How to change the default shell from the command line?

• iTerm2 version: 3.4.0beta2
• OS version: MacOS All
• Attach ~/Library/Preferences/com.googlecode.iterm2.plist here (drag-drop from finder into this window) N/A

How to change iTerm2 shell via CLI?

I want to change the user's iTerm2 preferences.

1. I've taken a snapshot of iTerm2's preferences:

   defaults read com.googlecode.iTerm2 > iTerm2.original.defaults

2. Then I've changed them in the GUI:
3. And then I've made another snapshot:

   defaults read com.googlecode.iTerm2 > iTerm2.updated.defaults

4. I can see that the difference is in "New Bookmarks" (though this seems like a rather odd name for a preference change...):

   {
       ...
       "New Bookmarks" =     (
           {
           ...
           Command = "/Users/aj/.local/bin/fish";
           "Custom Command" = "Custom Shell";
       )
   }

I have no idea how I can change an array like that - or if it's even possible with the defaults command. Is there another way?

For reference

I do know how to do this with Unix commands. That's NOT what I'm looking for.

I do NOT want to change /etc/shells and etc/passwd.

I've also asked this on SO: https://superuser.com/questions/1587484/how-to-change-default-shell-in-iterm2-via-command-line

Changing the user's default Shell

MY_FISH="$(which fish)"
if ! grep "$MY_FISH" /etc/shells; then
    sudo 'bash echo "$MY_FISH" >> /etc/shells'
fi
sudo chsh -s "$(which fish)" "$(whoami)"