NSFileManager & NSFilePosixPermissions

Question

I want to use the octal permissions (used for chmod) for NSFilePosixPermissions. Here is what I did now:

NSFileManager *manager = [NSFileManager defaultManager];
NSDictionary *attributes;

[attributes setValue:[NSString stringWithFormat:@"%d", 0777] 
             forKey:@"NSFilePosixPermissions"]; // chmod permissions 777
[manager setAttributes:attributes ofItemAtPath:@"/Users/lucky/Desktop/script" error:nil];

I get no error, but when I check the result with "ls -o" the permission are't -rwxrwxrwx.

What's wrong? Thanks for help.

Ruben

Answer 1

First, NSFilePosixPermissions is the name of a constant. Its value may also be the same, but that’s not guaranteed. The value of the NSFilePosixPermissions constant could change between framework releases, e. g. from @"NSFilePosixPermissions" to @"posixPermisions". This would break your code. The right way is to use the constant as NSFilePosixPermissions, not @"NSFilePosixPermissions".

Also, the NSFilePosixPermissions reference says about NSFilePosixPermisions:

The corresponding value is an NSNumber object. Use the shortValue method to retrieve the integer value for the permissions.

The proper way to set POSIX permissions is:

// chmod permissions 777

// Swift
attributes[NSFilePosixPermissions] = 0o777

// Objective-C
[attributes setValue:[NSNumber numberWithShort:0777] 
             forKey:NSFilePosixPermissions];

Despairing

Thanks! Only set the NSFilePermissions value don't work I did it so: NSMutableDictionary *attributes = [NSMutableDictionary dictionaryWithDictionary:[manager attributesOfItemAtPath:@"file..." error:nil]];Next step I set the permissions and it worked! – Ruben

You can also use literal notation to pass an NSNumber (and NSDictionary): @{NSFilePosixPermissions: @0777} – Gliwice

Note that the leading zero here is critical. That tells the C compiler that your constant is in Octal. Octal 0777 is 511 in decimal, not the same number! – Hilaria

You say a short. What if the octal I want is 0100640 instead of 0640? That's a perfectly valid octal for C's version of chmod(). However, a C short can only store a number up to 32767 if signed, and 65535 if unsigned. That's not enough to store 0100640. – Strother

No, it’s not me who’s saying that. Apple is.. Also, this attribute is about the file permissions. 0100000 isn’t a permission bit, however. It’s part of the mode. The permissions are a subset of the mode only. – Despairing

For anyone revisiting this with Swift, a leading 0 is not sufficient to indicate a number is octal, you must lead with 0o. NSFileManager.defaultManager.setAttributes([NSFilePosixPermissions: NSNumber(short: 0o777)], ofItemAtPath: "/tmp/foo") – Joule

Added a Swift example. Note that you don’t need to explicitly wrap the value in an NSNumber. Swift-Objective-C bridging will do that automatically. – Despairing

Has now been updated to posixPermissions in swift 3 developer.apple.com/reference/foundation/fileattributekey/… – Planogamete

Answer 2

Solution in Swift 3

let fm = FileManager.default

var attributes = [FileAttributeKey : Any]()
attributes[.posixPermissions] = 0o777
do {
    try fm.setAttributes(attributes, ofItemAtPath: path.path)
}catch let error {
    print("Permissions error: ", error)
}

Planogamete

Answer 3

There is now a FilePermissions type that can make this easier.

You can add an extension:

extension FileManager {

  func setUnixPermissions(_ permissions: FilePermissions, atPath: String) throws {
    try FileManager.default.setAttributes(
      [.posixPermissions: permissions.rawValue],
      ofItemAtPath: atPath
    )
  }

}

Example usage:

  let fm = FileManager.default
  let path = "/path/to/something"

  // FilePermissions.
  try fm.setUnixPermissions([.ownerRead, .groupRead], atPath: path)

  // Octal permissions.
  try fm.setUnixPermissions(FilePermissions(rawValue: 0x777), atPath: path)

Haematoid