update-desktop-database: 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/bin/update-desktop-database - from package: desktop-file-utils

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

On Debian possible default full paths are:

/usr/bin/update-desktop-database - from package: desktop-file-utils

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

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