It’s not uncommon to want to find a particular line in a file and print just a part of it.  For example, if we wanted to find the total memory on a machine we could use:

grep '^MemTotal:' /proc/meminfo | 
  sed -e 's/^MemTotal:\s*\([0-9]*\).*/\1/'

Which does the job but duplicates a bunch of the matching regex. If you’re using a reasonably recent gnu grep, you can use it’s support for perl regex syntax and the \K operator:

I wanted to create a CSV file showing the number of JUnit tests in our codebase vs the number of Spock tests over time. I can count the number of tests, along with the revision pretty easily with:

git svn find-rev HEAD
find src/test -name '*Test.java' | wc -l
find src/test -name '*Spec.groovy' | wc -l

But that outputs the results on three separate lines and I really want them on one line with some separator (comma, space or tab). The simplest way to achieve that is: