glxdemo: 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 Debian possible default full paths are:

/usr/bin/glxdemo - from package: mesa-utils

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

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