diff: 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/diff - from package: diffutils

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

On Debian possible default full paths are:

/usr/bin/diff - from package: diffutils
/usr/lib/plan9/bin/diff - from package: 9base

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

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