rev: 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/rev - from package: util-linux-ng

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

On Debian possible default full paths are:

/usr/bin/rev - from package: util-linux

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

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