I want to export all collections in MongoDB by the command:

mongoexport -d dbname -o Mongo.json

The result is:

No collection specified!

The manual says, if you don't specify a collection, all collections will be exported.

However, why doesn't this work?

http://docs.mongodb.org/manual/reference/mongoexport/#cmdoption-mongoexport--collection

My MongoDB version is 2.0.6.

I wrote bash script for that. Just run it with 2 parameters (database name, dir to store files).

#!/bin/bash

if [ ! $1 ]; then
        echo " Example of use: $0 database_name [dir_to_store]"
        exit 1
fi
db=$1
out_dir=$2
if [ ! $out_dir ]; then
        out_dir="./"
else
        mkdir -p $out_dir
fi

tmp_file="fadlfhsdofheinwvw.js"
echo "print('_ ' + db.getCollectionNames())" > $tmp_file
cols=`mongo $db $tmp_file | grep '_' | awk '{print $2}' | tr ',' ' '`
for c in $cols
do
    mongoexport -d $db -c $c -o "$out_dir/exp_${db}_${c}.json"
done
rm $tmp_file

In case you want to connect a remote mongoDB server like mongolab.com, you should pass connection credentials eg.

mongoexport -h id.mongolab.com:60599 -u username -p password -d mydb -c mycollection -o mybackup.json