Which of the following is NOT a valid permission type in Linux?

In Linux, file permissions are an essential part of the security and management of the file system. The three valid permission types are read, write, and execute.

Read permission allows a user to view the contents of a file or directory. If a user has read permission on a file, they can view (or read) that file's content. For directories, read permission allows a user to list the files and subdirectories contained within.

Write permission permits a user to modify the contents of a file or directory. A user with write permissions on a file can edit or overwrite the file's content. For directories, write permission allows a user to add, delete, or rename files within that directory.

Execute permission grants a user the ability to run a file as a program, or, in the case of a directory, to enter that directory and perform tasks within it.

The option "send" does not represent a valid permission type in Linux. Consequently, it stands apart from the standard permissions defined in the file system. Understanding these core permissions is crucial when configuring security settings for users and managing access control on a Linux system.