mongo --quiet databaseName --eval "var c = db.products.find(); while(c.hasNext()) {printjson(c.next())}" > products.json
create a Javascript file like the following:
vim query.js
use databaseName
db.products.find().pretty()
and execute from shell:
mongo < query.js > products.json
mongo --quiet databaseName << EOF > products.json
db.products.find()
EOF