Bolts usually works. You could try glue as well, but it may not hold for long.
The reason for the sarcasm is that your question is very minimalist and vague. Please provide an accurate description of your WHOLE problem. This is not Twitter, you are not limited to 124 characters.
Can not tell a whole lot from your 6 word question but going on the assumption that you are looking for hardware to mount an instrument, one option is B-Line Uni-strut.
You can buy uni-strut painted, galvanized, aluminum, or stainless steel.
Uni-strut also manufactures a number of accessories for their strut.
I have used this product in many environments and it has worked well.
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