Linux – grep command

grep is a command-line utility used for searching and filtering text files based on specified patterns or regular expressions. It displays the matching lines or filenames containing the pattern

Command	Description	Example	Output
grep pattern file	Search for a pattern in a single file.	grep "hello" file.txt	This is a sample file containing hello world. Another line with the word hello.
grep pattern file1 file2	Search for a pattern in multiple files.	grep "error" file1.txt file2.txt	file1.txt: An error occurred during the process. file2.txt: Error: Connection timeout.
grep -r pattern directory	Recursively search for a pattern in a directory and its subdirectories.	grep -r "TODO" ./src	./src/main.js: // TODO: Implement the feature. ./src/components/todo.jsx: <h1>TODO List</h1>
grep -i pattern file	Perform a case-insensitive search.	grep -i "apple" fruits.txt	There’s an Apple tree in the garden. I like apples and oranges.
grep -n pattern file	Display line numbers along with matching lines.	grep -n "function" script.js	1: function greet() { 4: const sayHello = function() {
grep -v pattern file	Invert the search and display non-matching lines.	grep -v "success" logs.txt	Error: Failed to load resource. Warning: Disk space low. Info: Application started.
grep -l pattern file1 file2	List filenames containing the pattern.	grep -l “error” *.log	logfile.txt
grep -w pattern file	Match whole words only.	grep -w “cat” animals.txt	The cat chased a mouse.
`grep -E “pattern1	pattern2″ file`	grep -E “apple|orange” fruits.txt	There’s an Apple tree in the garden. I like apples and oranges.
grep -A 2 pattern file	Display 2 lines after each match.	grep -A 2 “ERROR” logfile.txt	Line 72: ERROR – Null pointer exception. Line 73: Method call that caused the error. Line 74: Additional information about the error.
grep -B 1 pattern file	Display 1 line before each match.	grep -B 1 “warning” log.txt	Line 28: WARNING – This action is irreversible. Line 29: Proceed with caution.
grep -C 2 pattern file	Display 2 lines before and after each match.	grep -C 2 “important” notes.txt	Line 14: This is an important message. Line 15: Please take note of this information. Line 16: Also, remember the following details.
grep -o pattern file	Show only the matched part of the line.	grep -o “user[0-9]+” data.txt	user1234 user9876 user2468
grep -q pattern file	Quiet mode: suppress output, set exit status.	grep -q "pattern" file && echo "Found"	Found

file.txt

This is a sample file containing hello world.
Another line with the word hello.
Some random text.

file1.txt

This is file1.
An error occurred during the process.
Some other text here.

file2.txt

This is file2.
Error: Connection timeout.
Additional lines in file2.

fruits.txt

There's an Apple tree in the garden.
I like apples and oranges.
I prefer bananas.

script.js

function greet() {
  console.log("Hello!");
}

const sayHello = function() {
  console.log("Saying hello!");
};

// Some comments here.

log.txt

Error: Failed to load resource.
Warning: Disk space low.
Info: Application started.

notes.txt

Line 14: This is an important message.
Line 15: Please take note of this information.
Line 16: Also, remember the following details.
Line 17: Another note.

data.txt

user1234 data1
user9876 data2
user2468 data3