glx-free-context: 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/lib64/piglit/bin/glx-free-context - from package: piglit

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

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