rediff: 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/rediff - from package: patchutils

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

On Debian possible default full paths are:

/usr/bin/rediff - from package: patchutils

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

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