unset is a builtin Unix

Unix

Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...

 shell

Unix shell

A Unix shell is a command-line interpreter or shell that provides a traditional user interface for the Unix operating system and for Unix-like systems...

 command implemented by both the Bourne shell family of shells (sh, ksh, bash, etc.) and the C shell family of shells (csh, tcsh, etc.). It unsets a shell variable, removing it from memory and the shell's exported environment

Environment variable

Environment variables are a set of dynamic named values that can affect the way running processes will behave on a computer.They can be said in some sense to create the operating environment in which a process runs...

. It is implemented as a shell builtin

Shell builtin

In computing, a shell builtin is a command or a function, called from a shell, that is executed directly in the shell itself, instead of an external executable program which the shell would load and execute....

, because it directly manipulates the internals of the shell.

Read-only shell variables cannot be unset. If one tries to unset a read-only variable, the unset command will print an error message and return a non-zero exit code. For more information on read-only variables, see readonly

Readonly

- Unix Readonly Shell Command :The unix readonly command is used for writing a unix Shell script that uses variables. This command allows the programmer to set a variable stored in the Shell 's variable list. This command sets a specified shell variable to not be changeable. After this is done...

.