glsl-fwidth: command not found

Have You tried accessing the binary with the full path?

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-fwidth - from package: piglit

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