "foo.file"
    if intersectFileModes (fileMode fs) ownerReadMode == ownerReadMode
        then print "owner can read"
        else print "owner can not read"

To make it more convenient, we can define a function:

import System.Posix.Types(FileMode)
import System.Posix.Files.ByteString(intersectFileModes)

hasMode :: FileMode -> FileMode -> Bool
hasMode fa fb = intersectFileModes fa fb == fa

So then function hasMode ownerReadMode will check a filemode and return True if the filemode contains that permission, otherwise False.

Now we can rewrite the above program as:

import System.Posix.Types(FileMode)
import System.Posix.Files(getFileStatus,fileMode,ownerReadMode)
import System.Posix.Files.ByteString(intersectFileModes)

hasMode :: FileMode -> FileMode -> Bool
hasMode fa fb = intersectFileModes fa fb == fa

main = do
    fs <- getFileStatus "../README.md"
    if hasMode ownerReadMode (fileMode fs)
        then print "owner can read"
        else print "owner can not read"