null-rsync: 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/null-rsync - from package: mirrorbrain-tools

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

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