How to redirect output from "dbgs" to a special filename?

Question

I have written an llvm Pass to assign every basic block an stable ID, and output their name and parent name for next analysis. I use dbgs() to implement that. When I use -opt to run this pass, my command line as follow:

$ opt -load ../../build/Release+Debug+Asserts/lib/libdgutility.so \
-mergereturn -bbnum -dump-bbid -remove-bbnum -stats -debug \
unit1300.bc -o /dev/null 

it can output information to terminal, here is a part of my output stream:

FuncName : setcharset   16041 : if.end189
FuncName : setcharset   16042 : if.end190
FuncName : setcharset   16043 : if.end191
FuncName : setcharset   16044 : if.then196
FuncName : setcharset   16045 : land.lhs.true
FuncName : setcharset   16046 : lor.lhs.false209
.
.
.
FuncName : setcharset   16053 : while.end
FuncName : setcharset   16054 : if.else248

but I want to store this information in a file named *.log. I have tried redirection, but it does not work.