因为安装RMagick-2.1.0需要ImageMagick6.3.*，但是ubuntu的源上没有。
http://www.imagemagick.org下载ImageMagick源代码，解压缩，配置： tar xzvf ImageMagick-6.3.7.tar.gzcd tar xzvf ImageMagick-6.3.7./configure
make
sudo make install
这样执行convert -version 命令会出现错误：
convert: error while loading shared libraries:
libMagick.so.10: cannot open shared object file:
No such file or directory
编辑/etc/ld.so.conf，加入：/usr/local/lib执行命令：ldconfig将ImageMagick的库加入系统联接库
注销当前用户重新登录，便于环境变量修改生效。
solution to error:
/usr/lib/ruby/gems/1.8/gems/rmagick-2.1.0/lib/RMagick2.so: /usr/lib/ruby/gems/1.8/gems/rmagick-2.1.0/lib/RMagick2.so: undefined symbol: SketchImage - /usr/lib/ruby/gems/1.8/gems/rmagick-2.1.0/lib/RMagick2.so (LoadError)
Looks like your RMagick2 gem is compiled against the wrong version of ImageMagick. Could you try the last RMagic1 gem instead of the new v2?
With Ubuntu (7.10 tested only)
Install imagemagick, libmagick9, and libmagick9-dev
sudo apt-get install imagemagick libmagick9 libmagick9-dev
Install rmagick v1.15.12; assuming you have installed Ruby and RubyGems
sudo gem install rmagick -v 1.15.12
