Holding plywood between bars for shelves?
If it doesn't need to hold much weight a cheap way would be to use a U bolt and U bracket.

Attach the U bolt to the vertical pipe, with the U bracket held behind it and screwed to the sheet.
A bit stronger, and probably not much more money, would be a shelf bracket held on by two hose clamps. This would allow several screws into the wood.

If you want to get fancier there are special clamps of all sorts for fastening things to metal tubes.  Search for example for temporary handrail hardware.
Here's an example of one that is very expensive.