Try this command:

sudo chsh -s /bin/<shell of choice> username

To see which shells you have available to you, use this command:

chsh -l

From: PLUG-discuss [mailto:plug-discuss-bounces@lists.phxlinux.org] On Behalf Of Shawn Badger
Sent: Monday, May 1, 2017 4:50 PM
To: Main PLUG discussion list <plug-discuss@lists.phxlinux.org>
Subject: Ubuntu 16.04 Guest account shell

So i am using the guest account for a kiosk. So far it is great  because it locks the home directory down to a folder in /tmp. But it has some down sides. I can't seem to find a way to set the default shell to anything other then bash. I want a way to give that user a restricted shell in case the user finds a way to get access to a shell. Does anyone know where to modify the script that creates the guest user account is?

Also, as an FYI the guest account leaves the entries in passwd, group and shadow. You need to create a script to clean them out fi you are using it like this.