Umask

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Revision as of 12:05, 4 April 2009 by Kaiju (Talk | contribs) (Setting the UMASK)

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Original Web Page

http://www.cyberciti.biz/tips/understanding-linux-unix-umask-value-usage.html

What is UMASK

The user file-creation mode mask (umask) is use to determine the file permission for newly created files. It can be used to control the default file permission for new files. It is a four-digit octal number . Procedure to setup default umask.

Setting the UMASK

You can setup umask in /etc/bashrc or /etc/profile file for all users. By default most Linux distros set it to 0022 (022) or 0002 (002). Open /etc/profile (global) or ~/.bashrc file

# vi /etc/profile

OR

$ vi ~/.bashrc

Append/modify following line to setup a new umask: umask 022 Save and close the file. Changes will take effect after next login. But what is 0022 and 0002?

The default umask 0002 used for normal user. With this mask default directory permissions are 775 and default file permissions are 664.

The default umask for the root user is 0022 result into default directory permissions are 755 and default file permissions are 644.

For directories, the base permissions are (rwxrwxrwx) 0777 and for files they are 0666 (rw-rw-rw).

To calculate directory permission for 022 umask (root user):

Default Permissions: 777 Subtract umask value: 022 (-) Allowed Permissions: 755

To calculate file permission for 022 umask (root user):

Default Permissions: 666 Subtract umask value: 022 (-) Allowed Permissions: 644

The following example explains the steps needed to set umask for permissions 700 for user files. The idea very simply is that only the user is allowed to read or write the file.

Default Permissions: 777 Subtract umask value: 077 (-) Allowed Permissions: 700

$ umask 077 $ touch file.txt $ ls -l file.txt

Output:

-rw------- 1 vivek vivek 0 2007-02-01 02:21 file.txt

Sample umask values and permissions umask value User Group Others 0000 all all all 0007 all all none 0027 all r/w none

For more information, read the man page of bash: man bash help umask