Use the ieee.numeric_std library, and the appropriate vector type for the numbers you are working on (unsigned or signed).
Then the operators are `sla`/`sra` for arithmetic shifts (ie fill with sign bit on right shifts and lsb on left shifts) and `sll`/`srl` for logical shifts (ie fill with ‘0’s).
You pass a parameter to the operator to define the number of bits to shift:
A <= B srl 2; -- logical shift right 2 bits
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Update:
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I have no idea what I was writing above (thanks to Val for pointing that out!)
Of course the correct way to shift signed and unsigned types is with the shift_left and shift_right functions defined in ieee.numeric_std.
The shift and rotate operators sll, ror etc are for vectors of boolean, bit or std_ulogic, and can have interestingly unexpected behaviour in that the arithmetic shifts duplicate the end-bit even when shifting left.
And much more history can be found here:
http://jdebp.eu./FGA/bit-shifts-in-vhdl.html
However, the answer to the original question is still
sig <= tmp sll number_of_bits;