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

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