Mask that controls which file permissions are set for files and directories when they are created. It also refers to a function that sets the mask, and to the mask itself, which is formally known as the file mode creation mask.

Introduction

When a user creates a file or a directory under GNU/Linux or another Unix-like operating system, the default set of permissions are set by the umask command.

Set up the default umask

Usually the umask is set in the file:

 /etc/profile

or for bash shell:

~/.bashrc 
~/.bash_profile

for ksh/bash:

 ~/.profile 

for csh:

~/.cshrc 

for defining the user's environment at login:

~/.login

and it set with adding something like in the files:

umask 022

How is the umask calculated ?

The umask command uses octal values for determining the permissions of the files and directories. The default permissions for a file are calculated by subtracting the value of 666 like:

666 - 022 = 644  

This means the file permissions would be (rw-r--r--)

For directories the default permissions are calculated as follows:

777 - 022 = 755

This translates to base directory permissions (drwx-rw-rw-)

For more information on umask type in shell:

$ man umask

Reference

Wikipedia entry about umask

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