Joseph Herlant
version 1.0.0, 2013-10-19 : Initial version

This shell command returns the number of lines contained by a file.

perl -lne '}{print$.' file_input.txt

The above command is the exact same one but in a more human readable way:

perl -lne 'END{print$.}' file_input.txt

The $. variable contains the line number of the line currently been read. And as the END code block is executed after perl finished running the program and just before the interpreter is being exited, the $. variable contains the number of the last line of the file ⇒ the number of lines in the file. Easy! :)