I want to know the cumulative moving distance of robot from the bag file of the data which moved the robot.

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This will depend on what messages you have available to you in the bag file. If you're only got the relative movement (speed and angular rates) then you will have to integrate the speed as you described. But if you have odometry messages which include the absolute position or TF frames which describe the absolute position then you could use these instead.

You can use the rosbag info command described here to quickly see what topics you have available. Then you could visualise the topics in RVIZ to see how noisy they are to decide which one is best to use.

Hope this helps.