Bags are typically created by a tool like rosbag
They store the serialized message data in a file as it is received
http://pr.willowgarage.com/ 这里可以下载bag
使用bag文件作为静态输入的时候，要考虑bag和当前电脑的时间不一致的问题
参考了这篇博客关于时间问题的设置
http://blog.csdn.net/akunainiannian/article/details/44993391
The data stored within bag files is often very valuable, so bag files are also designed to be easily migrated when msg files are updated. The bag file format stores the msg file of the corresponding message data, and tools like rosbagmigration let you write rules to automatically update bag files when they become out of date.
rosbag: unified console tool for recording, playback, and other operations.
rqt_bag: graphical tool for visualizing bag file data.
rostopic: the echo and list commands are compatible with bag files.
原文：http://www.cnblogs.com/xy123001/p/6015218.html