Innobackupex: How to extract individual files from xbstream compressed backups
XtraBackup logs the most recent LSN not only in xtrabackup_info which you'll have to extract, but to STDERR, too:

150731 16:33:28  innobackupex: Executing FLUSH NO_WRITE_TO_BINLOG ENGINE LOGS...
150731 16:33:28  innobackupex: Waiting for log copying to finish

xtrabackup: The latest check point (for incremental): '1948062848'
xtrabackup: Stopping log copying thread.
.&gt;&gt; log scanned up to (1948062848)


The LSN would be 1948062848. Find string xtrabackup: The latest check point (for incremental): in the output. See http://bazaar.launchpad.net/~twindb-dev/twindb-agent/trunk/view/head:/twindb.py#L1888 for example:

def grep_lsn(output):
    """
    Finds LSN in XtraBackup output
    :param output: string with Xtrabackup output
    :return: LSN
    """
    lsn = None
    for line in output.split("\n"):
        if line.startswith("xtrabackup: The latest check point (for incremental):"):
            lsn = line.split("'")[1]
    return lsn


Then, pass 1948062848 with --incremental-lsn= option when you take an incremental copy. Example: http://bazaar.launchpad.net/~twindb-dev/twindb-agent/trunk/view/head:/twindb.py#L1429

if backup_type == 'incremental':
    last_lsn = job["params"]["lsn"]
    xtrabackup_cmd.append("--incremental")
    xtrabackup_cmd.append(".")
    xtrabackup_cmd.append("--incremental-lsn=%s" % last_lsn)