nyshell: 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/nyshell - from package: patchy

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

On Debian possible default full paths are:

/usr/bin/nyshell - from package: nypatchy

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

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