mosquitto_sub: 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/bin/mosquitto_sub - from package: mosquitto-clients

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

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