There has been quite a few questions about how to dress to an academic interview. However, no answer to these questions addresses what one should use to carry their things in. For example, is it acceptable to carry your things in a backpack, cheap shoulder/messenger bag made of synthetic materials, briefcase, expensive leather bag, purse etc? A backpack with a suit would look awfully strange. Maybe a cheap messenger bag would still look OK. Links to photos of acceptable/preferred bags would be nice. Is the etiquette different for post-docs vs. faculty positions?
I have seen them in all in faculty job talks in our department for the last few years. I have seen messenger bags, backpacks and briefcases; some of them have been made of expensive leather and looked really classy/professional and I have even seen them made of cheap, fraying materials.
Either way, it does not matter - except maybe in incredibly conservative departments. We hired one of the folks who had a pretty cheap looking backpack. The fact that they had multiple publications in the top journals and conferences in their area probably mattered more. They were all wearing suits though.
I think, at least in the UK (I couldn't be sure about other countries), this probably matters a lot less in academia than it would for a corporate job. The most important thing is probably that you feel comfortable with your choice. It's extremely unlikely that you would fail to get the job simply because of your choice of bag, but if you are feeling uncomfortable all day about, say, wearing a backpack with a suit, then that could end up having a negative effect on your interview performance.
When I interviewed for my current job, I carried my larger items in my backpack, and I did feel a bit silly wearing a backpack with a suit, but fortunately I was offered to leave my backpack in the secretaries' office for the day, and I had all the things I really needed to keep with me in a small leather handbag. I think I would have felt uncomfortable carrying the backpack around with me all day (even though it was a good-quality and quite new one), especially as the other candidates were quite smartly dressed and didn't have backpacks.
Bags, shoes, hats, it's all the same answer as for clothes. Post-docs vs faculty positions, it's the same answer. If you've any specific instructions from the panel who will interview you, follow them. Otherwise, follow local customs for smart professional attire. Err on the side of looking smarter and more professional.
If you wear a backpack on both shoulders with a suit jacket, it will make the suit bunch up in front and the sleeves look too short. Also, you will probably pop the button(s) if you wear the jacket closed. If you don't need to bring anything heavy to the interview and strongly prefer your backpack, then wear it on one shoulder only.
As someone else said, choose to bring what you are most comfortable with. By comfortable, I am referring to the ability to quickly find whatever you might need during the interview. It is awful to be dredging through a bag or purse, trying to find something while one or more potential employers are waiting! I have done that, and it was flustering, distracting. The same is true about latches on a briefcase or satchel, if you choose that.
An academic interview is so important. Even if you don't wear a suit, or don't have one, make sure that your shoes fit well, hair is combed, and your socks match. No one is likely to notice or care, but doing so costs you nothing and will minimize self-consciousness, especially if you need to stand to write something on a whiteboard.
Borrow a professional looking bag or briefcase. I borrowed a nice black (synthetic material) shoulder bag for my interviews that looked better than my Chrome bag I use for commuting on my bike. Even as they say it should not matter, other answers suggests that faculty remember how professional or non-professional the bag a interviewee brought was for long periods of time afterward.
In a job interview, you are trying to communicate that (a) you are professional and competent, (b) that you are taking the interview process seriously, and (c) that you the kind of person that faculty can imagine as their colleague (as opposed to their student). Looking professional and professorial is one way you can do all three things.
Keep in mind that the way you appear is a sort of a package deal. If you have an less than professional backpack that looks like something a student would bring, it's probably not going to tip the scale either way, but it's not going to help and it probably will be noticed. If you are a student or a post-doc, people understand that you don't have the cash for a fancy bag. That said, if there's any way you can borrow a nicer looking bag from somebody for your interviews, the downside is low enough that I don't see why you wouldn't.