ef: 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/ef - from package: ElectricFence

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

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