在MacOS Mojave上成功测试:

卸载所有旧的ruby版本(假设你有2.00和2.3.0): $ RVM卸载2.0.0 $ RVM卸载2.3.0 安装全新的ruby版本: 安装ruby 设置默认别名为您的版本: $ RVM别名创建默认ruby 重新启动您的系统，因为这是您的计算机加载最新安装的新ruby版本的最安全的方式。

完成以上步骤后，您可以成功地运行任何gem命令。

安装rbenv;

然后在~/的末尾加上eval "$(rbenv init -)"。Bash_profile(或~/.zshrc MacOS);

打开一个新终端，运行gem install ***将工作!

检查你的Ruby版本是否正确。如果不是，那就改变它。

这对我来说很管用:

$ rbenv global 1.9.3-p547
$ gem update --system

正如bobbdelsol所指出的，rehash对我来说很管用:

==> which ruby
/usr/bin/ruby

==> rbenv install 1.9.3-p551
Downloading ruby-1.9.3-p551.tar.bz2...
-> https://cache.ruby-lang.org/pub/ruby/1.9/ruby-1.9.3-p551.tar.bz2
Installing ruby-1.9.3-p551...
Installed ruby-1.9.3-p551 to /Users/username/.rbenv/versions/1.9.3-p551


==> which ruby
/Users/username/.rbenv/shims/ruby

==> which gem
/Users/username/.rbenv/shims/gem

==> gem install compass
ERROR:  While executing gem ... (Gem::FilePermissionError)
    You don't have write permissions for the /Library/Ruby/Gems/2.0.0 directory.


==> ruby -v
ruby 2.0.0p648 (2015-12-16 revision 53162) [universal.x86_64-darwin15]


==> rbenv global 1.9.3-p551


==> ruby -v
ruby 2.0.0p648 (2015-12-16 revision 53162) [universal.x86_64-darwin15]


==> rbenv global 1.9.3-p551


==> rbenv rehash


==> ruby -v
ruby 1.9.3p551 (2014-11-13 revision 48407) [x86_64-darwin15.4.0]


==> gem install compass
Fetching: sass-3.4.22.gem (100%)
Fetching: multi_json-1.11.3.gem (100%)
Fetching: compass-core-1.0.3.gem (100%)
Fetching: compass-import-once-1.0.5.gem (100%)
Fetching: chunky_png-1.3.5.gem (100%)
Fetching: rb-fsevent-0.9.7.gem (100%)
Fetching: ffi-1.9.10.gem (100%)
Building native extensions.  This could take a while...
Fetching: rb-inotify-0.9.7.gem (100%)
Fetching: compass-1.0.3.gem (100%)
    Compass is charityware. If you love it, please donate on our behalf at http://umdf.org/compass Thanks!
Successfully installed sass-3.4.22
Successfully installed multi_json-1.11.3
Successfully installed compass-core-1.0.3
Successfully installed compass-import-once-1.0.5
Successfully installed chunky_png-1.3.5
Successfully installed rb-fsevent-0.9.7
Successfully installed ffi-1.9.10
Successfully installed rb-inotify-0.9.7
Successfully installed compass-1.0.3
9 gems installed
Installing ri documentation for sass-3.4.22...
Installing ri documentation for multi_json-1.11.3...
Installing ri documentation for compass-core-1.0.3...
Installing ri documentation for compass-import-once-1.0.5...
Installing ri documentation for chunky_png-1.3.5...
Installing ri documentation for rb-fsevent-0.9.7...
Installing ri documentation for ffi-1.9.10...
Installing ri documentation for rb-inotify-0.9.7...
Installing ri documentation for compass-1.0.3...
Installing RDoc documentation for sass-3.4.22...
Installing RDoc documentation for multi_json-1.11.3...
Installing RDoc documentation for compass-core-1.0.3...
Installing RDoc documentation for compass-import-once-1.0.5...
Installing RDoc documentation for chunky_png-1.3.5...
Installing RDoc documentation for rb-fsevent-0.9.7...
Installing RDoc documentation for ffi-1.9.10...
Installing RDoc documentation for rb-inotify-0.9.7...
Installing RDoc documentation for compass-1.0.3...

我的问题是，我之前从zshell切换到bash，没有登录:

/bin/bash --login

虽然我已经安装了rvm，但它无法切换到我新安装的rvm ruby版本，并且仍然试图使用默认的mac安装的ruby二进制。因此我的困惑(用户错误!!)和持续的权限问题…

这对我很管用。另外，如果你之前以根用户的身份安装了gems，它会通过将所有权交还给你来解决这个问题(更安全)。

sudo chown -R `whoami` /Library/Ruby/Gems