How many lines of source code are there in the C netcat.c source file I downloaded? (Here, we use a small part of the popular open source netcat utility code base.) We use the wc utility. Before we go further, what's wc? word count (wc) is a filter: it reads input from stdin, counts the number of lines, words, and characters in the input stream, and writes this result to its stdout. Further, as a convenience, one can pass filenames as parameters to it; passing the -l option switch has wc only print the number of lines:

$ wc -l src/netcat.c
618 src/netcat.c
$ 

Here, the input is a filename passed as a parameter to wc.

Interestingly, we should by now realize that if we do not pass it any parameters, wc would read its input from stdin, which by default is the keyboard device. For example is shown as follows:

$ wc -l
hey, a small
quick test
 of reading from stdin
by wc!
^D
4
$ 

Yes, we typed in 4 lines to stdin; thus the result is 4, written to stdout – the terminal device by default.

Here is the beauty of it:

$ wc -l < src/netcat.c > num
$ cat num
618
$

As we can see, wc is a great example of a Unix filter.