Can I revert to sh later?

Yes, you can revert to sh later. To change the user's shell back to sh, you can use the chsh command. Here’s how you can do it:

chsh -s /bin/sh joker

Make sure to replace /bin/sh with the correct path to the sh shell on your system if it's different. After running this command, the user 'joker' will have their shell changed back to sh. You may need to log out and log back in for the changes to take effect.