y4munsharp: 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/y4munsharp - from package: mjpegtools

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

Error in other languages:
y4munsharp: bevel nie gevind nie
y4munsharp: comandă negăsită
y4munsharp: comando não encontrado
y4munsharp: commande introuvable
y4munsharp: command not found
y4munsharp: käsku ei ole
y4munsharp: không tìm thấy lệnh
y4munsharp: komanda nerasta
y4munsharp: Kommando nicht gefunden.
y4munsharp: kommandot finns inte
y4munsharp: komut yok
y4munsharp: nie znaleziono polecenia
y4munsharp: níor aimsíodh an t-ordú
y4munsharp: no se encontró la orden
y4munsharp: no s'ha trobat l'ordre
y4munsharp: opdracht niet gevonden
y4munsharp: parancs nem található
y4munsharp: perintah tidak ditemukan
y4munsharp: príkaz nenájdený
y4munsharp: příkaz nenalezen
y4munsharp: команда не найдена
y4munsharp: командата не е открита
y4munsharp: 命令找不到
y4munsharp: コマンドが見つかりません