以下是两条命令的输出结果：
第一条：

dong@Ubuntu:~/octopress$ rake generate
## Generating Site with Jekyll
    write source/stylesheets/screen.css
/home/dong/.rvm/gems/ruby-1.9.3-p551/gems/execjs-2.3.0/lib/execjs/runtimes.rb:45:in `autodetect': Could not find a JavaScript runtime. See https://github.com/sstephenson/execjs for a list of available runtimes. (ExecJS::RuntimeUnavailable)
    from /home/dong/.rvm/gems/ruby-1.9.3-p551/gems/execjs-2.3.0/lib/execjs.rb:5:in `<module:ExecJS>'
    from /home/dong/.rvm/gems/ruby-1.9.3-p551/gems/execjs-2.3.0/lib/execjs.rb:4:in `<top (required)>'
    from /home/dong/.rvm/gems/ruby-1.9.3-p551/gems/coffee-script-2.3.0/lib/coffee_script.rb:1:in `require'
    from /home/dong/.rvm/gems/ruby-1.9.3-p551/gems/coffee-script-2.3.0/lib/coffee_script.rb:1:in `<top (required)>'
    from /home/dong/.rvm/gems/ruby-1.9.3-p551/gems/coffee-script-2.3.0/lib/coffee-script.rb:1:in `require'
    from /home/dong/.rvm/gems/ruby-1.9.3-p551/gems/coffee-script-2.3.0/lib/coffee-script.rb:1:in `<top (required)>'
    from /home/dong/.rvm/gems/ruby-1.9.3-p551/gems/jekyll-coffeescript-1.0.1/lib/jekyll-coffeescript.rb:2:in `require'
    from /home/dong/.rvm/gems/ruby-1.9.3-p551/gems/jekyll-coffeescript-1.0.1/lib/jekyll-coffeescript.rb:2:in `<top (required)>'
    from /home/dong/.rvm/gems/ruby-1.9.3-p551/gems/jekyll-2.5.3/lib/jekyll/deprecator.rb:46:in `require'
    from /home/dong/.rvm/gems/ruby-1.9.3-p551/gems/jekyll-2.5.3/lib/jekyll/deprecator.rb:46:in `block in gracefully_require'
    from /home/dong/.rvm/gems/ruby-1.9.3-p551/gems/jekyll-2.5.3/lib/jekyll/deprecator.rb:44:in `each'
    from /home/dong/.rvm/gems/ruby-1.9.3-p551/gems/jekyll-2.5.3/lib/jekyll/deprecator.rb:44:in `gracefully_require'
    from /home/dong/.rvm/gems/ruby-1.9.3-p551/gems/jekyll-2.5.3/lib/jekyll.rb:166:in `<top (required)>'
    from /home/dong/.rvm/gems/ruby-1.9.3-p551/gems/jekyll-2.5.3/bin/jekyll:6:in `require'
    from /home/dong/.rvm/gems/ruby-1.9.3-p551/gems/jekyll-2.5.3/bin/jekyll:6:in `<top (required)>'
    from /home/dong/.rvm/gems/ruby-1.9.3-p551/bin/jekyll:23:in `load'
    from /home/dong/.rvm/gems/ruby-1.9.3-p551/bin/jekyll:23:in `<main>'
    from /home/dong/.rvm/gems/ruby-1.9.3-p551/bin/ruby_executable_hooks:15:in `eval'
    from /home/dong/.rvm/gems/ruby-1.9.3-p551/bin/ruby_executable_hooks:15:in `<main>'

