d.histogram: 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.histogram - 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.histogram - from package: grass-core

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

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