i.his.rgb: 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/i.his.rgb - 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/i.his.rgb - from package: grass-core

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

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