jack_disconnect: 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_disconnect - 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_disconnect - from package: jackd1
/usr/bin/jack_disconnect - from package: jackd2

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

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