Merge rosbags. Source: https://answers.ros.org/question/10683/is-there-a-way-to-merge-bag-files/

mergebag.py

#!/usr/bin/env python
import sys
import roslib;
import rospy
import rosbag
from rospy import rostime
import argparse
import os

def parse_args():
    parser = argparse.ArgumentParser(
        prog = 'bagmerge.py',
        description='Merges two bagfiles.')
    parser.add_argument('-o', type=str, help='name of the output file', 
        default = None, metavar = "output_file")
    parser.add_argument('-t', type=str, help='topics which should be merged to the main bag', 
        default = None, metavar = "topics")
    parser.add_argument('-i', help='reindex bagfile', 
        default = False, action="store_true")
    parser.add_argument('main_bagfile', type=str, help='path to a bagfile, which will be the main bagfile')
    parser.add_argument('bagfile', type=str, help='path to a bagfile which should be merged to the main bagfile')
    args = parser.parse_args()
    return args

def get_next(bag_iter, reindex = False, 
        main_start_time = None, start_time = None, 
        topics = None):
    try:
        result = bag_iter.next()
        if topics != None:
            while not result[0] in topics:
                result = bag_iter.next()
        if reindex:
            return (result[0], result[1], 
                result[2] - start_time + main_start_time)
        return result
    except StopIteration:
        return None

def merge_bag(main_bagfile, bagfile, outfile = None, topics = None, 
        reindex = True):
    #get min and max time in bagfile
    main_limits = get_limits(main_bagfile)
    limits = get_limits(bagfile)
    #check output file
    if outfile == None:
        pattern = main_bagfile + "_merged_%i.bag"
        outfile = main_bagfile + "_merged.bag"
        index = 0
        while (os.path.exists(outfile)):
            outfile = pattern%index
            index += 1
    #output some information
    print "merge bag %s in %s"%(bagfile, main_bagfile)
    print "topics filter: ", topics
    print "writing to %s."%outfile
    #merge bagfile
    outbag = rosbag.Bag(outfile, 'w')
    main_bag = rosbag.Bag(main_bagfile).__iter__()
    bag = rosbag.Bag(bagfile).__iter__()
    main_next = get_next(main_bag)
    next = get_next(bag, reindex, main_limits[0], limits[0], topics)
    try:
        while main_next != None or next != None:
            if main_next == None:
                outbag.write(next[0], next[1], next[2])
                next = get_next(bag, reindex, main_limits[0], limits[0], topics)
            elif next == None:
                outbag.write(main_next[0], main_next[1], main_next[2])
                main_next = get_next(main_bag)
            elif next[2] < main_next[2]:
                outbag.write(next[0], next[1], next[2])
                next = get_next(bag, reindex, main_limits[0], limits[0], topics)
            else:
                outbag.write(main_next[0], main_next[1], main_next[2])
                main_next = get_next(main_bag)
    finally:
        outbag.close()

def get_limits(bagfile):
    print "Determine start and end index of %s..."%bagfile
    end_time = None
    start_time = None

    for topic, msg, t in rosbag.Bag(bagfile).read_messages():
        if start_time == None or t < start_time:
            start_time = t
        if end_time == None or t > end_time:
            end_time = t
    return (start_time, end_time)
    
if __name__ == "__main__":
    args = parse_args()
    print args
    if args.t != None:
        args.t = args.t.split(',')
    merge_bag(args.main_bagfile, 
        args.bagfile, 
        outfile = args.o,
        topics = args.t,
        reindex = args.i)

Here is the same code but with changes to work in Python3

#!/usr/bin/env python
import sys
import roslib;
import rospy
import rosbag
from rospy import rostime
import argparse
import os

def parse_args():
    parser = argparse.ArgumentParser(
        prog = 'bagmerge.py',
        description='Merges two bagfiles.')
    parser.add_argument('-o', type=str, help='name of the output file', 
        default = None, metavar = "output_file")
    parser.add_argument('-t', type=str, help='topics which should be merged to the main bag', 
        default = None, metavar = "topics")
    parser.add_argument('-i', help='reindex bagfile', 
        default = False, action="store_true")
    parser.add_argument('main_bagfile', type=str, help='path to a bagfile, which will be the main bagfile')
    parser.add_argument('bagfile', type=str, help='path to a bagfile which should be merged to the main bagfile')
    args = parser.parse_args()
    return args

def get_next(bag_iter, reindex = False, 
        main_start_time = None, start_time = None, 
        topics = None):
    try:
        result = next(bag_iter)
        if topics != None:
            while not result[0] in topics:
                result = next(bag_iter)
        if reindex:
            return (result[0], result[1], 
                result[2] - start_time + main_start_time)
        return result
    except StopIteration:
        return None

def merge_bag(main_bagfile, bagfile, outfile = None, topics = None, 
        reindex = True):
    #get min and max time in bagfile
    main_limits = get_limits(main_bagfile)
    limits = get_limits(bagfile)
    #check output file
    if outfile == None:
        pattern = main_bagfile + "_merged_%i.bag"
        outfile = main_bagfile + "_merged.bag"
        index = 0
        while (os.path.exists(outfile)):
            outfile = pattern%index
            index += 1
    #output some information
    print(f"merge bag {bagfile} in {main_bagfile}")
    print(f"topics filter: {topics}")
    print(f"writing to {outfile}.")
    #merge bagfile
    outbag = rosbag.Bag(outfile, 'w')
    main_bag = rosbag.Bag(main_bagfile).__iter__()
    bag = rosbag.Bag(bagfile).__iter__()
    main_next = get_next(main_bag)
    next = get_next(bag, reindex, main_limits[0], limits[0], topics)
    try:
        while main_next != None or next != None:
            if main_next == None:
                outbag.write(next[0], next[1], next[2])
                next = get_next(bag, reindex, main_limits[0], limits[0], topics)
            elif next == None:
                outbag.write(main_next[0], main_next[1], main_next[2])
                main_next = get_next(main_bag)
            elif next[2] < main_next[2]:
                outbag.write(next[0], next[1], next[2])
                next = get_next(bag, reindex, main_limits[0], limits[0], topics)
            else:
                outbag.write(main_next[0], main_next[1], main_next[2])
                main_next = get_next(main_bag)
    finally:
        outbag.close()

def get_limits(bagfile):
    print (f"Determine start and end index of {bagfile}...")
    end_time = None
    start_time = None

    for topic, msg, t in rosbag.Bag(bagfile).read_messages():
        if start_time == None or t < start_time:
            start_time = t
        if end_time == None or t > end_time:
            end_time = t
    return (start_time, end_time)
    
if __name__ == "__main__":
    args = parse_args()
    print(args)
    if args.t != None:
        args.t = args.t.split(',')
    merge_bag(args.main_bagfile, 
        args.bagfile, 
        outfile = args.o,
        topics = args.t,
        reindex = args.i)