rbenv can't change global ruby version

Question

My Mac OS X has a default ruby.

$ ruby -v
ruby 2.0.0p451 (2014-02-24 revision 45167) [universal.x86_64-darwin13]

I installed rbenv and ruby ver

Answer 1

instead of:
rbenv global 2.7.1
do:
sudo rbenv global 2.7.1