grep

`grep`#

reading

We will loosely follow the Software Carpentry Finding Things lesson from The Unix Shell

The final handy Unix command we’ll look at (for now) is grep. grep searches for patterns in files (or input that is piped to it).

Let’s start in ~/shell-lesson-data/exercise-data/writing/

Counting chapters#

The file LittleWomen.txt has the entire novel Little Women (along with some copyright text indicating it is public domain).

Let’s figure out how many chapters there are in Little Women. We imagine that the word Chapter should appear. So let’s do:

grep Chapter LittleWomen.txt

This returns nothing.

If we do:

man grep

we see that if we use the -i flag, then it will do the search as case-insensitive. So maybe that’s the problem. Let’s try again, now as:

grep -i Chapter LittleWomen.txt

Now we see a lot of output. Some of it looks like:

CHAPTER ONE
CHAPTER TWO
CHAPTER THREE
CHAPTER FOUR

These are clearly what we were looking for. But other instance look like:

the chapter, and don't be impertinent, miss'."

This is a case where the characters in the book were talking about chapters. That’s not what we want.

In this case, we can recognize that the actual book chapters are all upper case, so we can do:

grep CHAPTER LittleWomen.txt

And we get:

CHAPTER ONE
CHAPTER TWO
CHAPTER THREE
CHAPTER FOUR
CHAPTER FIVE
CHAPTER SIX
CHAPTER SEVEN
CHAPTER EIGHT
CHAPTER NINE
CHAPTER TEN
CHAPTER ELEVEN
CHAPTER TWELVE
CHAPTER THIRTEEN
CHAPTER FOURTEEN
CHAPTER FIFTEEN
CHAPTER SIXTEEN
CHAPTER SEVENTEEN
CHAPTER EIGHTEEN
CHAPTER NINETEEN
CHAPTER TWENTY
CHAPTER TWENTY-ONE
CHAPTER TWENTY-TWO
CHAPTER TWENTY-THREE
CHAPTER TWENTY-FOUR
CHAPTER TWENTY-FIVE
CHAPTER TWENTY-SIX
CHAPTER TWENTY-SEVEN
CHAPTER TWENTY-EIGHT
CHAPTER TWENTY-NINE
CHAPTER THIRTY
CHAPTER THIRTY-ONE
CHAPTER THIRTY-TWO
CHAPTER THIRTY-THREE
CHAPTER THIRTY-FOUR
CHAPTER THIRTY-FIVE
CHAPTER THIRTY-SIX
CHAPTER THIRTY-SEVEN
CHAPTER THIRTY-EIGHT
CHAPTER THIRTY-NINE
CHAPTER FORTY
CHAPTER FORTY-ONE
CHAPTER FORTY-TWO
CHAPTER FORTY-THREE
CHAPTER FORTY-FOUR
CHAPTER FORTY-FIVE
CHAPTER FORTY-SIX
CHAPTER FORTY-SEVEN

and we can just read off from this that there are 47 chapters.

Of course, we could also do:

grep CHAPTER LittleWomen.txt  | wc -l

What if we weren’t so lucky and the word “chapter” wasn’t distinguishable from its use in the text?

In this case, we could recognize that the “chapter” we are interested in always appears at the very start of the line. We can use the ^ character in our search to indicate that we want to start at the beginning of the line, e.g,:

grep -i "^Chapter" LittleWomen.txt

There are many other patterns that we’ll see can be useful, and we’ll introduce those as we need them later in the course.

Counting character appearances#

One of the main characters is “Jo”. If we do:

grep Jo LittleWomen.txt

we see all of her appearances, but we also see a character “John”.

In this case, we can use the -w option to match on words. If we do:

grep -w Jo LittleWomen.txt

then it no longer matches “John”.

Remembering a command from history#

Previously we compiled our hello.cpp code using g++. If we want to find the command we used, we can look at our history, using the history command as we previously saw.

But that can be a lot to scroll through.

Instead, we can pipe the output from history into grep, like:

history | grep g++