停止rake preview 任务，用ctrl+c，别用ctrl+z，可以避免这个问题

在执行命令

rake preview

时，输出：

dong@Ubuntu:~/octopress$ rake preview  
Starting to watch source with Jekyll and Compass. Starting Rack on port 4000  
>>> Compass is watching for changes. Press Ctrl-C to Stop.  
Configuration file: /home/dong/octopress/_config.yml  
[2015-03-05 13:22:19] INFO  WEBrick 1.3.1  
[2015-03-05 13:22:19] INFO  ruby 1.9.3 (2014-11-13) [i686-linux]  
[2015-03-05 13:22:19] WARN  TCPServer Error: Address already in use - bind(2)  
/home/dong/.rvm/rubies/ruby-1.9.3-p551/lib/ruby/1.9.1/webrick/utils.rb:85:in `initialize': Address already in use - bind(2) (Errno::EADDRINUSE)
    from /home/dong/.rvm/rubies/ruby-1.9.3-p551/lib/ruby/1.9.1/webrick/utils.rb:85:in `new'
    from /home/dong/.rvm/rubies/ruby-1.9.3-p551/lib/ruby/1.9.1/webrick/utils.rb:85:in `block in create_listeners'
    from /home/dong/.rvm/rubies/ruby-1.9.3-p551/lib/ruby/1.9.1/webrick/utils.rb:82:in `each'
    from /home/dong/.rvm/rubies/ruby-1.9.3-p551/lib/ruby/1.9.1/webrick/utils.rb:82:in `create_listeners'
    from /home/dong/.rvm/rubies/ruby-1.9.3-p551/lib/ruby/1.9.1/webrick/server.rb:82:in `listen'
    from /home/dong/.rvm/rubies/ruby-1.9.3-p551/lib/ruby/1.9.1/webrick/server.rb:70:in `initialize'
    from /home/dong/.rvm/rubies/ruby-1.9.3-p551/lib/ruby/1.9.1/webrick/httpserver.rb:45:in `initialize'
    from /home/dong/.rvm/gems/ruby-1.9.3-p551/gems/rack-1.6.0/lib/rack/handler/webrick.rb:32:in `new'
    from /home/dong/.rvm/gems/ruby-1.9.3-p551/gems/rack-1.6.0/lib/rack/handler/webrick.rb:32:in `run'
    from /home/dong/.rvm/gems/ruby-1.9.3-p551/gems/rack-1.6.0/lib/rack/server.rb:286:in `start'
    from /home/dong/.rvm/gems/ruby-1.9.3-p551/gems/rack-1.6.0/lib/rack/server.rb:147:in `start'
    from /home/dong/.rvm/gems/ruby-1.9.3-p551/gems/rack-1.6.0/bin/rackup:4:in `<top (required)>'
    from /home/dong/.rvm/gems/ruby-1.9.3-p551/bin/rackup:23:in `load'
    from /home/dong/.rvm/gems/ruby-1.9.3-p551/bin/rackup: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>'
        Source: source
   Destination: public
      Generating... 
                done.
    write public/stylesheets/screen.css
 Auto-regeneration: enabled for 'source'

解决方法：
执行：

sudo netstat -tulpn |grep 4000

如果提示：

1

sudo: netstat：找不到命令

运行：

1

netstat -tulpn |grep 4000

然后：

1
2
3
4

dong@Ubuntu:~/octopress$ netstat -tulpn |grep 4000
Command 'netstat' not found, but can be installed with:
sudo apt install net-tools
dong@Ubuntu:~/octopress$ sudo apt install net-tools

再运行：

1

sudo netstat -tulpn |grep 4000

得到：

dong@Ubuntu:~/octopress$ sudo netstat -tulpn | grep 4000
[sudo] password for dong: 
tcp        0      0 127.0.0.1:4000          0.0.0.0:*               LISTEN      2889/rackup  

执行：

kill -9 2889
#更改2889处对应的数字，前面的-9好像不变

得到：

dong@Ubuntu:~/octopress$ kill -9 2889
dong@Ubuntu:~/octopress$

done.