How to use GREP command in Shell

Introduction

Sometimes you remember a phrase or a given word or words, you put in a document, but you do not remember the name of the document. How to find it?, well do a search of all your documents, looking for that word or words or sentence, in other words, look for a string or strings.

Now, thanks to computers this is easier now, than it was in our parents' days.

If you are using Linux, you have grep to help on this job.

Look for a document, containing a given string

First the easy case, you know the exact sentence, you are looking for, and you at least remember the folder where the file is.

grep "sentence to look for" /home/user/docs/

Now, let's suppose you do not know if the sentence was in uppercase or in lowercase, so ask grep to ignore case.

grep -i "sentence to look for" /home/user/docs/

Well, let's suppose you have a lot of sub-folders, and you do not remember where your file is.

grep -r "sentence to look for" /home/user/docs/

Let's see an example of the output in my PC

grep -ri "Introduction" /home/user/post/

The output is:

/home/user/post/monthy-newsletter.txt:###Introduction###
/home/user/post/monthy-newsletter.txt:I'm no expert in MySQL, but anyway I have written three introduction-type MySQL posts, something we all need to know to start [how to create databases][1], [How to list those databases][2] and [How to create tables in MySQL][3]
/home/user/post/how-to-debug-bash-shell-scripts.txt:##Introduction##
/home/user/post/how-to-setup-dns-bind-master-slave-linux.txt:##Introduction
/home/user/post/interview-raphael-hertzog.txt:I also have plans for bigger changes concerning Debian, and among them is the introduction of Debian Rolling, a distribution similar to testing but with some design choices to make it more usable at any point in time.
/home/user/post/four-years-with-debian-testing.txt:##Introduction##
Binary file /home/user/post/.find-a-documents-with-given-string.txt.swp matches
/home/user/post/how-to-change-the-priority-of-Linux-processes.txt:##Introduction##

As you can see there are some binary files, also scanned, if we want to avoid that:

grep -riI "introduction" /home/user/post/

The output now is:

/home/user/post/monthy-newsletter.txt:###Introduction###
/home/user/post/monthy-newsletter.txt:I'm no expert in MySQL, but anyway I have written three introduction-type MySQL posts, something we all need to know to start [how to create databases][1], [How to list those databases][2] and [How to create tables in MySQL][3]
/home/user/post/how-to-debug-bash-shell-scripts.txt:##Introduction##
/home/user/post/how-to-setup-dns-bind-master-slave-linux.txt:##Introduction
/home/user/post/interview-raphael-hertzog.txt:I also have plans for bigger changes concerning Debian, and among them is the introduction of Debian Rolling, a distribution similar to testing but with some design choices to make it more usable at any point in time.
/home/user/post/four-years-with-debian-testing.txt:##Introduction##
/home/user/post/how-to-change-the-priority-of-Linux-processes.txt:##Introduction##

Finally, I just want the file names, and not the sentences where the sentence or word appears.

grep -riIl "introduction" /home/user/post/

The output will be:

/home/user/post/monthy-newsletter.txt
/home/user/post/how-to-debug-bash-shell-scripts.txt
/home/user/post/how-to-setup-dns-bind-master-slave-linux.txt
/home/user/post/interview-raphael-hertzog.txt
/home/user/post/four-years-with-debian-testing.txt
/home/user/post/how-to-change-the-priority-of-Linux-processes.txt

Conclusion

I know, you may use other ways, maybe find, but I like this method, and use it a lot, just like the advanced search function in Gmail :