unbound-host: 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/unbound-host - from package: unbound

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

On Debian possible default full paths are:

/usr/bin/unbound-host - from package: unbound-host

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

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