你没有写/Library/Ruby/Gems/1.8目录的权限。

也就是说，你没有权限在那里写东西。

这是苹果安装的Ruby版本，供他们自己使用。虽然如果您知道自己在做什么，可以对其进行一些小的修改，因为您不确定权限问题，但我认为继续这样做并不是个好主意。

相反，我强烈建议您考虑使用rbenv或RVM来管理一个单独的Ruby，将其安装到您的主目录中的沙箱中，这样您就可以修改/折叠/主轴/更改，而不必担心会弄乱系统Ruby。

在这两者之间，我使用rbenv，尽管我过去经常使用RVM。rbenv采用了一种更加“不干涉”的方法来管理Ruby安装。RVM有很多特性并且非常强大，但是，结果是更具有侵入性。无论哪种情况，在开始安装之前，都要多阅读几次安装文档。

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

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

给用户$whoami在这些文件夹中创建一些东西

sudo chown -R user /Library/Ruby/Gems/2.0.0

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…

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...