File ownership and permissions

Each file and directory belongs to a specific user ID and group ID, and has a set of permissions (also referred to as modes) associated with it.

You can use these utilities to control ownership and permissions:

To: Use:
Specify the permissions for a file or directory chmod
Change the owner (and optionally the group) for a file or directory chown
Change the group for a file or directory chgrp

For details, see the Utilities Reference.

You can change the permissions and ownership for a file or directory only if you're its owner or you're logged in as root. If you want to change both the permissions and the ownership, change the permissions first. Once you've assigned the ownership to another user, you can't change the permissions.

Permissions are divided into these categories:

u
Permissions for the user (i.e., the owner)
g
Permissions for the group.
o
Permissions for others (i.e., everyone who isn't in the group).

Each set of permissions includes:

r
Read permission.
w
Write permission.
x
Execute permission. For a directory, this is permission to list or search the directory.
s or S
Setuid or setgid (see below).
t or T
Sticky bit (see below).

For example, if you list your home directory (using ls -al), you might get output like this: 

total 94286
drwxr-xr-x 18 barney    techies        6144 Sep 26 06:37 ./
drwxrwxr-x  3 root      root           2048 Jul 15 07:09 ../
drwx------  2 barney    techies        4096 Jul 04 11:17 .AbiSuite/
-rw-rw-r--  1 barney    techies         185 Oct 27  2000 .Sig
-rw-------  1 barney    techies          34 Jul 05  2002 .cvspass
drwxr-xr-x  2 barney    techies        2048 Feb 26  2003 .ica/
-rw-rw-r--  1 barney    techies         320 Nov 11  2002 .kshrc
-rw-rw-r--  1 barney    techies           0 Oct 02 11:17 .lastlogin
drwxrwxr-x  3 barney    techies        2048 Oct 17  2002 .mozilla/
drwxrwxr-x 11 barney    techies        2048 Sep 08 09:08 .ph/
-rw-r--r--  1 barney    techies         254 Nov 11  2002 .profile
drwxrwxr-x  2 barney    techies        4096 Jul 04 09:06 .ws/
-rw-rw-r--  1 barney    techies        3585 Dec 05  2002 123.html

The first column is the set of permissions. A leading d indicates that the item is a directory; see Types of files, earlier in this chapter.

If the permissions are followed by a plus sign (+), the file or directory has an access control list that further specifies the permissions. For more information, see Access Control Lists (ACLs), below.

You can also use octal numbers to indicate the modes; see chmod in the Utilities Reference.

Related links

  • Working with Files

    In a QNX Neutrino system, almost everything is a file; devices, data, and even services are all typically represented as files. This lets you work with local and remote resources easily from the command line, or through any program that works with files.

  • Setuid and setgid

    Some programs, such as passwd, need to run as a specific user in order to work properly:

  • Sticky bit

    The sticky bit is an access permission that affects the handling of executable files and directories.

  • Default file permissions

    Use the umask command to specify the mask for setting the permissions on new files.

  • Access Control Lists (ACLs)

    Some filesystems, such as the Power-Safe (fs-qnx6.so) filesystem, extend file permissions with Access Control Lists, which are based on the withdrawn IEEE POSIX 1003.1e and 1003.2c draft standards.