Umask in Linux and Setting Up Default Umask

Table of Contents

In this article, I’ll give you a walkthrough of what is umask in Linux and how to set up default umask. Let’s get started

What is umask in Linux?

User file-creation mode mask (umask) determines the file permissions for newly generated files. It is possible to use it to change the default file permissions for new files. It is an octal number of four digits. A umask can be specified or represented as follows:

Octal Values
Symbolic Values

You can use umask command to set default file permissions on Linux or Unix-like systems.

Moreover,

When a user creates a file or directory in Linux or UNIX, the permissions are set to the defaults. For file-sharing reasons, the system defaults may be left open or loosened in most circumstances. For example, if a text file has 666 permissions, it grants everyone read and write access. Similarly, a directory with 777 permissions allows everyone to read, write, and execute.

Prerequisites

Basics of Linux systems and commands
Basic understanding of file/directory permissions in Linux or Unix-like systems

How to set up default umask on Linux

First, you can find the current value of the shell’s umask by running the umask command without any arguments:

$ umask

Set up default umask on Linux

Umask can be configured for all users in the /etc/bashrc or /etc/profile files. Most Linux distributions set it to 0022 (022) or 0002 by default (002). To modify the system defaults, edit the /.bashrc file in your HOME directory: To override the system defaults for ALL USERS, it is preferable to create or change the /etc/profile.d/umask-for-all-users.sh file on all recent Linux distributions:

$ vi /etc/profile.d/umask-for-all-users.sh

OR you can edit ~/.bashrc or ~/.bash_profile file as:

$ vi ~/.bashrc

Add the following lines to the file:

umask 022

Then save and exit the editor.

You have to re-login so that the changes will take effect

Explanation: octal umask mode 022 and 002

As I mentioned earlier, if the default settings are not changed, the files will be created with access mode 666 and directories will be created with access mode 777.

Here,

Since the default umask for normal users is 002, default file permissions will be 664 and default directory permissions will be 775.
Since the default umask for the root user is 022, the default file permissions will be 644 and the default directory permission will be 755.
The base permissions for the directories are 0777 (rwxrwxrwx) and for the files are 0666 (rw-rw-rw)

i.e.

With umask of 002, you can share data with other users in the same group. Also, group of users can create and modify data files but those who are outside the group can only read the data file. You can set umask to 007 to exclude users completely who are not group members.
With umask of 022, only you can write data, but anyone can read the data.
With umask of 077, No other user can read or write your data.

Calculating umask value

The octal umasks can be calculated via the bitwise AND of the unary complement of the argument using bitwise NOT.

Value notations

Octal Value	Permission
0	read, write and execute
1	read and write
2	read and execute
3	read-only
4	write and execute
5	write only
6	execute only
7	no permissions

Calculation

If you set up umask to 077 the file permissions can be calculated as:

Bit	Targeted at	File permission
0	Owner	read, write and execute
7	Group	no permissions
7	Others	no permissions

Examples

Let’s have an example so that you can understand more easily.

First note down all the default permissins of files and directories in the current shell using following ls -l command:

$ umask

0002

$ ls -l

total 0

Since we don’t have any files and directories, let’s create one file and one directory:

$ touch example.txt
$ mkdir example
$ ls -l

total 4
drwxrwxr-x 2 cloudyfox cloudyfox 4096 दिसम्बर 12 16:33 example
-rw-rw-r-- 1 cloudyfox cloudyfox    0 दिसम्बर 12 16:33 example.tx

Now, let’s set umask to 0 and see what happens:

$ umask 0
$ ls -l
$ touch example2.txt
$ mkdir example2
$ ls -l

Here, you can see for the file example2.txt, file permissions for other changed from read to read and write. Also for the directory example2, directory permissions for other changed from read and execute to read, write and execute.

Next, let’s set umask to 077 and see what happens:

$ umask 077
$ ls -l
$ touch example3.txt
$ mkdir example3
$ ls -l

Calculating final FILE permissions

Now, you can simply subtract the umask from the base permission and you’ll get the final file permission:

666–022 = 644

File base permissions : 666
umask value : 022
subtract to get permissions of new file (666–022) : 644 (rw-r–r–)

Calculating final DIRECTORIES permissions

As above you can simply subtract the umask from the base permission and you’ll get the final directory permission:

777–022 = 755

Directory base permissions : 777
umask value : 022
subtract to get permissions of new directory (777–022) : 755 (rwxr-xr-x)

Set umask using symbolic values

Notations

r : read
w : write
x : execute
u : User ownership (the user who owns the file)
g : group ownership (the permissions granted to other users who are members of the file’s group)
o : other ownership (the permissions granted to users that are in neither of the two preceding categories)

Example

The following command will set umask to 077 i.e. a umask set to u=rwx,g=,o= will result in new files having the modes -rw-------, and new directories having the modes drwx------:

$ umask u=rwx,g=,o=
$ ls -l
$ touch example4.txt
$ mkdir example4
$ ls -l

umask Values and File Creation Permission

umask Value	User Permission	Group Permission	Others Permission
000	all	all	all
007	all	all	none
027	all	read/execute	none

Here, all = read, write and execute

Limitations of umask

This umask command can restrict the permissions.
You cannot use umask command to grant extra permissions beyond what is specified by the program that creates the file or directory. You need to use chmod command to make permission changes to the existing file.

Conclusion

In this, you get to learn about what is umask in Linux and how to set up default umask. For more information you can read the manual using the following shell command:

$ man chmod
$ helm umask
$ man bash

Thanks for reading!

Ref: ClassicSysAdmin