glsl-unused-varying: 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-unused-varying - from package: piglit

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

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