rngtest: 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/rngtest - from package: rng-tools

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

On Debian possible default full paths are:

/usr/bin/rngtest - from package: rng-tools

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

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