d.extend: command not found

Have You tried accessing the binary with the full path?

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/lib64/grass-6.4.4/bin/d.extend - from package: grass

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

On Debian possible default full paths are:

/usr/lib/grass64/bin/d.extend - from package: grass-core

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