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Managing Permissions via Command Line

Viewing and modifying file system permissions in the command line is both much richer and more complicated than in the Finder. The Finder has streamlined ownership, permissions, and ACLs, providing only the most common features that users require. However, the command line offers every conceivable ownership and permissions option. Further, the command line often provides more than one method for performing identical permissions tasks.

Viewing Traditional UNIX Permissions

Once again, the ls command is your primary tool for viewing file and folder information in the command line. The ls command has many options for viewing nearly any file or folder attribute. You can learn more about all the options available to ls from its manual entry page. Here, you will be presented with a few fundamental permissions viewing options.

The most basic ls option for viewing file and folder ownership and permissions is –l:

MyMac:~ michelle$ ls -l
total 0
drwx------+ 5 michelle staff 170 Aug 20 15:49 Desktop
drwx------+ 3 michelle staff 102 Aug 20 01:08 Documents
drwx------+ 3 michelle staff 102 Aug 20 01:08 Downloads
drwx------ 19 michelle staff 646 Aug 20 01:08 Library
drwx------+ 3 michelle staff 102 Aug 20 01:08 Movies
drwx------+ 3 michelle staff 102 Aug 20 01:08 Music
drwx------+ 4 michelle staff 136 Aug 20 01:08 Pictures
drwxr-xr-x+ 7 michelle staff 238 Aug 20 15:29 Public
drwxr-xr-x 5 michelle staff 170 Aug 20 01:08 Sites

The first string of characters at the beginning of each line is shorthand for the item type and permissions. The following information appears from left to right: the number of hard links associated with the item (for most users, this particular bit will be trivial information), the assigned owner, the assigned group, the last modification date, and finally the item’s name.

The syntax for the abbreviated information section is:

  • The first character is item type: - for file, d for folder, and l for symbolic link.
  • The next three characters indicate the owner’s permissions: - for no access, r for read access, w for write access, and x for file execute access or folder browsing access.
  • The middle set of three rwx or - characters indicate the group’s permissions.
  • The final set of three rwx or - characters indicate everyone else’s permissions.
  • Optionally, there may be a + at the end to indicate that the item has ACL rules applied to it, or an @ at the end to indicate that the item has extended attributes.

The execute privilege attribute x has not been introduced yet, but it is the third standard UNIX privilege attribute after read and write. The execute privilege is enabled on files that are commands and applications (or folders that contain application bundles), to indicate that the item contains executable software code. The execute privilege is also required on normal folders to access the contents of the folder. The Finder doesn’t show you when the execute privilege is used, but it will properly manage the execute privilege when you make permissions changes using the Get Info window.

Viewing Access Control Lists (ACLs)

The ACL technology is more advanced than traditional UNIX-style permissions because it allows for an arbitrary number of user and group permissions rules per item. Each permissions rule is known as an Access Control Entry (ACE). Every file and folder on the system can have an unlimited list of ACE rules, hence the “list” in ACLs.

From the command line, Mac OS X’s ACL implementation provides more than a dozen unique privilege attribute types and lets you define each as a specific allow or deny rule. In other words, you can assign an item an unlimited number of user or group rules, or ACEs, which can be used to allow or deny any of the following privilege attributes:

  • Administration—Administration attributes, which define a user’s or group’s ability to make permissions changes, include change privileges and change ownership.

  • Read—Read attributes define a user or group’s ability to read items and include read attributes, read extended attributes, read file data or list folder contents, execute file or traverse folder, and read permissions.

  • Write—Write attributes define a user’s or group’s ability to make changes and include write attributes, write extended attributes, read file data or create files in folder, append file data or create new subfolder inside folder, delete item, and delete subfolders and files.

Furthermore, each ACE for a folder can include a static inheritance rule that defines whether the folder’s permissions also apply to new items placed in the folder. Inheritance attributes include the following: no inheritance, apply to just new items in this folder, apply to any new child folders, apply to any new child files, and apply to all descendants of this folder.

