cow-shell: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On Debian possible default full paths are:

/usr/bin/cow-shell - from package: cowdancer

You may install the required package with command apt-get install {package}

Error in other languages:
cow-shell: bevel nie gevind nie
cow-shell: comandă negăsită
cow-shell: comando não encontrado
cow-shell: commande introuvable
cow-shell: command not found
cow-shell: käsku ei ole
cow-shell: không tìm thấy lệnh
cow-shell: komanda nerasta
cow-shell: Kommando nicht gefunden.
cow-shell: kommandot finns inte
cow-shell: komut yok
cow-shell: nie znaleziono polecenia
cow-shell: níor aimsíodh an t-ordú
cow-shell: no se encontró la orden
cow-shell: no s'ha trobat l'ordre
cow-shell: opdracht niet gevonden
cow-shell: parancs nem található
cow-shell: perintah tidak ditemukan
cow-shell: príkaz nenájdený
cow-shell: příkaz nenalezen
cow-shell: команда не найдена
cow-shell: командата не е открита
cow-shell: 命令找不到
cow-shell: コマンドが見つかりません