pi: 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/pi - from package: puppet

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

On Debian possible default full paths are:

/usr/bin/pi - from package: pi

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

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