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:

grep ppp /proc/net/dev | sed "s/:/ /" | awk '($2+$10>1000000000){print $1}' | xargs --max-args=1 --no-run-if-empty ppp-off

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.