aws-rb: 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/lib/ruby/gems/1.8/gems/aws-sdk-1.40.3/bin/aws-rb - from package: rubygem-aws-sdk

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

On Debian possible default full paths are:

/usr/bin/aws-rb - from package: ruby-aws-sdk

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

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