How to save the poses which on the global_path topic into a txt file?

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hello everyone ,

I want to save the path from start pose to goal published by the global path planner like Dijkstar in the navfn package , so i am ready to write a node which subscribes that topic.In the callback , i want to save those poses into a txt file,which Matlab can read in , so i can plot them in the Matlab and also analyse the path length and the smoothness .

my question is simple :

in the callback ,how can i save that messages into a txt file or something else,sorry ,i don't know how to write the body of the callback.

Thank you in advance.