How to escape history expansion exclamation mark ! inside a double quoted string?

Question

EDIT: the command substitution is not necessary for the surprising behavior, although it is the most common use case. The same question applies to just echo \"\'!b\'\"

Answer 1

In your last example,

echo "$(echo '!b')"

the exclamation point is not single-quoted. Because history expansion occurs so early in the parsing process, the single quotes are just part of the double-quoted string; the parser hasn't recognized the command substitution yet to establish a new context where the single quotes would be quoting operators.

To fix, you'll have to temporarily turn off history expansion:

set +H
echo "$(echo '!b')"
set -H