To view an item’s ACLs alongside their permissions, simply add the -e option to the -l option:

MyMac:~ michelle$ ls -le
total 0
drwx------+ 5 michelle staff 170 Aug 20 15:49 Desktop
 0: group:everyone deny delete
drwx------+ 3 michelle staff 102 Aug 20 01:08 Documents
 0: group:everyone deny delete
drwx------+ 3 michelle staff 102 Aug 20 01:08 Downloads
 0: group:everyone deny delete
drwx------ 19 michelle staff 646 Aug 20 01:08 Library
drwx------+ 3 michelle staff 102 Aug 20 01:08 Movies
 0: group:everyone deny delete
drwx------+ 3 michelle staff 102 Aug 20 01:08 Music
 0: group:everyone deny delete
drwx------+ 4 michelle staff 136 Aug 20 01:08 Pictures
 0: group:everyone deny delete
drwxr-xr-x+ 7 michelle staff 238 Aug 20 15:29 Public
 0: group:everyone deny delete
drwxr-xr-x 5 michelle staff 170 Aug 20 01:08 Sites

Modifying File Permissions via Command Line

You will use two primary commands for changing file and folder permissions in the command line: chown for changing ownership and chmod for changing privileges.

Changing Ownership via Command Line

Short for “change ownership,” chown will let you change the owner and group associated with a file or folder. Using chown often requires root access, so this command is almost always preceded by the sudo command. To use chown, enter the new owner’s name, followed optionally by a colon and the new group name, and then finish with the item’s path. In the following example, Michelle will use the chown command to change testfile1’s ownership to the user account “kevin” and the group account “admin.”

MyMac:~ michelle$ ls -l Desktop/
total 0
-rw-r--r-- 1 michelle staff 0 Aug 20 15:49 testfile1
drwxr-xr-x 4 michelle staff 136 Aug 20 15:47 testfolder
MyMac:~ michelle$ sudo chown kevin:admin Desktop/testfile1
Password:
MyMac:~ michelle$ ls -l Desktop/
total 0
-rw-r--r-- 1 kevin admin 0 Aug 20 15:49 testfile1
drwxr-xr-x 4 michelle staff 136 Aug 20 15:47 testfolder

Changing Privileges via Command Line

Short for “change file mode,” chmod will let you change the privileges associated with a file or folder. Using chmod on files you don’t own requires root access, so the chmod command is often preceded by the sudo command. To use chmod, enter the new privileges first followed by the item’s path.

As for changing privileges, there are two basic methods when using the chmod command:

  • Using alphanumeric abbreviations—The basic syntax goes: account type, modifier, and then privilege. Account types include u for owner, g for group, and o for everyone else. Modifiers include + for allow, - for deny, and = for exact setting. Privileges are as expected with r for read, w for write, and x for execute or folder access. For example, if you’re using this method to allow full access for the owner and group but read-only access for everyone else, you’d enter ug=rwx,o=r.

  • Using octal notation—As you can see, chmod extensively uses shortcuts and abbreviations. To save even more keystrokes you can use octal notation, which uses numeric abbreviations for defining privileges. The basic syntax for octal notation is to use a single-digit number for the user first, followed by a single number for the group, and then a last single number for everyone else. Octal notation uses 0, for no access; 1, for execution only; 2, for write-only; and 4, for read-only. To use mixed permissions, simply add the numbers together. For example, if you’re using this method to allow for full access to a folder for the owner and group but read-only access for everyone else, you’d type 775.

In the following example, Michelle will use the chmod command to change the permissions of testfile1 and testfolder to allow read and write access for the owner and the group but read-only access for everyone else. She will first use alphanumeric abbreviations, and then octal privilege equivalents.

