How can I find out how many lines a text file contains without viewing it?

Indeed there is. It is called wc, originally for word count, I believe, but it can do lines, words, characters, bytes (and with some implementations, the length in bytes of the longest line or the display width of the widest one). The -l option tells it to count lines (in effect, it counts the newline characters, so only properly delimited lines):

wc -l mytextfile

Or to only output the number of lines:

wc -l < mytextfile

(beware that some implementations insert blanks before that number).

Another option would be to use grep to find the number of times a pattern is matched:

grep --regexp="$" --count /path/to/myfile.txt

In this example,

• $ is an expression that evaluates to a new line (enter button is pressed)
• --count suppresses normal output of matches, and displays the number of times it was matched.
• The /path/to/myfile.txt is pretty obvious, I hope :)

EDIT: As mentioned by @hesse in the comments, this can be shortened to

grep -c $ path/to/file

Which would also make it standard and portable to non-GNU grep implementations.

I would also add that it is quite easy to do this in pure awk if you, for some reason, wished to not use wc.

$ awk 'END { print NR }' /path/to/file

The above prints the number of records (NR) present in the file at /path/to/file.

Note: unlike wc, this will not print the name of the file. So, if you only wanted the number, this would be a good alternative to cat file | wc -l.