I'm using Linux Mint, I set a hot key that ` will open a new terminal window. But now I found that when writing markdown, I need this key. How could I map ~ to `?
You can map keys using xbindkeys :
sudo apt-get install xbindkeys # write default configuration xbindkeys --defaults > /home/jylam/.xbindkeysrc
You can then edit ~/.xbindkeysrc to add you own bindings, or install xbindkeys-config as well to use the graphical editor.
I strongly suggest NOT to use ~ as a shortcut, as it represents your home directory (~/). Use modifiers, like ctrl or alt.
I use an application called Guake for this. Instead of opening up a terminal window, it has a persistent drop-down terminal that appears when you press the hotkey. By default it is set to F12, but it can be changed to other keys. When I first started using it, I set it to `, but found that didn't work very well, because, as Ravachol said, you use that key for other things.