qv4l2: 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/qv4l2 - from package: qv4l2

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

On Debian possible default full paths are:

/usr/bin/qv4l2 - from package: qv4l2

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

Error in other languages:
qv4l2: bevel nie gevind nie
qv4l2: comandă negăsită
qv4l2: comando não encontrado
qv4l2: commande introuvable
qv4l2: command not found
qv4l2: käsku ei ole
qv4l2: không tìm thấy lệnh
qv4l2: komanda nerasta
qv4l2: Kommando nicht gefunden.
qv4l2: kommandot finns inte
qv4l2: komut yok
qv4l2: nie znaleziono polecenia
qv4l2: níor aimsíodh an t-ordú
qv4l2: no se encontró la orden
qv4l2: no s'ha trobat l'ordre
qv4l2: opdracht niet gevonden
qv4l2: parancs nem található
qv4l2: perintah tidak ditemukan
qv4l2: príkaz nenájdený
qv4l2: příkaz nenalezen
qv4l2: команда не найдена
qv4l2: командата не е открита
qv4l2: 命令找不到
qv4l2: コマンドが見つかりません