ds9: 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/ds9 - from package: saods9

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

Error in other languages:
ds9: bevel nie gevind nie
ds9: comandă negăsită
ds9: comando não encontrado
ds9: commande introuvable
ds9: command not found
ds9: käsku ei ole
ds9: không tìm thấy lệnh
ds9: komanda nerasta
ds9: Kommando nicht gefunden.
ds9: kommandot finns inte
ds9: komut yok
ds9: nie znaleziono polecenia
ds9: níor aimsíodh an t-ordú
ds9: no se encontró la orden
ds9: no s'ha trobat l'ordre
ds9: opdracht niet gevonden
ds9: parancs nem található
ds9: perintah tidak ditemukan
ds9: príkaz nenájdený
ds9: příkaz nenalezen
ds9: команда не найдена
ds9: командата не е открита
ds9: 命令找不到
ds9: コマンドが見つかりません