Intro To 'grep' Command In Linux

2024-07-22 - By Robert Elder

I use the 'grep' command to search for things in text files. For example, if I run the 'grep' command on the document 'interject.txt':

I'd just like to interject for a moment.  What you're
referring to as Linux, is in fact, GNU/Linux, or as I've
recently taken to calling it, GNU plus Linux.  Linux is
not an operating system unto itself, but rather another
free component of a fully functioning GNU system made
useful by the GNU corelibs, shell utilities and vital
system components comprising a full OS as defined by POSIX.

like this:

grep system interject.txt

the output will show only the lines that contain the word 'system':

not an operating system unto itself, but rather another
free component of a fully functioning GNU system made
system components comprising a full OS as defined by POSIX.

Filtering Dating Matches

One day, I was feeling lonely, so I decided to check out my favourite terminal-based online dating web site:

 ____ _____ _______   __    _     _____     _______   ___  _   _ _     ___ _   _ _____
/ ___|_   _|_   _\ \ / /   | |   / _ \ \   / / ____| / _ \| \ | | |   |_ _| \ | | ____|
\___ \ | |   | |  \ V /____| |  | | | \ \ / /|  _|  | | | |  \| | |    | ||  \| |  _|
 ___) || |   | |   | |_____| |__| |_| |\ V / | |___ | |_| | |\  | |___ | || |\  | |___
|____/ |_|   |_|   |_|     |_____\___/  \_/  |_____(_)___/|_| \_|_____|___|_| \_|_____|

The world's most popular site for singles who conform to the 80 column line limit.

                        ***********************************

                                   - Match Search -

I'm a

 ___________ (Select One: Vim, Emacs, Nano, Editor-fluid) user

Seeking A

 ___________ (Select One: Vim, Emacs, Nano, Editor-fluid) user

[ Find Matches ]

I can query the site for dating matches using the 'curl' command:

curl -s http://stty-love.online

Lily:       I'm a cat gurl ;) Come and pet me, Meow!
Ava:        I enjoy activism that consists of destroying priceless artwork.
Victoria:   I have incredibly poor spending habits and no intentions of changing them.
Alice:      I only pursue people who ignore me and reject anyone who gives me attention.
Miley:      Doing my taxes and obeying the law are my favourite activites.
Sophia:     I only vote for a specific political party, no matter how unqualified they are.
Emma:       I project past relationship trauma on everyone around me.
Lucy:       I enjoying drinking, partying, and not taking responsibility for my actions.
Nora:       I am very mentally stable, and have normal social relationships.
Sally:      I like to spread dangerous misinformation for my own amusement.

This web site shows me a list of potential dating matches, but unfortunately, most of the matches are not very wholesome!

I can use the 'grep' command to solve this problem, by piping the list of matches directly into grep so that I can filter the matches based on desired keywords:

curl -s http://stty-love.online | grep obey

Miley:      Doing my taxes and obeying the law are my favourite activites.

Filter Using Regular Expressions

Using the '-P' flag, I can filter using regular expressions (although, this is not POSIX-compliant):

curl -s http://stty-love.online | grep -P 'obey|stable'

Miley:      Doing my taxes and obeying the law are my favourite activites.
Nora:       I am very mentally stable, and have normal social relationships.

Invert Matches

The '-v' flag can be used to only show lines that don't match instead:

curl -s http://stty-love.online | grep -P -v 'danger|poor|trauma|party'

Lily:       I'm a cat gurl ;) Come and pet me, Meow!
Ava:        I enjoy activism that consists of destroying priceless artwork.
Alice:      I only pursue people who ignore me and reject anyone who gives me attention.
Miley:      Doing my taxes and obeying the law are my favourite activites.
Nora:       I am very mentally stable, and have normal social relationships.

Show Line Numbers

The '-n' flag can be used to also show the line numbers in the results as well:

curl -s http://stty-love.online | grep -P -n 'stable|relationship'

1:Lily:       I'm a cat gurl ;) Come and pet me, Meow!
2:Ava:        I enjoy activism that consists of destroying priceless artwork.
4:Alice:      I only pursue people who ignore me and reject anyone who gives me attention.
5:Miley:      Doing my taxes and obeying the law are my favourite activites.
9:Nora:       I am very mentally stable, and have normal social relationships.

Searching Multiple Files & Folders

You can also use grep to recursively search through many files and folders at once using the '-R' flag. For example, I can use these commands to search for 'love' in the Linux kernel source code:

#  Frist, clone source code for Linux kernel to ~/linux
cd ~/linux/arch
grep -R -n -P '\blove\b' *

And that's why the 'grep' command is my favourite Linux command.

A Surprisingly Common Mistake Involving Wildcards & The Find Command Published 2020-01-21	$1.00 CAD Terminal Block Mining Simulation Game	A Guide to Recording 660FPS Video On A $6 Raspberry Pi Camera Published 2019-08-01	The Most Confusing Grep Mistakes I've Ever Made Published 2020-11-02
Use The 'tail' Command To Monitor Everything Published 2021-04-08	An Overview of How to Do Everything with Raspberry Pi Cameras Published 2019-05-28	An Introduction To Data Science On The Linux Command Line Published 2019-10-16	Using A Piece Of Paper As A Display Terminal - ed Vs. vim Published 2020-10-05

Why Bother Subscribing?

Free Software/Engineering Content. I publish all of my educational content publicly for free so everybody can make use of it. Why bother signing up for a paid 'course', when you can just sign up for this email list?
Read about cool new products that I'm building. How do I make money? Glad you asked! You'll get some emails with examples of things that I sell. You might even get some business ideas of your own :)
People actually like this email list. I know that sounds crazy, because who actually subscribes to email lists these days, right? Well, some do, and if you end up not liking it, I give you permission to unsubscribe and mark it as spam.