sgmldiff: 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/sgmldiff - from package: docbook-utils

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

On Debian possible default full paths are:

/usr/bin/sgmldiff - from package: docbook-utils

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

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