我对添加宝石很困惑。我列出了已安装的宝石：
mypath$ gem list
*** LOCAL GEMS ***
bouncy-castle-java (1.5.0147)
cassandra-driver (2.1.3 java)
cql-rb (2.0.4)
domain_name (0.5.23)
http-cookie (1.0.2)
httpclient (2.6.0.1)
imagesize (0.1.1)
ione (1.2.0)
jruby-openssl (0.9.7 java, 0.9.6 java)
json (1.8.2 java)
mime-types (2.4.3)
net-scp (1.2.1)
net-ssh (2.9.2)
netrc (0.10.2)
rest-client (1.8.0, 1.7.2)
ruby-debug-base (0.10.5.rc10 java, 0.10.4 java)
ruby-debug-ide (0.4.24)
soap4r (1.5.8)
unf (0.1.4 java)
xml-simple (1.1.5)
mypath$
我想将minitest-junit gem添加到已安装的gem中，所以我安装了它并且它说gem安装成功了。
myPath$sudo gem install minitest-junit
Successfully installed minitest-junit-0.2.0
1 gem installed
Installing ri documentation for minitest-junit-0.2.0...
Installing RDoc documentation for minitest-junit-0.2.0...
mypath$
当我尝试使用它时，我得到了宝石未知的错误，所以我再次运行gem list命令并且没有列出gem：
mypath$ gem list
*** LOCAL GEMS ***
bouncy-castle-java (1.5.0147)
cassandra-driver (2.1.3 java)
cql-rb (2.0.4)
domain_name (0.5.23)
http-cookie (1.0.2)
httpclient (2.6.0.1)
imagesize (0.1.1)
ione (1.2.0)
jruby-openssl (0.9.7 java, 0.9.6 java)
json (1.8.2 java)
mime-types (2.4.3)
net-scp (1.2.1)
net-ssh (2.9.2)
netrc (0.10.2)
rest-client (1.8.0, 1.7.2)
ruby-debug-base (0.10.5.rc10 java, 0.10.4 java)
ruby-debug-ide (0.4.24)
soap4r (1.5.8)
unf (0.1.4 java)
xml-simple (1.1.5)
mypath$
我在这里缺少什么？如果宝石没有安装，那么不应该出错吗？