twiddle_bag.py

#!/bin/env python3

import rosbag
import sys

_, inputfile, outputfile = sys.argv
bag = rosbag.Bag(inputfile)
message_count = bag.get_message_count()
c = 0
with rosbag.Bag(outputfile, "w") as output:
    for topic, msg, t in bag.read_messages():
        if c < message_count - 1:
            output.write(topic, msg, msg.header.stamp if msg._has_header else t)
            c += 1