The command used to change the permissions of a file or directory in Linux is chmod (change mode).
The chmod command allows users to set or modify the access permissions for file system objects (files and directories). These permissions determine the actions that can be performed by different classes of users: the file owner, members of the file's group, and others. The command syntax typically includes the permissions to be set, which can be expressed in either symbolic or numeric format, and the name of the target file or directory.
References: The use of the chmod command is a fundamental concept covered in the EC-Council's Certified Network Defender (CND) program, as it pertains to securing data by correctly setting file and directory permissions as part of system hardening practices123.