错误如下：
$ sudo rosdep init
ERROR: default sources list file already exists:
/etc/ros/rosdep//
Please delete if you wish to re-initialize
Then you can execute
$ rosdep update
If you get
reading in sources list data from /etc/ros/rosdep/
Hit /ros/rosdistro/master/rosdep/
Hit /ros/rosdistro/master/rosdep/
ERROR: unable to process source [/ros/rosdistro/master/rosdep/]:
<urlopen error timed out> (/ros/rosdistro/master/rosdep/)
then you can execute the following command:
解决方法：
命令行中输入以下代码：
sudo gedit /etc/修改，改成下面两句：
nameserver 8.8.8.8 #google域名服务器
nameserver 8.8.4.4 #google域名服务器