It's an easy (but not clear, please refer to the Note of wc below) question, but sill quite funny.
Tested under the Bash shell on Ubuntu 12.04 32-bit.
(1) wc
cat sample.txt | wc -l
or
wc sample.txt | awk '{print $1}'
awk is used to extract the 1st field.
Note: wc only counts the newline characters! The last line without newline is not counted. For example:
echo -ne "ab\ncd" | wc -l
would generate 1 (line), not 2.
