jack_server_control: 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/bin/jack_server_control - from package: jack-audio-connection-kit-example-clients

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

On Debian possible default full paths are:

/usr/bin/jack_server_control - from package: jackd1
/usr/bin/jack_server_control - from package: jackd2

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

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