ping: 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:

/bin/ping - from package: iputils

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

On Debian possible default full paths are:

/bin/ping - from package: inetutils-ping
/bin/ping - from package: iputils-ping

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

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