How can I backup or sync an Amazon S3 bucket?

The AWS CLI supports this now.

aws s3 cp s3://first-bucket-name s3://second-bucket-name --recursive

I prefer to backup locally using sync where only changes are updated. That is not the perfect backup solution but you can implement periodic updates later as you need:

s3cmd sync --delete-removed s3://your-bucket-name/ /path/to/myfolder/

If you never used s3cmd, install and configure it using:

pip install s3cmd
s3cmd --configure

Also there should be S3 backup services for $5/month but I would also check Amazon Glacier which lets you put nearly 40 GB single archive file if you use multi-part upload.

http://docs.aws.amazon.com/amazonglacier/latest/dev/uploading-archive-mpu.html#qfacts

Remember, if your S3 account is compromised, you have chance to lose all of your data as you would sync an empty folder or malformed files. So, you better write a script to archive your backup few times, for e.g by detecting start of the week.

Update 01/17/2016:

Python based AWS CLI is very mature now.

Please use: https://github.com/aws/aws-cli
Example: aws s3 sync s3://mybucket .

This script backs up an S3 bucket:

#!/usr/bin/env python
from boto.s3.connection import S3Connection
import re
import datetime
import sys
import time

def main():
    s3_ID = sys.argv[1]
    s3_key = sys.argv[2]
    src_bucket_name = sys.argv[3]
    num_backup_buckets = sys.argv[4]
    connection = S3Connection(s3_ID, s3_key)
    delete_oldest_backup_buckets(connection, num_backup_buckets)
    backup(connection, src_bucket_name)

def delete_oldest_backup_buckets(connection, num_backup_buckets):
    """Deletes the oldest backup buckets such that only the newest NUM_BACKUP_BUCKETS - 1 buckets remain."""
    buckets = connection.get_all_buckets() # returns a list of bucket objects
    num_buckets = len(buckets)

    backup_bucket_names = []
    for bucket in buckets:
        if (re.search('backup-' + r'\d{4}-\d{2}-\d{2}' , bucket.name)):
            backup_bucket_names.append(bucket.name)

    backup_bucket_names.sort(key=lambda x: datetime.datetime.strptime(x[len('backup-'):17], '%Y-%m-%d').date())

    # The buckets are sorted latest to earliest, so we want to keep the last NUM_BACKUP_BUCKETS - 1
    delete = len(backup_bucket_names) - (int(num_backup_buckets) - 1)
    if delete <= 0:
        return

    for i in range(0, delete):
        print 'Deleting the backup bucket, ' + backup_bucket_names[i]
        connection.delete_bucket(backup_bucket_names[i])

def backup(connection, src_bucket_name):
    now = datetime.datetime.now()
    # the month and day must be zero-filled
    new_backup_bucket_name = 'backup-' + str('%02d' % now.year) + '-' + str('%02d' % now.month) + '-' + str(now.day);
    print "Creating new bucket " + new_backup_bucket_name
    new_backup_bucket = connection.create_bucket(new_backup_bucket_name)
    copy_bucket(src_bucket_name, new_backup_bucket_name, connection)


def copy_bucket(src_bucket_name, dst_bucket_name, connection, maximum_keys = 100):
    src_bucket = connection.get_bucket(src_bucket_name);
    dst_bucket = connection.get_bucket(dst_bucket_name);

    result_marker = ''
    while True:
        keys = src_bucket.get_all_keys(max_keys = maximum_keys, marker = result_marker)

        for k in keys:
            print 'Copying ' + k.key + ' from ' + src_bucket_name + ' to ' + dst_bucket_name

            t0 = time.clock()
            dst_bucket.copy_key(k.key, src_bucket_name, k.key)
            print time.clock() - t0, ' seconds'

        if len(keys) < maximum_keys:
            print 'Done backing up.'
            break

        result_marker = keys[maximum_keys - 1].key

if  __name__ =='__main__':main()

I use this in a rake task (for a Rails app):

desc "Back up a file onto S3"
task :backup do
     S3ID = "AKIAJM3FAKEFAKENRWVQ"
     S3KEY = "0A5kuzV+F1pbaMjZxHQAZfakedeJd0dfakeNpry"
     SRCBUCKET = "primary-mzgd"
     NUM_BACKUP_BUCKETS = 2

     Dir.chdir("#{Rails.root}/lib/tasks")
     system "./do_backup.py #{S3ID} #{S3KEY} #{SRCBUCKET} #{NUM_BACKUP_BUCKETS}"
end