hydrogen: 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/hydrogen - from package: hydrogen

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

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