$screen sage  Then you do what you want in Sage, and when you want to hide it, you "detach" the screen (I think the default command for that is "ctrl-A d"). Later on, you can reattach the screen using $ screen -r

For example, when I'm building Sage on some remote machine, I log in, type screen make ptestlong, detach the screen, and then log out. While it's still building, I can reattach the screen to see what's going on (or I can just look at \$SAGE_ROOT/spkg/logs). When the build process finishes, the screen goes away automatically. If I were doing some long computation in Sage, I might set it up to write output to some file so that I can just look at the file instead of reattaching the screen.
Alternatively, I could just run screen with no arguments. This starts a new shell in which I could execute commands, like make ptestlong or sage. This screen won't go away until I exit from the shell.