Command output is given to the console. As in the following case -

wc -l filename
produces following output
nn filename

From this, I am only interested with nn(rowcount). Cant we simply extract it thr console?

I also tried with null device, I know as - /dev/null

wc -l filename > /dev/null | cut -d " " -f 1 /dev/null

This also didnt help me.

Will I have to use temporary file, in order to retrieve only rowcount?

The easiest way is to pipe it through awk
You can omit the filename by supplying the file as STDIN, e.g.
but you'll still get leading whitespace. So the awk solution is easiest.

EDIT: Looking at this, it'd be less expensive to just do the whole thing with awk...
Cheers
ZB