If you've come across a file with the .bz2 extension or a .tar.bz2, you may be wondering what it is and whether it's safe to open. The bz2 format is a less usual type of compression, so it's understandable that you might not have anticipated getting a file in this format.
Before we look at whether you should open a .bz2 file, it's important to know a bit more about this file format since it may have a bearing on your decision-making process.
What is a bz2 file?
A file with the .bz2 extension means it has been compressed with the bzip2 compression format. This is a similar concept to a .zip file, which you've probably seen before. The difference is that bzip2 compresses according to a different algorithm than zip.
Therefore, you need different software to unzip a bz2 than you would for a zip file.
.bz2 (or bzip2) is a compression format that's mainly used on Linux
Before you saw the file, when it was being compressed by whoever compressed it, that compression process is likely to have happened on a Linux system. This is because bzip2 is a compression format used on Linux but not often used on other operating systems. While it's true that you can install software to compress (and extract) bz2 files on other operating systems such as Windows and Mac, it's not built-in like other alternatives, such as the zip software.
So your bz2 file most likely was generated on Linux, although it's certainly possible that it could have been done on Mac or Windows if someone installed the bzip2 compression software there.
What is a .tar.bz2 file?
If you have a .tar.bz2 file instead of just a .bz2 file, it means that another step has happened first before bzip2. You might be wondering why the other step was needed. The answer is that the bzip2 compression algorithm has a significant limitation: it can only work on a single file, not on multiple files and folders.
Therefore, for a collection of files, the original collection of files and folders would need to be archived via tar. After that process, the collection will be in .tar format, which can be treated as a single file. Then this .tar file can be compressed by bzip2. This is how you have wound up with a .tar.bz2 file.
The unzipping would therefore need to be done in 2 steps: unzipping the bz2 and then un-archiving the tar. By contrast, if you just had a .bz2 file you could just do 1 step: unzipping the bz2.
Is it safe to unzip a .bz2 file?
This brings us to the question of whether it's safe to open a bz2 file (or a .tar.bz2). The answer to this is the same as for any compressed file: it depends on whether you trust the source. There are some sources that you should definitely not trust, we'll get to those in a minute.
When you're about to unzip any compressed file (whether it's .bz2 or .zip or any other format) you have no idea of the contents of it. You might have an idea of what it should be, but whether it actually is that, you can't tell. The only way to find out is by unzipping - and if it's malware, you don't want to do that!
This is why the question of the source is so important. The .bz2 file extension is not inherently "good" or "bad". It's just a compression format. It is a less typical format, as outlined above, so you should first stop to consider the question of why they used bzip2 instead of a more common compression such as zip.
The next questions to ask yourself are: do you personally know the person giving it to you? How computer-savvy are they - is it possible they got tricked into thinking this file is OK when it's not? Again, these are the same questions you should ask of any compressed files that come your way too (.e.g .zip files), not just .bz2.
When you definitely should not open a bz2 file
You should definitely not open a .bz2 file that you found by randomly looking around on the internet. You don't know what's in it and you don't personally know the author of it. Anything could be in it, and it might even be malware.
When it's probably safe to open a .bz2 file
If your co-worker sent you a .bz2 file that you need for your job, it's probably safe.
I say "probably" because if you want to be really safe, you might want to first contact that co-worker separately on another channel (e.g. voice call) to make sure you're really reaching that person, they really sent you the file, their email hasn't been hacked, etc.
It's a good idea to do these steps for any compressed file that any co-worker sends you unless you were actually expecting such a file from them ahead of time.
Situations where it's unclear whether you should open a bzip2 file
If you requested a bunch of family photos from your cousin Bob, and he sent you a .tar.bz2 file saying the photos are in there, it's probably safe to open - but only if you know that Bob is a reliable and honest guy. On the other hand, if Bob is known for being a bit of a trickster who likes to play practical jokes, you might want to re-think the idea of opening it.
Conclusion
As you can see, the question of whether it's safe to open a .bz2 file all comes down to how trustworthy you know the source to be. A bz2 file has been compressed, which is neither good nor bad, but it means you're not able to see ahead of time what's in it. This issue is not just specific to bz2 files, it's common to any compressed format.
Ultimately the decision is yours whether to open a .bz2 file, but understand that you have no way of knowing for sure what's in there. You should never open a .bz2 file that you found randomly on the internet, and the same goes for files with any other compression format.
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