d.mapgraph: 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/lib64/grass-6.4.4/bin/d.mapgraph - 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.mapgraph - from package: grass-core

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

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