= From the command line (flake style)

....
$# echo '$ nix shell nixpkgs#hello nixpkgs#cowsay --command bash'
$# nix shell nixpkgs#hello nixpkgs#cowsay --command bash <<EOL
$ hello
$ cowsay "moo"
$# EOL
....

= From the command line (non-flake style)

....
$# echo '$ nix-shell -p "[hello cowsay]"'
$# nix-shell -p "[hello cowsay]" --run bash <<EOL
$ hello
$ cowsay "moo"
$# EOL
....