attach is one of the most useful features of tmux. If you have no interest in tmux for pane and window management, you should use tmux for this. It can be a life saver if you ever get disconnected from the host machine or accidently close your terminal you can connect right back into the session you were just in using attach.
this command will simply attach back to tmux if you are ever disconnected
If you ever run long running tasks on a remote machine by sshing into this you should be doing it inside tmux, or something like tmux so that you do not loose your work.
If you have multiple sessions running you can select the session that you want to attach to by passing -t .
tmux attach -t scratch
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Also check out this long form post for more about how I use tmux.