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.