Export all MongoDB collections as csv without specifying fields (tested only on non-empty collections with plain scheme).

mongo-dump-csv.sh

OIFS=$IFS
IFS=","

# fill in your details here
dbname=<DBNAME>
user=<USERNAME>
pass=<PASSWORD>
host=<HOST>
port=<PORT>
authdb=<AUTHDB>

# first get all collections in the database
collectionNames=`mongo $dbname --host $host --port $port -u $user -p $pass --authenticationDatabase $authdb --eval 'db.getCollectionNames().join('');' --quiet`

# uncomment for debug
# echo $collectionNames

for col in $collectionNames
do
    echo 'exporting collection' $col
    # get comma-separated list of keys
    keys=`mongo $dbname --host $host --port $port -u $user -p $pass --authenticationDatabase $authdb --eval "rs.slaveOk(); let keys = ''; for (let key in db.getCollection('$col').findOne()) { keys += (key + ','); }; keys;" --quiet`
    mongoexport --host $host --port $port -u $user -p $pass --authenticationDatabase $authdb -d $dbname -c $col --fields "${keys::-1}" --type=csv --out $dbname.$col.csv
done

IFS=$OIFS