readonly: readonly [-af] [name[=value] ...] or readonly -p

Mark shell variables as unchangeable.

Mark each NAME as read-only; the values of these NAMEs may not be changed by subsequent assignment. If VALUE is supplied, assign VALUE before marking as read-only.

Options
-a refer to indexed array variables -A refer to associative array variables -f refer to shell functions -p display a list of all readonly variables and functions

An argument of `--' disables further option processing.

Exit Status:
Returns success unless an invalid option is given or NAME is invalid.