I never found the need to study those common unix utilities like sed, awk, xargs. Even my knowledge of grep didn't go further than blah | grep foo. Until yesterday, when I needed to accomplish a task that just happened to involve all of these utilities:
First, grep filters all lines from /proc/net/dev that contain ppp.
Then, sed changes the colon into a space using the regex s/:/ /.
Next, awk parses the values of the 2nd and 10th column [bytes sent and received] and if the sum exceeds 1GB it prints the value in the first column [the interface].
Finally, xargs invokes ppp-off for every interface returned.
Why I needed this is left as an exercise to the reader.