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3.4.2.3. The file mask

When a new file is saved somewhere, it is first subjected to the standard security procedure. Files without permissions don't exist on Linux. The standard file permission is determined by the mask for new file creation. The value of this mask can be displayed using the umask command:


bert:~> umask
0002

Instead of adding the symbolic values to each other, as with chmod, for calculating the permission on a new file they need to be subtracted from the total possible access rights. In the example above, however, we see 4 values displayed, yet there are only 3 permission categories: user, group and other. The first zero is part of the special file attributes settings, which we will discuss in Section 3.4.2.4 and Section 4.1.6. It might just as well be that this first zero is not displayed on your system when entering the umask command, and that you only see 3 numbers representing the default file creation mask.

Each UNIX-like system has a system function for creating new files, which is called each time a user uses a program that creates new files, for instance, when downloading a file from the Internet, when saving a new text document and so on. This function creates both new files and new directories. Full read, write and execute permission is granted to everybody when creating a new directory. When creating a new file, this function will grant read and write permissions for everybody, but set execute permissions to none for all user categories. This, before the mask is applied, a directory has permissions 777 or rwxrwxrwx, a plain file 666 or rw-rw-rw-.

The umask value is subtracted from these default permissions after the function has created the new file or directory. Thus, a directory will have permissions of 775 by default, a file 664, if the mask value is (0)002. This is demonstrated in the example below:


bert:~> mkdir newdir

bert:~> ls -ld newdir
drwxrwxr-x    2 bert    bert            4096 Feb 28 13:45 newdir/

bert:~> touch newfile

bert:~> ls -l newfile
-rw-rw-r--    1 bert    bert               0 Feb 28 13:52 newfile

If you log in to another group using the newgrp command, the mask remains unchanged. Thus, if it is set to 002, files and directories that you create while being in the new group will also be accessible to the other members of that group; you don't have to use chmod.

The root user usually has stricter default file creation permissions:


[[email protected] root]# umask
022

These defaults are set system-wide in the shell resource configuration files, for instance /etc/bashrc or /etc/profile. You can change them in your own shell configuration file, see Chapter 7 on customizing your shell environment.

Introducing Linux
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