How to type "Control" on Termux? There is a good App "Termux" which can provide a powerful shell environment without gain root permission. However, the touch-screen keyboard doesn't include "Control" and "Alt" button. But some interactive program (for example, nano, emacs) require these button.

Is there any way to add this button on, or use some other way to replace this button?
You will want to use Hackers Keyboard (available on playstore as well as on F-Droid) for that, which was explicitly written for such purpose:


Hacker's Keyboard (source: Google Play; click image for larger variant)

That would be the most convenient approach – though the app hasn't been updated lately. But if you don't want that, just take a look at the corresponding Termux help page (or the matching Termux Wiki page): Termux utilizes the volume keys for "special combos". You can also have the most common keys shown on the screen to be "tap ready":



The docu writes: To enable the extra keys view you have to long tap on the keyboard button in the left drawer menu. You can also press Volume Up+Q.

Or, as Death Mask Salesman stated in a now deleted comment: "Volume Down equals Control, while Volume Up is the same as Alt."