Overview of BSD Permissions

For each folder and file in the file system, BSD has three categories of users: owner, group, and other. For each of these types of user, three specific permissions affect access to the file or folder: read, write, and execute. If a user does not have read permissions in any of the categories for a file, the user cannot read the file. Similarly, if a user does not have execute permissions for an application, the user cannot run the application.

• The owner of a folder or file is generally the user who created it. Owners typically have full privileges (read, write, and execute) for that file or folder. The owner of a file can set the permissions for other classes of users. The root user is the only user that can transfer ownership of files.

• Every user on the system also belongs to one or more groups. A group is a named collection of users that have something in common. Every file has a group owner associated with it. The group owner can give additional permissions to a particular set of users. For example, if you had a group containing the engineers for a specific project, you would likely give that entire group write permissions for the source files in the project’s code repository.

  The system administrator is responsible for setting up groups.

• Users in the other category are just that—everyone who is neither the owner of a file or a member of the group associated with a file. Permissions for this type of user are generally the most restrictive.

Viewing File Permissions

In a Terminal window, if you enter the command ls -l for some location in the file system (say, ~steve/Documents), you get results similar to these:

total 3704

drwxrwxrwx  5 steve  staff  264 Oct 24 20:56 General

drwxrwxr-x  5 steve  admin  264 Oct 21 21:47 ProjectDocs

drwxr-xr-x  6 steve  staff  160 Oct 25 12:00 Planning

drwx--x--x  6 steve  staff  160 Oct 21 15:22 Private

-rwxrwxrwx  1 steve  staff    0 Oct 23 09:55 picture clipping

[sponge:~/Documents] steve%

The results show, among other things, the owner and primary group for a file or folder and the permissions for each type of user. The owner of a file is shown in the third column and the primary group is shown in the fourth. Thus, in the preceding listing, the General folder is owned by user steve and the group is staff.

The first column in a detailed file listing is a set of ten characters. The first character indicates the type of the item. A hyphen indicates an ordinary file, a d indicates a folder (directory), and an l indicates a symbolic link. The remaining nine characters fall into three implicit groups representing the permissions for the owner, group, other user types. The r, w, and x characters, if present, indicate that read, write, and execute permissions are turned on for the type of user the set of bits applies to.

To give an example of how to read the permissions of a file, the following listing shows the permissions for the Planning folder in ~steve/Documents:

drwxr-xr-x  6 steve  staff  160 Oct 25 12:00 Planning

In this example, the item is a folder. The owner permissions are rwx, so the owner view the contents of the directory, can copy files and folders to this directory, can make it his or her current working directory (through the cd command), and can execute any program in it. The permissions for both the staff group and other users are both r-x, meaning that those users can read files in the directory and make it their current working directory; however, they cannot write files to the directory or modify or delete files in it.

Changing File Permissions

If you have the appropriate permissions, you can change owner, group, and individual permissions from a Terminal shell using, respectively, the chown, chgrp, and chmod commands. (See the associated man pages for details.) You can also see the same ownership and permissions information for a selected file or folder in the Privileges pane in an Info window in the Finder. If you are the owner of the file or folder, you can also change permissions directly from the Info window.

To change the individual permissions for a file, you must be able to specify the desired permissions as three-digit integer. In this scenario, each read, write, execute triplet is represented as a decimal number between 0 and 7. Table 1 lists the relationships between each number and triplet.

For example, suppose you have a directory whose contents are visible only to the owner. In this case, the directory listing might look something like this:

drwx--x--x  6 steve  staff  160 Oct 21 15:22 Private

To get the permissions specified here, the owner would have had to execute a command similar to this one:

chmod 711 ./Private

To break this command down, the chmod command changes the individual permissions for the specified item. The number 711 represents the overall permissions for the directory. The first digit (7) represents the triplet rwx. The second and third digits (both 1) represent the triplet --x.

Permissions for Applications and Documents

When you build an application with Xcode, the build subsystem automatically sets the permissions of the executable file to -rwxr-xr-x; this setting enables the owner—that is, the person who installs the application—to execute and write to the application, whereas all others can only execute it. Other IDEs set similar permissions on built executables.

The -rwxr-xr-x setting should suffice except in the rare situations where an application requires privileged (root) access. An example would be an application such as a disk repairer that required low-level hardware access through the kernel. In such cases, you could use the setuid command to acquire root access for the application. You could then use the features of the NetInfo Kit and System frameworks that allow you to authenticate administrators. For more information on setuid, consult the setuid (2) and chmod (1) man pages.

Although the permission set of the application determines who can launch an application, once it is launched, the application process is owned by the user who launched it. This means the application has the same access rights as the logged-in user and inherits the permissions of that user account. These permissions affect many aspects of the application, such as where the application can save user documents. The application can save files only to locations for which the user has appropriate permissions.

When a Carbon, Cocoa, or Java application saves a document, the respective application environment automatically sets the permissions of the document to (-rw-r--r--) by default, giving the owner read and write access but limiting other users to only read-only access. If you want different permissions for the documents created by your application, you must set those permissions using an appropriate file-management interface. For Carbon, use the interfaces of the File Manager. For Cocoa, use the NSFileManager class.

Administrative and Root Accounts

In Mac OS X, administrative and root accounts give you extended access to modifying the file system.