webdiff: 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/webdiff - from package: websec

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

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