db.execute: 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/db.execute - 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/db.execute - from package: grass-core

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

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