umask is a fundamental command in Linux that sets the default permissions for newly created files and directories. It's a mask that determines which permission bits will not be set when a file or directory is created.

How umask Works

When a file or directory is created, Linux assigns default permissions to it. For files, the default permissions are typically 666 (read and write for everyone), and for directories, it's 777 (read, write, and execute for everyone). However, these default permissions are modified by the umask value, which acts as a subtractive mask.

Here's a simple way to understand how umask modifies permissions:

• The umask value is an octal (base-8) number that represents permissions you do not want to be set.

• To determine the actual permissions of a new file or directory, subtract the umask value from the default permissions.

• For example, if the umask value is 022, the actual permissions for a new file would be 666 - 022 = 644 (read and write for the owner, read-only for the group and others).

Setting umask

You can view the current umask value by simply typing umask in the terminal. To change the umask value for the current session, you can use the same command followed by the new value, like umask 027. This will affect all files and directories created in that session.

For a more permanent change, you can set the umask value in global configuration files like /etc/profile or /etc/bash.bashrc, or in a user's shell configuration files such as ~/.bashrc or ~/.profile.