alex: 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/alex - from package: alex

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

On Debian possible default full paths are:

/usr/bin/alex - from package: alex

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

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