Tried printf(1)?
printf(1)
printf "%x\n" 3422
There are probably ways of doing that with builtin functions in all shells but it would be less portable. I've not checked the POSIX sh specs to see whether it has such capabilities.
echo "obase=16; 34" | bc
If you want to filter a whole file of integers, one per line:
( echo "obase=16" ; cat file_of_integers ) | bc
Hexidecimal to decimal:
$ echo $((0xfee10000))4276158464
Decimal to hexadecimal:
$ printf '%x\n' 261a