xdm: 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/xdm - from package: xorg-x11-xdm

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

On Debian possible default full paths are:

/usr/bin/xdm - from package: xdm

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

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