Mastering the grep Command in Linux

Introduction

The grep (Global Regular Expression Print) command is one of the most powerful text processing tools in Linux. It allows you to search through files or input streams using regular expressions, making it indispensable for programmers, system administrators, and anyone working with text data.

Why Learn grep?

• Quickly find information in files
• Filter log files efficiently
• Combine with other commands using pipes
• Search recursively through directories
• Powerful pattern matching with regular expressions

Basic Syntax

The basic syntax of grep is:

grep [options] pattern [file...]

Common Options Cheat Sheet

Practical Examples

Sample Files Setup

First, let’s create some sample files to work with.

echo "Linux is awesome.
Welcome to Ubuntu.
linux is case-sensitive.
ERROR: File not found.
Another line with error." > sample.txt

mkdir -p test_dir
echo "Grep examples in subdirectory
Linux command examples" > test_dir/subfile.txt

Example 1: Basic Search

grep "Linux" sample.txt

Linux is awesome.

Example 2: Case-insensitive Search (-i)

grep -i "linux" sample.txt

Example 3: Inverse Match (-v)

grep -v "error" sample.txt

Example 4: Show Line Numbers (-n)

grep -n "Ubuntu" sample.txt

2:Welcome to Ubuntu.

Example 5: Count Matches (-c)

grep -c "linux" sample.txt

1

Example 6: Recursive Search (-r)

grep -r "examples" .

Example 7: Whole-word Match (-w)

grep -w "Linux" sample.txt

Linux is awesome.

Example 8: Combine Options (-in)

grep -in "error" sample.txt

Regular Expressions with grep

1. Anchoring

grep '^Linux' sample.txt  # Lines starting with Linux

Linux is awesome.

2. Character Classes

grep '[A-Z]' sample.txt  # Lines containing uppercase letters

3. Quantifiers

grep -E 'e{2}' sample.txt  # Lines with double 'e'

4. Alternation

grep -E 'Linux|Ubuntu' sample.txt

Pro Tips

1. Combine with other commands:

      ps aux | grep 'nginx'

   karna     202119  0.0  0.0   6408  3984 ?        SN   15:55   0:00 grep nginx

2. Search multiple patterns:

      grep -e 'error' -e 'warning' log.txt

3. Use context control:

      grep -A2 -B1 'critical' server.log