xhydra: 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/xhydra - from package: hydra-frontend

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

On Debian possible default full paths are:

/usr/bin/xhydra - from package: hydra-gtk

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

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