Write a hosts file based on all the current instances in 'running' state on EC2

dumphosts.py

import sys
import os
import boto

from optparse import OptionParser
from contextlib import closing

parser = OptionParser()
parser.add_option('-a', '--access-key', default=None, dest='access_key',
                  help="AWS access key (required)")
parser.add_option('-d', '--domain', default=None, dest='domain',
                  help="Top-level domain")
parser.add_option('-s', '--secret-key', default=None, dest='secret_key',
                  help="AWS secret key (required)")
parser.add_option('-o', '--output', default=None, dest='output',
                  help="Output file (defaults to stdout)")

def open_output(f):
    if f == None or f == '-':
        fd = sys.stdout.fileno()
        return os.fdopen(fd, 'w')
    else:
        return open(f, 'w')

def write_hosts(access, secret, domain, output):
    conn = boto.connect_ec2(access, secret)
    instances = conn.get_all_instances()
    fmt = ''
    if domain:
        fmt = '%(ip)s %(name)s.int %(name)s.int.%(domain)s'
    else:
        fmt = '%(ip)s %(name)s.int'

    lines = ['127.0.0.1 localhost localhost.localdomain']
    for ins in instances:
        for i in ins.instances:
            if i.state == 'running' and 'Name' in i.tags:
                if len(i.tags['Name']) == 0:
                    continue

                lines.append(fmt % {'ip': i.private_ip_address,
                                    'name': i.tags['Name'],
                                    'domain': domain})
    if len(lines) > 1:
        with closing(open_output(output)) as out:
            out.write('\n'.join(lines))
            out.write('\n')
        return True
    return False

if __name__ == '__main__':
    (options, args) = parser.parse_args()
    output = None
    
    if len(args) > 0:
        sys.stderr.write("ERROR: no extra args needed!\n")
        parser.print_help()
        raise SystemExit()

    if not options.access_key and 'AWS_ACCESS_KEY' in os.environ:
        options.access_key = os.environ['AWS_ACCESS_KEY']

    if not options.secret_key and 'AWS_SECRET_KEY' in os.environ:
        options.secret_key = os.environ['AWS_SECRET_KEY']

    if not options.secret_key or not options.access_key:
        sys.stderr.write("ERROR: must supply both an access key "
                         "and secret key!\n")
        parser.print_help()
        raise SystemExit()

    write_hosts(options.access_key, options.secret_key, \
                    options.domain, options.output)