Using UNIX Permissions to Protect Files
Sticky Bit
The sticky bit is a permission bit that protects the files within a directory. If the directory has the
sticky bit set, a file can be deleted only by the file owner, the directory owner, or by a privileged
user. The root user is an example of a privileged user. The sticky bit prevents a user from
deleting other users' files from public directories such as /tmp:
drwxrwxrwt 7 root sys 400 Sep 3 13:37 tmp
Be sure to set the sticky bit manually when you create a swap file or set up a public directory
on a TMPFS file system. For instructions, see Example 5, “Setting Special File Permissions in
Absolute Mode,” on page 24.
Default umask Value
When you create a file or directory, you create it with a default set of permissions. The system
defaults are open. A text file has 666 permissions, which grants read and write permission to
everyone. A directory and an executable file have 777 permissions, which grants read, write,
and execute permission to everyone. Typically, users override the system defaults in their shell
initialization files, such as .bashrc and .kshrc.user. An administrator can also set defaults in
the /etc/profile file.
The value that the umask command assigns is subtracted from the default. This process has
the effect of denying permissions in the same way that the chmod command grants them. For
example, the chmod 022 command grants write permission to group and others. The umask 022
command denies write permission to group and others.
The following table shows some typical umask values and their effect on an executable file.
TABLE 3
umask Settings for Different Security Levels
Level of Security
umask Setting
Permissions Disallowed
Permissive (744) 022 w for group and others
Moderate (751) 026 w for group, rw for others
Strict (740) 027 w for group, rwx for others
Severe (700) 077 rwx for group and others
For more information about setting the umask value, see the umask(1) man page.
Chapter 1 • Controlling Access to Files 13
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