hydra: 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/bin/hydra - from package: hydra

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

On Debian possible default full paths are:

/usr/bin/hydra - from package: hydra

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

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