sudo chown -R $USER /Library/Ruby/Gems/

我需要做一个rbenv rehash，这样它就会指向我的本地Gem库。

看起来您已经让您的gem管理器指向系统库，因此，与其混淆权限，不如为您的管理器执行等效的“rehash”以获得指向本地的内容。

sudo chown -R $USER /Library/Ruby/Gems/

该错误的原因是您没有以root用户登录终端。

如果你已经在终端类型的mac上启用了根用户

$ su

如果您没有root用户，您需要使用以下步骤启用它

From the Apple menu choose System Preferences…. From the View menu choose Users & Groups. Click the lock and authenticate as an administrator account. Click Login Options…. Click the “Edit…” or “Join…” button at the bottom right. Click the “Open Directory Utility…” button. Click the lock in the Directory Utility window. Enter an administrator account name and password, then click OK. Choose Enable Root User from the Edit menu. Enter the root password you wish to use in both the Password and Verify fields, then click OK.

更多内容请登录http://support.apple.com/kb/ht1528

在被困了几个小时后，至少它对我有用。

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...

您可以更改GEM_HOME。您还在您的主目录下有一个gem文件夹来检查它的使用

$ gem env

结果如下。不相关的部分省略。

...
  - GEM PATHS:
     - /Users/xxx/.gem/ruby/2.6.0
     - /Library/Ruby/Gems/2.6.0
     - /System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/gems/2.6.0
...
 

你可以使用你的/Users/xxx/.gem/ruby/2.6.0文件夹。

vim ~/.bash_profile

添加以下行

export GEM_HOME=~/.gem/ruby/2.6.0/

之后你就可以使用了

source ~/.bash_profile