Which command can change permissions recursively on a directory?

The command that can change permissions recursively on a directory is the one that uses 'chmod' with the '-R' option. The '-R' flag stands for recursive, which means that the operation will be applied not just to the specified directory, but also to all of its contents, including all subdirectories and files within that directory. This allows for efficient management of file permissions across large directory trees, ensuring that every file and subdirectory has the updated permissions as required.

The structure of the command is such that you specify the desired permissions, followed by the directory in which you want those permissions to be applied. This is particularly useful for scenarios where you need to ensure consistent access levels for users across all files and folders under a particular directory.

The other commands listed do not serve the purpose of changing file permissions. While 'chown' is used for changing the owner of a file or directory, and 'chgrp' is for changing the group ownership, neither directly modifies the read, write, or execute permissions of the files. Furthermore, 'perm' does not exist as a valid command in Linux for setting permissions. Thus, the use of 'chmod -R' is the correct and logical choice for changing permissions recursively.