tethereal: 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/sbin/tethereal - from package: wireshark

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

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