How can I make a "read-only variable"?

Question

In Bash, you can create a read-only variable

declare -r somevar='bla'

I tried to find something similar in POSIX sh, but the only thing that comes close is this phrase in the set documentation:

[...] read-only variables cannot be reset.

How can I create such a read-only variable?

Answer 1

You can make use of readonly:

$ var="hello"
$ readonly var
$ echo $var
hello
$ var="bye"
sh: var: readonly variable