libvirt.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 Debian possible default full paths are:

/usr/lib/ruby/vendor_ruby/fog/bin/libvirt.rb - from package: ruby-fog

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

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