Depends on what you are using to add users. If it's the
adduser command, then edit your
/etc/adduser.conf file -- there you will find the default login shell option, namely:
If, on the other hand, you prefer to use
useradd then use it with the
-s <shell of choice> parameter.
If you wish to change the shell of existing users you can simply edit
/etc/passwd and change it there.
sed is a perfect choice in case you need to apply changes to numerous lines. Run
sed 's/bash$/zsh/g' /etc/passwd and see what it does. You should see the contents of your
/etc/passwd with every
bash replaced by
zsh. If you are satisfied with the result you can run the same command with -i option:
sed -i 's/bash$/zsh/g' /etc/passwd. Don't forget to make a copy of the original file, just in case :)