BAG File

What are BAG files and how to open them

Are you having problems opening a BAG file or are you simply curious about its contents? We're here to explain the properties of these files and provide you with software that can open or handle your BAG files.

What is a BAG file?

BAG files have multiple uses, and AOL Instant Messenger buddy list is one of them. Read more about the other uses further down the page.

AOL Instant Messenger buddy list

These BAG files contain the list of your frequent correspondences, also known as Buddies, on the AOL Instant Messenger application. These lists are organized into groups depending on the relationship with the correspondence. These lists get saved on AOL servers and locally on your device.

How to open BAG files

Important: Different programs may use files with the BAG file extension for different purposes, so unless you are sure which format your BAG file is, you may need to try a few different programs.

While we have not verified the apps ourselves yet, our users have suggested two different BAG openers which you will find listed below.

Last updated: August 23, 2026

All known file formats using extension .BAG

While AOL Instant Messenger buddy list is a popular type of BAG-file, we know of 4 different uses of the .BAG file extension. Different software may use files with the same extension for different types of data.

Bag archive

These BAG files are archive files created in Bagger, a graphical user interface used to package a group of data files to create archives. The packages are called 'bags'. It is used in Archivematica, an open-source digital preservation system, to store AIPs (Archival Information Package). AIPs describe the structure and content of an archive file.

Bathymetric Attributed Grid

Bathymetric Attributed Grid is an open, non-proprietary standard for storing seafloor elevation data with associated vertical uncertainty and metadata. It was developed by the Open Navigation Surface Working Group in response to a need for a gridded format that not only captures bathymetry but also provides confidence estimates for those measurements and enough metadata to trace origin and use. It is commonly used by hydrographic agencies such as NOAA for public release of survey data.

Files in this format hold data in a regular two-dimensional grid: one layer for seafloor (elevation or depth) values, and a co-located layer describing vertical uncertainty at each node. Mandatory contents include metadata describing spatial reference, vertical datum, and bounds; mandatory structural elements also include a change-tracking list for edits and a digital signature to validate file integrity. Optional datasets may include additional "auxiliary" layers such as nominal elevation.

The format is implemented using the Hierarchical Data Format version 5 (HDF5) for internal structure. Elevation and uncertainty grids are stored as HDF5 datasets (typically 32-bit floats) with positional metadata defining origin, spacing, and coordinate reference. Metadata is encoded as an XML stream conforming to ISO 19115/19115-2 profile and stored within the HDF5 file. A separate certification block appended to the file provides a digital signature and checksum for validation.

A distinguishing technical signature near the beginning of a file includes the ASCII string ".HDF" at offset zero and multiple recognizable fields such as "BagVersion," "EX_GEOGRAPHICBOUNDINGBOX," "MAXIMUM ELEVATION VALUE," "MINIMUM UNCERTAINTY VALUE," etc.

ROS Bag

ROS Bag files store streams of messages exchanged within the Robot Operating System software. They are produced by the rosbag tool when writing data from one or more ROS topics into a file for later analysis, debugging, or playback.

The format captures all messages along with precise timestamps and metadata. Users can record selected topics or all active topics, then replay that data later, or inspect contents, topics, time ranges, and message counts. In robotic development workflows it allows offline examination of sensor logs, replaying recorded actions, synchronizing different data sources, and testing software without needing live hardware.

Internally the file begins with a version header like "#ROSBAG V2.0", followed by a sequence of records. Each record has a header length, header, data length, and data. Records include file header record (which points to where index/connection information is located), chunk records (which group many messages, possibly compressed), connection records (metadata about topics and message datatypes), message data records, index data (so seeking by time is efficient), and finally chunk info records (offsets plus start and end times for each chunk).

Various apps that use files with this extension

These apps are known to open certain types of BAG files. Remember, different programs may use BAG files for different purposes, so you may need to try out a few of them to be able to open your specific file.

Help us help others

File.org helps thousands of users every day, and we would love to hear from you if you have additional information about BAG file formats, example files, or compatible programs. Please use the links below or email us at submit@file.org to discuss further.