An interface to the umask function is a built-in command in the sh, ksh, and csh shell programs. (If umask were a separate program, then typing “umask” wouldn’t change the umask value for the shell’s process! See Appendix B if you are unsure why this scenario is so.) There is also a umask( ) system call for programs that wish to further change their umask.

The most common umask values are 022, 027, and 077. A umask value of 022 lets the owner both read and write all newly created files, but everybody else can only read them.

 0666               Default file-creation mode
 (0022)             Umask
___________________________________________________
 0644               Resultant mode

A umask value of 077 lets only the file’s owner read all newly created files.

 0666                 Default file-creation mode
 (0077)               Umask
____________________________________________________
 0600                 Resultant mode

A simple way to calculate umask values is to remember that the number 2 in the umask turns off write permission, while 7 turns off read, write, and execute permissions.

A umask value of 002 is commonly used by people who are working on group projects. If you create a file with your umask set to 002, anyone in the file’s group will be able to read or modify the file. Everybody else will only be allowed to read it.

 0666                  Default file-creation mode
 (0002)                Umask
______________________________________________________
 0664                  Resultant mode

If you use the Korn shell, ksh , then you can set your umask symbolically. You do this with the same general syntax as the chmod command. In the ksh, the following two commands would be equivalent:

% umask u=rwx,g=x,o=
% umask 067

On many Unix systems, the default umask is 022. This is inherited from the init process, as all processes are descendants of init (see Appendix B). Some systems may be configured to use another umask value, or a different value may be set in the startup files.

The designers of these systems chose this umask value to foster sharing, an open computing environment, and cooperation among users. Most prototype user accounts shipped with Unix operating systems specify 022 as the default umask, and many computer centers use this umask when they set up new accounts. Unfortunately, system administrators frequently do not make a point of explaining the umask to novice users, and many users are not aware that most of the files they create are readable by every other user on the system.

Another approach is to set up new accounts with a umask of 077, so a user’s files will, by default, be unreadable by anyone else on the system unless the user makes a conscious choice to make them readable.

Table 6-13 shows some common umask values and their effects.