在执行gem install redis时 
提示:

    gem install redis
    ERROR:  Error installing redis:
            redis requires Ruby version >= 2.2.2.

原因，ruby版本小于了2.2.2。
1.安装RVM:

gpg2 --keyserver hkp://keys.gnupg.net --recv-keys D39DC0E3
curl -L get.rvm.io | bash -s stable
find / -name rvm -print

/usr/local/rvm 
/usr/local/rvm/src/rvm 
/usr/local/rvm/src/rvm/bin/rvm 
/usr/local/rvm/src/rvm/lib/rvm 
/usr/local/rvm/src/rvm/scripts/rvm 
/usr/local/rvm/bin/rvm 
/usr/local/rvm/lib/rvm 
/usr/local/rvm/scripts/rvm

source /usr/local/rvm/scripts/rvm

2.查看rvm库中已知的ruby版本

 rvm list known

MRI Rubies 
[ruby-]1.8.6[-p420] 
[ruby-]1.8.7[-head] # security released on head 
[ruby-]1.9.1[-p431] 
[ruby-]1.9.2[-p330] 
[ruby-]1.9.3[-p551] 
[ruby-]2.0.0[-p648] 
[ruby-]2.1[.10] 
[ruby-]2.2[.6] 
[ruby-]2.3[.3] 
[ruby-]2.4[.0] 
ruby-head 
…. 
3.安装一个ruby版本

rvm install 2.3.3

No checksum for downloaded archive, recording checksum in user configuration. 
ruby-2.3.3 - #extracting rubygems-2.6.12…. 
ruby-2.3.3 - #removing old rubygems……… 
ruby-2.3.3 - #installing rubygems-2.6.12……………………. 
ruby-2.3.3 - #gemset created /usr/local/rvm/gems/ruby-2.3.3@global 
ruby-2.3.3 - #importing gemset /usr/local/rvm/gemsets/global.gems…………..| 
ruby-2.3.3 - #generating global wrappers…….. 
ruby-2.3.3 - #gemset created /usr/local/rvm/gems/ruby-2.3.3 
ruby-2.3.3 - #importing gemsetfile /usr/local/rvm/gemsets/default.gems evaluated to empty gem list 
ruby-2.3.3 - #generating default wrappers…….. 
ruby-2.3.3 - #adjusting #shebangs for (gem irb erb ri rdoc testrb rake). 
Install of ruby-2.3.3 - #complete 
Ruby was built without documentation, to build it run: rvm docs generate-ri 
4.使用一个ruby版本

rvm use 2.3.3

Using /usr/local/rvm/gems/ruby-2.3.3 
[5].设置默认版本

rvm use 2.3.3 --default

Using /usr/local/rvm/gems/ruby-2.3.3 
[6].卸载一个已知版本

rvm remove 2.0.0

查看ruby版本:

ruby --version

ruby 2.3.3p222 (2016-11-21 revision 56859) [x86_64-linux]

安装redis:

gem install redis

Fetching: redis-4.0.0.gem (100%) 
Successfully installed redis-4.0.0 
Parsing documentation for redis-4.0.0 
Installing ri documentation for redis-4.0.0 
Done installing documentation for redis after 1 seconds 
1 gem installed

 上面的步骤完事了，接下来运行一下redis-trib.rb