Use grep to report back only line numbers

try:

grep -n "text to find" file.ext | cut -f1 -d:

If you're open to using AWK:

awk '/textstring/ {print FNR}' textfile

In this case, FNR is the line number. AWK is a great tool when you're looking at grep|cut, or any time you're looking to take grep output and manipulate it.