mongoexport: 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/mongoexport - from package: mongodb

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

On Debian possible default full paths are:

/usr/bin/mongoexport - from package: mongodb-clients

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

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