MyMac:~ michelle$ ls -l Desktop/
total 0
-rw-r--r-- 1 michelle staff 0 Aug 20 15:49 testfile1
drwxr-xr-x 4 michelle staff 136 Aug 20 15:47 testfolder
MyMac:~ michelle$ chmod ug=rw,o=r Desktop/testfile1
MyMac:~ michelle$ ls -l Desktop/
total 0
-rw-rw-r-- 1 michelle staff 0 Aug 20 15:49 testfile1
drwxr-xr-x 4 michelle staff 136 Aug 20 15:47 testfolder
MyMac:~ michelle$ chmod 775 Desktop/testfolder/
MyMac:~ michelle$ ls -l Desktop/
total 0
-rw-rw-r-- 1 michelle staff 0 Aug 20 15:49 testfile1
drwxrwxr-x 4 michelle staff 136 Aug 20 15:47 testfolder

Using the Sticky Bit

As mentioned previously in this chapter, the /Users/Shared folder has a unique permission setting that allows all local users to read and write items into the folder yet prevents other users from being able to delete files that they didn’t originally put in this folder. This special permissions configuration is brought to you courtesy of the “sticky bit.” Essentially, enabling the sticky bit on a folder defines it as an append-only destination, or, more accurately, a folder in which only the owner of the item can delete the item.

You can clearly see the sticky bit setting of the /Users/Shared folder when you view its ownership and permissions. Note the t on the end of the permissions information, which indicates that the sticky bit is enabled:

MyMac:~ michelle$ ls -l /Users/
total 0
drwxrwxrwt   7 root     wheel  238 Aug 10 18:49 Shared
drwxr-xr-x+ 16 cadmin   staff  544 Aug 17 00:10 cadmin
drwxr-xr-x+ 16 logan    staff  544 Aug 19 00:06 logan
drwxr-xr-x+ 17 kevin    staff  578 Aug 17 00:14 kevin
drwxr-xr-x+ 15 michelle staff  510 Aug 20 16:43 michelle

You can enable sticky bit functionality similar to the /Users/Shared folder on any other folder using a special octal notation with the chmod command. In the following example, Michelle has already created a new folder named NewShared. She then uses the chmod command with +t to set sharing for all users with sticky bit functionality:

MyMac:~ michelle$ chmod -R +t NewShared/
MyMac:~ michelle$ ls -l
total 0
drwx------+ 5 michelle staff 170 Aug 20 15:49 Desktop
...
drwxrwxrwt 2 michelle staff 68 Aug 20 17:20 NewShared
...

Managing Locked Items via Command Line

As mentioned previously, Mac OS X includes a special file system lock feature that prevents anyone but the owner of an item from making changes to that item. Any user can easily lock a file or folder he owns from the Finder’s Get Info window, also covered earlier in this chapter.

The problem with the file system lock is that the Finder prevents even other administrative users from making changes or even unlocking items they don’t own. In fact, this file system lock extends to the command line as well. Even with sudo access, an administrator is not allowed to change a locked item—with one important exception, the chflags command. This command allows the administrator to change file system flags, which among other things allows you to lock or unlock any file or folder on the system.

In the following example, Michelle needs to change the permissions of a folder owned by another user so the folder can be shared. However, even using sudo she is denied access from doing this, indicating the file is locked. She verifies this by using ls –lO to view the file listing with file flags, which indeed returns that the folder is locked, “uchg”. She then uses the chflags command with the nouchg option to unlock the folder. Finally, she is able to make changes to the previously locked file.

MyMac:Shared michelle$ sudo chmod go+rx SecureFolder/
chmod: Unable to change file mode on SecureFolder/: Operation not permitted
MyMac:Shared michelle$ ls -lO
total 0
drwx------  7 cadmin  wheel  -     238 Jul  7 13:18 AnotherFolder
drwx------  7 cadmin  wheel  uchg  238 Jul  7 13:18 SecureFolder
MyMac:Shared michelle$ sudo chflags nouchg SecureFolder/
MyMac:Shared michelle$ sudo chmod go+rx SecureFolder/
MyMac:Shared michelle$ ls –lO
total 0
drwx------  7 cadmin  wheel  -     238 Jul  7 13:18 AnotherFolder
drwxr-xr-x  7 cadmin  wheel  -     238 Jul  7 13:18 SecureFolder
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