readonly [-aAf] [name[=value] ...] or readonly -p
Mark shell variables as not changeable. If no ARGUMENTs are given,
or if -p' is given, a list of all readonly variables is printed. An argument of--' disables further option processing.
This is equivalent to declare -r.
Reference: https://cgit.git.savannah.gnu.org/cgit/bash.git/plain/builtins/declare.def?h=bash-5.3