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
[tab]refer to indexed array variables
-A
[tab]refer to associative array variables
-f
[tab]refer to shell functions
-p
[tab]display a list of all readonly variables or functions, depending on
whether or not the -f option is given
An argument of `--' disables further option processing.
Exit Status:
Returns success unless an invalid option is given or NAME is invalid.
