How to choose a fixed location for created bag files ?

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For sure the best way to do this is to specify full file paths for the bag file names. Here are a couple of thoughts that would go along with implementing this:

• The first question is what directory to place the bag files in? This depends a bit on exactly what you are doing, but likely the default behavior would be to write the bag files to a particular ROS package directory or some pre-defined directory. If using a ROS package directory, you could use rospkg, if using a predefined directory, you'd likely want to use os.path.expanduser("~") to get the user's home directory (see example below).
• Secondly, how can the user specify a different directory? You could allow the user to use Qt interfaces to specify the directory, or the user could pass a command line argument or set a ROS private parameter to overwrite the default directory.

Example:

import rospkg
import os
fname = "file_name.bag"

# first method
rospack = rospkg.RosPack()
pkgpath = rospack.get_path('my_package')
fullpath = os.path.join(pkgpath, fname)

# second method
pkgpath = os.path.expanduser("~")
fullpath = os.path.join(pkgpath, fname)