xdpyinfo: 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/bin/xdpyinfo - from package: xorg-x11-utils

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

On Debian possible default full paths are:

/usr/bin/xdpyinfo - from package: x11-utils

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

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