when I want to install Ubuntu restricted extras, it says that
to install Ubuntu restricted extras, these items must be removed: Libav codec library
libavcodec54.
I want to know that what will happen if the libavcodec54 moved? If it will generate some errors in my system?
You do not have to worry about the message. From this guide:
Tip: If you get a warning dialog says “to install ubuntu restricted extras, these items must be removed”, just click the ignore because -extra versions of these libraries will be installed instead, to provide additional functionality.
