Command Shell
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:
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:
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:
users.users.myuser.shell = pkgs.fish;
You can also choose whether or not a user should use the default shell:
users.users.myuser.useDefaultShell = true;
Changing /bin/sh
/bin/sh is a symlink to your default POSIX-compliant shell. It's used when writing shell scripts, so that the script works on all machines independently of which shell the user is using. /bin/sh doesn't have to be the same as your interactive shell (i.e. the one you use in your terminal). For example, some people set their interactive shells to zsh/fish but set /bin/sh to dash, because it's fast and scripts don't need any of those fancy zsh/fish features.
To change your default POSIX shell on NixOS, use
# Dash is just an example, you can use whatever you want
environment.binsh = "${pkgs.dash}/bin/dash";