Rdiff: 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/rdiff - from package: librsync
/usr/lib64/R/bin/Rdiff - from package: R-core
/usr/lib/R/bin/Rdiff - from package: R-core

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

On Debian possible default full paths are:

/usr/bin/rdiff - from package: rdiff
/usr/lib/R/bin/Rdiff - from package: r-base-core

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

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