glreadtest: 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/glreadtest - from package: VirtualGL

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

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