出现的问题： [root@localhost ~]# gem install redis
Fetching: redis-4.0.1.gem (100%)
ERROR: Error installing redis:
redis requires Ruby version >= 2.2.2.

原因:ruby版本太低，要求2.2.2版本+

解决：安装rbuy高版本

需要用rvm 来安装ruby

[root@localhost ~]# gpg --keyserver hkp://keys.gnupg.net --recv-keys 409B6B1796C275462A1703113804BB82D39DC0E3
gpg: keyring '/root/.gnupg/secring.gpg' created
gpg: requesting key D39DC0E3 from hkp server keys.gnupg.net

gpg: /root/.gnupg/trustdb.gpg: trustdb created
gpg: key D39DC0E3: public key "Michal Papis (RVM signing) <mpapis@gmail.com>" imported
gpg: no ultimately trusted keys found
gpg: Total number processed: 1
gpg:               imported: 1  (RSA: 1)

[root@localhost ~]# curl -sSL https://get.rvm.io | bash -s stable

Downloading https://github.com/rvm/rvm/archive/1.29.3.tar.gz
curl: (52) Empty reply from server

Could not download 'https://github.com/rvm/rvm/archive/1.29.3.tar.gz'.
  curl returned status '52'.

Downloading https://bitbucket.org/mpapis/rvm/get/1.29.3.tar.gz
Downloading https://bitbucket.org/mpapis/rvm/downloads/1.29.3.tar.gz.asc
gpg: Signature made Mon 11 Sep 2017 04:59:29 AM CST using RSA key ID BF04FF17
gpg: Good signature from "Michal Papis (RVM signing) <mpapis@gmail.com>"
gpg:                 aka "Michal Papis <michal.papis@toptal.com>"
gpg:                 aka "[jpeg image of size 5015]"
gpg: WARNING: This key is not certified with a trusted signature!
gpg:          There is no indication that the signature belongs to the owner.
Primary key fingerprint: 409B 6B17 96C2 7546 2A17  0311 3804 BB82 D39D C0E3
     Subkey fingerprint: 62C9 E5F4 DA30 0D94 AC36  166B E206 C29F BF04 FF17
GPG verified '/usr/local/rvm/archives/rvm-1.29.3.tgz'
Creating group 'rvm'

Installing RVM to /usr/local/rvm/
Installation of RVM in /usr/local/rvm/ is almost complete:

  * First you need to add all users that will be using rvm to 'rvm' group,
    and logout - login again, anyone using rvm will be operating with `umask u=rwx,g=rwx,o=rx`.

  * To start using RVM you need to run `source /etc/profile.d/rvm.sh`
    in all your open shell windows, in rare cases you need to reopen all shell windows.

[root@localhost ~]# source /etc/profile.d/rvm.sh 

[root@localhost ~]# rvm -v
rvm 1.29.3 (latest) by Michal Papis, Piotr Kuczynski, Wayne E. Seguin [https://rvm.io]

查询rvm库中ruby的版本

[root@localhost ~]# 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[.7]
[ruby-]2.3[.4]
[ruby-]2.4[.1]
ruby-head

# for forks use: rvm install ruby-head-<name> --url https://github.com/github/ruby.git --branch 2.2

# JRuby
jruby-1.6[.8]
jruby-1.7[.27]
jruby[-9.1.13.0]
jruby-head

# Rubinius
rbx-1[.4.3]
rbx-2.3[.0]
rbx-2.4[.1]
rbx-2[.5.8]
rbx-3[.84]
rbx-head

# Opal
opal

# Minimalistic ruby implementation - ISO 30170:2012
mruby-1.0.0
mruby-1.1.0
mruby-1.2.0
mruby-1[.3.0]
mruby[-head]

# Ruby Enterprise Edition
ree-1.8.6
ree[-1.8.7][-2012.02]

# Topaz
topaz

# MagLev
maglev[-head]
maglev-1.0.0

# Mac OS X Snow Leopard Or Newer
macruby-0.10
macruby-0.11
macruby[-0.12]
macruby-nightly
macruby-head

# IronRuby
ironruby[-1.1.3]
ironruby-head

安装最新的2.4版本ruby

[root@localhost ~]# rvm install 2.4
Searching for binary rubies, this might take some time.
Found remote file https://rvm_io.global.ssl.fastly.net/binaries/centos/7/x86_64/ruby-2.4.1.tar.bz2
Checking requirements for centos.
Installing requirements for centos.
Installing required packages: patch, autoconf, automake, bison, bzip2, libffi-devel, libtool, patch, readline-devel, sqlite-devel, zlib-devel, li...............
Requirements installation successful.
ruby-2.4.1 - #configure
ruby-2.4.1 - #download
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 14.1M  100 14.1M    0     0   117k      0  0:02:02  0:02:02 --:--:--  121k
No checksum for downloaded archive, recording checksum in user configuration.
ruby-2.4.1 - #validate archive
ruby-2.4.1 - #extract
ruby-2.4.1 - #validate binary
ruby-2.4.1 - #setup
ruby-2.4.1 - #gemset created /usr/local/rvm/gems/ruby-2.4.1@global
ruby-2.4.1 - #importing gemset /usr/local/rvm/gemsets/global.gems..............................
ruby-2.4.1 - #generating global wrappers........
ruby-2.4.1 - #gemset created /usr/local/rvm/gems/ruby-2.4.1
ruby-2.4.1 - #importing gemsetfile /usr/local/rvm/gemsets/default.gems evaluated to empty gem list
ruby-2.4.1 - #generating default wrappers........

切换使用2.4版本ruby

[root@localhost ~]# rvm use 2.4
Using /usr/local/rvm/gems/ruby-2.4.1

移除2.0.0 版本ruby

[root@localhost ~]# rvm remove 2.0.0
ruby-2.0.0-p648 - #already gone
Using /usr/local/rvm/gems/ruby-2.4.1

验证ruby版本

[root@localhost ~]# ruby -v
ruby 2.4.1p111 (2017-03-22 revision 58053) [x86_64-linux]