Getting only used memory from free command every few seconds

It was explained e.g. here: https://unix.stackexchange.com/questions/68526/get-separate-used-memory-info-from-free-m-command how to cut the output of free command. But I want to do this every few seconds and log it to a file. So I tried: free -g -s2 | sed -n 's/^Mem:\s\+[0-9]\+\s\+\([0-9]\+\)\s.\+/\1/p' >> memory2.log but the file stays empty. Why is that and how to fix it?