If you want, you can export all collections to csv without specifying --fields (will export all fields).
From http://drzon.net/export-mongodb-collections-to-csv-without-specifying-fields/ run this bash script
OIFS=$IFS;
IFS=",";
# fill in your details here
dbname=DBNAME
user=USERNAME
pass=PASSWORD
host=HOSTNAME:PORT
# first get all collections in the database
collections=`mongo "$host/$dbname" -u $user -p $pass --eval "rs.slaveOk();db.getCollectionNames();"`;
collections=`mongo $dbname --eval "rs.slaveOk();db.getCollectionNames();"`;
collectionArray=($collections);
# for each collection
for ((i=0; i<${#collectionArray[@]}; ++i));
do
echo 'exporting collection' ${collectionArray[$i]}
# get comma separated list of keys. do this by peeking into the first document in the collection and get his set of keys
keys=`mongo "$host/$dbname" -u $user -p $pass --eval "rs.slaveOk();var keys = []; for(var key in db.${collectionArray[$i]}.find().sort({_id: -1}).limit(1)[0]) { keys.push(key); }; keys;" --quiet`;
# now use mongoexport with the set of keys to export the collection to csv
mongoexport --host $host -u $user -p $pass -d $dbname -c ${collectionArray[$i]} --fields "$keys" --csv --out $dbname.${collectionArray[$i]}.csv;
done
IFS=$OIFS;