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Disks

# Permissions

OS X has a UNIX-style foundation that uses permissions in the file system. Every file and folder on your hard disk has an associated set of permissions that determines who can read, write, or execute them. In addition to traditional UNIX permissions OS X Tiger (10.4) introduced ACL (access control lists) for more fine grained permission management. In OS X Leopard (10.5) ACL is used by default in for example the user's home folder.
Incorrect permissions may cause unexpected behaviour, for example, applications appearing in the Dock as question marks and/or are not able to connect to the Internet. Software installed while logged in as one user will be inaccessible when logged in as another user, files created in Mac OS 9 may appear in OS X with root ownership, backup software may not back up OS X system files with root as its owner. You might not be able to delete folders from the trash without write permission. Please be advised that under certain circumstances repairing a disks permissions can take up to 15 minutes to complete.
You can chose to repair the current user or every users file permissions.

To repair home directory permissions and ACLs:

1. Choose disk in the drop-down menu.
2. Select current or/and every user.
3. Click the Repair button.
4. Click the output button to view log. (optional)
5. Done!

When installing from a package (whose filename extension is ".pkg") OS X stores a "Bill of Materials" file (whose filename extension is ".bom") in the the package's receipt file, which is kept in /Library/Receipts folder. A "Bill of Materials" file contains a list of files installed by the package, and the proper permissions for each file. These files don't take up much disk space and you shouldn't put them in the Trash.
When you repair disk permissions, the system reviews each of the “Bill of Materials” files in the /Library/Receipts/ folder and compares its list to the actual permissions on each file listed. If the permissions differ, they are corrected (all modifications to the privileges will be lost).
Files that aren't installed as part of an installer package are not listed in a receipt and, therefore, are not checked. For example, if you install an application using a non-Apple installer application or by copying it from a disk image, network volume, or other disk instead of installing it via Installer, a receipt file is not created. Also, certain files whose permissions can be changed during normal usage without affecting their function are deliberately not checked.
You can view the output provided by the system while repairing disk permissions in the Outputs window (choose Outputs from the Window menu, then click the Repair Permissions button).

Online information

- Access control list by Wikipedia
- Repair permissions by Wikipedia