rdfns: command not found

Have You tried accessing the binary with the full path?

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH

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/rdfns - from package: librdf-ns-perl

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