Command Shell
From NixOS Wiki
A shell is a program that translates text commands (like ls
, vim
, reboot
etc) into instructions for your computer. The default shell on NixOS is bash, but it can be easily changed.
Enable
When adding a new shell, always enable the shell system-wide, even if it's already enabled in your Home Manager configuration, otherwise it won't source the necessary files.
For example, for Zsh:

/etc/nixos/configuration.nix
programs.zsh.enable = true;
Changing the default shell
For all users
To set a command shell as the default for all users, use the defaultUserShell
option.
For example, to set Zsh as the default user shell for all users:

/etc/nixos/configuration.nix
users.defaultUserShell = pkgs.zsh;
For a specific user
To set a command shell as the default for a particular user, use the <name>.shell
option.
For example, to set user "myuser"'s shell to fish:

/etc/nixos/configuration.nix
users.users.myuser.shell = pkgs.fish;
You can also choose whether or not a user should use the default shell:

/etc/nixos/configuration.nix
users.users.myuser.useDefaultShell = true;