Which command is used to change file permissions in Linux?

The command used to change file permissions in Linux is chmod. This command allows users to specify the level of access that different users have for a file or directory by modifying its permission settings. Permissions can be changed for the owner of the file, the group associated with the file, and all other users. The syntax allows users to set permissions using either symbolic (using letters to represent actions) or numeric (using numbers to represent permissions) representations.

For example, using chmod with the numeric mode, 'chmod 755 filename' grants the owner read, write, and execute permissions while giving the group and others read and execute permissions. This command is fundamental for managing security and access control in a Linux environment.

The other options mentioned serve different functions. The chown command is used to change the ownership of a file or directory, meaning it alters who the file belongs to, but does not affect its permissions. The chgrp command changes the group associated with a file, also not modifying the permissions directly. Lastly, the mkdir command is utilized to create new directories, which again does not involve changing permissions of existing files. Understanding the specificity of these commands helps ensure effective file management in Linux systems.