How to cmd_vel a 90 degree turn?

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You can create a relatively simple ROS node (or function) to do this with the following steps:

• Create a TF listener and cmd_vel publisher
• Ask TF the current odometric position (odom -> base link)
• In a while loop:
  • publish some rotation command to cmd_vel, for instance 0.5 radians / second
  • Check TF to see the current pose, compute the amount you've rotated
  • If within a threshold, break from the loop