$ brew info readline readline: stable 6.3.6 (bottled) http://tiswww.case.edu/php/chet/readline/rltop.html
This formula is keg-only. OS X provides the BSD libedit library, which shadows libreadline. In order to prevent conflicts when programs look for libreadline we are defaulting this GNU Readline installation to keg-only.
$ brew switch readline 6.2.2 Cleaning /usr/local/Cellar/readline/6.2.2 Cleaning /usr/local/Cellar/readline/6.3.6 Opt link created for /usr/local/Cellar/readline/6.2.2
3. 再度link
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$ brew link --force readline Linking /usr/local/Cellar/readline/6.2.2... 12 symlinks created