unknown: 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/unknown - from package: fastlink

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

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