How to bring a background program to foreground

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http://stackoverflow.com – I am working on CentOS6, and running a django server for my development on a tmux session as: python manage.py runserver So I can read my debug string print on it. While there is an unknown reason which made my tmux session lost, and I cannot to bak to my working session using "tmux attach" command. I still can find my server is running by ps command, but I have no permission to kill this in order to run a new server on the same port. So, I want to call this backgound server to frontgound again. As I never use ctrl+z or other ways to take process background, so even I use (HowTos)