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.BAK files are often left by file editing programs as backups of the previous version of a file. I'll look at how it works and touch on its history.

When I go to empty my Recycle Bin, I notice there are a lot of files in there called “BAK files”. What are they, are they important, where do they come from & is it okay to delete them with everything else that is in there?

".BAK" files are perhaps one of the oldest file types known to computing.

That's not to say that they're all the same thing, though. Far from it. It's very likely that you have many different kinds of .BAK files.

It all starts with the fact that BAK is short for BAcKup.

I'll describe where they come from.

Where do .BAK files come from?

.BAK files come from programs that make changes to files.

"In general, .BAK files are safe to delete ..."

A good example is perhaps a text editor or word processor. It may work like this:

  • You fire up the program to edit a document. Let's call it "example.doc".

  • You make many edits.

  • You click Save, or you close the program which automatically saves the document.

  • Before saving, the editing program renames the original unmodified document file from "example.doc" to "example.bak".

  • The program then writes out your edited document to "example.doc".

What you're left with are two documents: example.doc - your modified document, and example.bak - the document as it was before you edited it.

In other words, you have your new document and a backup of the old.

Now, I use a mythical "text editor or word processor" above because not all programs do what I just described, and not all that do use the ".BAK" extension when they do.

Some programs actually have options on whether to provide this extra layer of safety at all. It's up to you.

Are they safe to delete?

Well, to be clear, if they're in your recycle bin, someone already did say "delete this file" to Windows. Most likely, it was you, but it's possible that other programs may have sent them to the recycle bin as well.

The file is still there simply because you haven't emptied the recycle bin yet.

In general, .BAK files are safe to delete, particularly if you've already said "delete this file" and placed it in the recycle bin.

Naturally, there could be exceptions - but as long as things are working properly and the master copies are still around, there's typically no reason not to delete the ".BAK" backup files.

A bit of history

Imagine that your disk was really, really, REALLY slow. And small. Very, very VERY small.

Thirty years ago or so, that's what they were, compared to what most of us take for granted today. The first floppy disk that I worked with had a capacity of 128 kilobytes. To put that in perspective, that's enough to hold just a tad over 1 second of CD audio.

And it would take several seconds to read the entire disk.

One approach to editing programs was to have two drives and two disks - the text editor would read the original from one disk and write the new changed file to the other.

Upon completion, the editor would simply rename the file on the first disk to whatever.bak, so that you didn't have two disks that had the exact same filename.

The next time that you edited, the process was duplicated in the opposite direction, often automatically deleting any pre-existing .BAK file to make room.

It's not uncommon that programs that have been around for a long, long time still retain the ".BAK" mechanism as an artifact of their roots long ago.

Article C4878 - July 15, 2011 « »

Leo Leo A. Notenboom has been playing with computers since he was required to take a programming class in 1976. An 18 year career as a programmer at Microsoft soon followed. After "retiring" in 2001, Leo started Ask Leo! in 2003 as a place for answers to common computer and technical questions. More about Leo.

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1 Comment
July 26, 2011 5:42 PM

How do i read Bak files? After a computer "glitch" some files went Bak and I seem to have lost the originals.
Thank you ,

There is no standard formata for ".bak" files - it depends entirely on the program that created them in the first place. If you know that you can often rename the .bak back to the original file extension and have it work. But you need to know what the original extension was.

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