How do I count text lines?

Try sed:

sed '/^$/d' file.txt | wc -l

If you have any lines containing only spaces or tabs and if you want to ignore them as well from count:

sed '/^[[:blank:]]*$/d' file.txt | wc -l

The above answer are correct but slightly different, you can use the grep to for easier code such grep -vc '^$' file.txt

For example(A):file.txt

$grep -vc '^$' file.txt

1    First line         #This is two tabs to comment.
2    
4
3    Fourth line        #Another two tabs to comment.

$2

For example(B):file.txt

$sed '/^$/d' file.txt | wc -l

1    First line         #This is two tabs to comment.
2    
4
3    Fourth line        #Another two tabs to comment.

$4

Notice the result is 4! when we want to expect only two. but this counts the tabs in between the content and comment as well.

Notice the counts from 0 and counts from 1 its different from the grep to the sed as I remember for more details search for grep or sed.

Using grep:

grep -vc '^$' file  # or
grep -vc '^\s*$' file