rear: 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/sbin/rear - from package: rear

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

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