ryu: 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/ryu - from package: ryu-bin

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

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