How to turn current job in background?

I've ssh-ed to my ubuntu 15 server box.

And I run apt-get xxx. It takes too long to download, and I don't want to stop it.

How can I turn this job in background just like from the start I run it with "&"

You can background a process by typing C-z, at that point, the process is suspended and doesn't do anything other than wait. Then execute bg to allow the process to continue in the background.

jobs will list all the process running/waiting in the background.

testarossa :: ~ % jobs                            
[1]  + suspended  sudo apt-get update

To bring it back to the foreground you can just execute fg. In case you have many processes running in the background you can specify which one to bring to the foreground (or continue in the background) by specifying the process id (e.g., [1] in the above example). So given the above example, bg 1 and bg are equivalent. Note that this is not the pid!

Using a terminal multiplexer like screen and tmux would have prevented this. Might want to check those out.

Example

testarossa :: ~ % sudo apt-get update                                                                                            1 ↵
[sudo] password for christophe: 
Hit http://ppa.launchpad.net trusty InRelease
Hit http://ppa.launchpad.net trusty InRelease                                  
Hit http://ppa.launchpad.net trusty InRelease                                  
Hit http://ppa.launchpad.net trusty InRelease                                  
Hit http://ppa.launchpad.net trusty/main amd64 Packages                        
Hit http://ppa.launchpad.net trusty/main i386 Packages                         
Hit http://ppa.launchpad.net trusty/main Translation-en                        
Ign http://us.archive.ubuntu.com trusty InRelease                              
Hit http://ppa.launchpad.net trusty/main amd64 Packages                        
Hit http://ppa.launchpad.net trusty/main i386 Packages                         
Get:1 https://get.docker.com docker InRelease                                  
100% [Waiting for headers] [Connecting to security.ubuntu.com (91.189.91.14)] [^Z
[1]  + 16963 suspended  sudo apt-get update
testarossa :: ~ % bg                                                                                                            20 ↵
[1]  + 16963 continued  sudo apt-get update

Do mind that output from the process appears on your terminal! You can, however, just do normal stuff on the commandline as you wish.