sheep: 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 CentOS possible default full paths are:

/usr/sbin/sheep - from package: sheepdog

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/sbin/sheep - from package: sheepdog

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

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