I just added a stem extender and trekker bars to an old hardtail mountain bike with twist grip shifters, and I need to replace 3 of the cables and housings because they're not long enough. (For some reason, the existing rear brake cable and housing seem to be ok as is... There's not a lot of slack in the housing, but it seems to work.)

I bought a couple of these brake cables and housings: https://www.amazon.com/gp/product/B000C128MK, and was wondering if I could use one of the cables (not the housing) for my rear shifter. I have a set of brand new (Campagnolo) shifter cable housings that are good quality and can be used, but am short one long cable to use for the rear shifter.

Is there a difference between brake cables and shifter cables? Do I need to buy a shifter cable? (Normally, I would just go ahead and order one, but I'm anxious to get this bike back on the road...)

Thanks in advance...

You need shifted cable housing for the shifter cables and brake cable housings for the brake cables. They are not the same size.
Be sure to replace the short housing found at the rear dérailleur. It's the first to wear out and the most critical for good shifting performance.

There is a difference in both shift and brake cables as well as the cable housings so be sure to get the appropriate ones. Brake cables are larger diameter. If you buy sets you will likely have to cut the cables and housings to length. That is best done with a proper cutter but if you are careful you could do it without one.

You can always move the existing rear cables to the front and just buy new cables for the rear. As the previous posters stated shifter cable/housing is not interchangible with brake cable/housing.

Tony: Do yourself a favor and buy yourself a high-quality set of shift cables and then install them properly. This includes preparing the housing ends correctly by cutting them cleanly, filing or grinding them square, opening the ends and using the correct ferrules. Size the loop at the rear derailleur properly. Consult the Park Tool or Sheldon Brown Web sites if you need guidance. Doing it right rather than half-a**ed and in a hurry will save you a lot of grief and head-scratching down the road. Correct installation of brake cables is even more important from a safety standpoint.

Thanks for the replies. I was afraid that there was some difference in the cables... sigh. I guess I'll get busy and order some shifter cables and housings. And maybe the cable cutter while I'm at it. Does the cable cutter also work for the housings?

For the actual cables themselves (both derailleur cable and brake cable), you can use a diagonal cutter/plier. When cutting, try to apply pressure to cut through the cable fast - this will result in a cleaner cut and no fraying. If it frays a bit, don't worry as that's what crimping cable ends on are for.

Use the special cable cutter (Park Tool, et al) for shifter housing. If you by chance have a dremel tool already at your disposal, buy a reinforced cut-off wheel - this worked well for me. If using the dremel, try to cut through it fast as well as the heat may melt the plastic covering a bit.

You can use the diagonal cutter/plier for the brake housing.

For either of the housings, after you cut, make sure it's a good perpendicular cut. If not, file/sand it down. Then use a poke tool or awl to open up the hole all the way, as cutting may have squished the hole up.