第二条：

dong@Ubuntu:~/octopress$ rake preview
Starting to watch source with Jekyll and Compass. Starting Rack on port 4000
/home/dong/.rvm/gems/ruby-1.9.3-p551/gems/execjs-2.3.0/lib/execjs/runtimes.rb:45:in `autodetect': Could not find a JavaScript runtime. See https://github.com/sstephenson/execjs for a list of available runtimes. (ExecJS::RuntimeUnavailable)
    from /home/dong/.rvm/gems/ruby-1.9.3-p551/gems/execjs-2.3.0/lib/execjs.rb:5:in `<module:ExecJS>'
    from /home/dong/.rvm/gems/ruby-1.9.3-p551/gems/execjs-2.3.0/lib/execjs.rb:4:in `<top (required)>'
    from /home/dong/.rvm/gems/ruby-1.9.3-p551/gems/coffee-script-2.3.0/lib/coffee_script.rb:1:in `require'
    from /home/dong/.rvm/gems/ruby-1.9.3-p551/gems/coffee-script-2.3.0/lib/coffee_script.rb:1:in `<top (required)>'
    from /home/dong/.rvm/gems/ruby-1.9.3-p551/gems/coffee-script-2.3.0/lib/coffee-script.rb:1:in `require'
    from /home/dong/.rvm/gems/ruby-1.9.3-p551/gems/coffee-script-2.3.0/lib/coffee-script.rb:1:in `<top (required)>'
    from /home/dong/.rvm/gems/ruby-1.9.3-p551/gems/jekyll-coffeescript-1.0.1/lib/jekyll-coffeescript.rb:2:in `require'
    from /home/dong/.rvm/gems/ruby-1.9.3-p551/gems/jekyll-coffeescript-1.0.1/lib/jekyll-coffeescript.rb:2:in `<top (required)>'
    from /home/dong/.rvm/gems/ruby-1.9.3-p551/gems/jekyll-2.5.3/lib/jekyll/deprecator.rb:46:in `require'
    from /home/dong/.rvm/gems/ruby-1.9.3-p551/gems/jekyll-2.5.3/lib/jekyll/deprecator.rb:46:in `block in gracefully_require'
    from /home/dong/.rvm/gems/ruby-1.9.3-p551/gems/jekyll-2.5.3/lib/jekyll/deprecator.rb:44:in `each'
    from /home/dong/.rvm/gems/ruby-1.9.3-p551/gems/jekyll-2.5.3/lib/jekyll/deprecator.rb:44:in `gracefully_require'
    from /home/dong/.rvm/gems/ruby-1.9.3-p551/gems/jekyll-2.5.3/lib/jekyll.rb:166:in `<top (required)>'
    from /home/dong/.rvm/gems/ruby-1.9.3-p551/gems/jekyll-2.5.3/bin/jekyll:6:in `require'
    from /home/dong/.rvm/gems/ruby-1.9.3-p551/gems/jekyll-2.5.3/bin/jekyll:6:in `<top (required)>'
    from /home/dong/.rvm/gems/ruby-1.9.3-p551/bin/jekyll:23:in `load'
    from /home/dong/.rvm/gems/ruby-1.9.3-p551/bin/jekyll:23:in `<main>'
    from /home/dong/.rvm/gems/ruby-1.9.3-p551/bin/ruby_executable_hooks:15:in `eval'
    from /home/dong/.rvm/gems/ruby-1.9.3-p551/bin/ruby_executable_hooks:15:in `<main>'
>>> Compass is watching for changes. Press Ctrl-C to Stop.
[2015-03-05 12:32:30] INFO  WEBrick 1.3.1
[2015-03-05 12:32:30] INFO  ruby 1.9.3 (2014-11-13) [i686-linux]
[2015-03-05 12:32:30] INFO  WEBrick::HTTPServer#start: pid=2889 port=4000
    write public/stylesheets/screen.css
127.0.0.1 - - [05/Mar/2015:12:32:49 +0800] "GET / HTTP/1.1" 404 - 0.0610
127.0.0.1 - - [05/Mar/2015:12:32:50 +0800] "GET /favicon.ico HTTP/1.1" 404 - 0.0015
127.0.0.1 - - [05/Mar/2015:12:32:53 +0800] "GET / HTTP/1.1" 404 - 0.0013
127.0.0.1 - - [05/Mar/2015:12:37:50 +0800] "GET / HTTP/1.1" 404 - 0.0007
127.0.0.1 - - [05/Mar/2015:12:37:51 +0800] "GET / HTTP/1.1" 404 - 0.0007
^Z
[1]+  已停止               rake preview

解决方法：
执行命令：

gem install execjs
gem install therubyracer

may fail,check this ,or

sudo apt-get install nodejs

done.