ex: 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:

/bin/ex - from package: vim-minimal

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

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