glsl-light-model: 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/glsl-light-model - from package: piglit

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

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