How to only get the number of lines of a file

Try this

wc -l < sample.txt

wc doesn't print out the filename if it reads the file through standard input. The < feeds the file via standard input.

Other single commands to get the number of lines in a file without filename.

sed:

$ sed -n '$=' filename

awk:

$ awk 'END{print NR}' filename

If you want to strip the whitespace out too, use sed.

wc -l < file | sed 's/ //g'