glxspheres: 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/glxspheres - from package: VirtualGL

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

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