Grep

Text search and pattern matching utility for searching plain-text data sets for lines that match a regular expression.

tools linux cli text search

Overview

Grep is a command-line utility for searching plain-text data sets for lines that match a regular expression. It’s one of the most powerful and frequently used tools in Unix/Linux systems.

Basic Usage

grep "pattern" filename

Example:

# Search for the word "error" in log file
grep "error" /var/log/system.log

Common Options

Case & Matching

-i - Ignore case
-v - Invert match (exclude)
-w - Match whole words
-x - Match whole lines
-F - Fixed strings (no regex)
-E - Extended regex

Output Control

-n - Show line numbers
-c - Count matches
-l - List matching files
-L - List non-matching files
-o - Show only matches
-q - Quiet mode

Context Options

# Show 3 lines after match
grep -A 3 "pattern" file

# Show 3 lines before match
grep -B 3 "pattern" file

# Show 3 lines around match
grep -C 3 "pattern" file

Regular Expressions

Basic Patterns

^ - Start of line
$ - End of line
. - Any character
* - Zero or more
+ - One or more (with -E)
? - Zero or one (with -E)

Character Classes

[abc] - Any of a, b, c
[^abc] - Not a, b, c
[a-z] - Any lowercase
[0-9] - Any digit
\d - Digit (with -P)
\w - Word character (with -P)

Practical Examples

Search for IP addresses:

grep -E "[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}" file.txt

Search for email addresses:

grep -E "[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}" file.txt

Search in multiple files:

grep -r "TODO" /path/to/project

Search with file patterns:

grep -r --include="*.js" "console.log" .

Count occurrences:

grep -c "error" /var/log/system.log

Show only matching part:

grep -o "[0-9]\{1,3\}\.[0-9]\{1,3\}\.[0-9]\{1,3\}\.[0-9]\{1,3\}" access.log

File Operations

Recursive Search

grep -r "pattern" /path
grep -R "pattern" /path

Include/Exclude Files

grep -r --include="*.txt" "pattern" .
grep -r --exclude="*.log" "pattern" .

Useful Combinations

Search for functions in code:

grep -n "function.*(" *.js

Find files containing pattern:

grep -l "pattern" *.txt

Search case-insensitive with line numbers:

grep -in "error" /var/log/system.log

Exclude binary files:

grep -I "pattern" *

Search for multiple patterns:

grep -E "error|warning|critical" /var/log/system.log

Exit Codes

0 - Match found
1 - No match found
2 - Error occurred

Tips and Tricks

Use grep --help for complete option list
Combine with other tools: ps aux | grep process_name
Use aliases for common patterns: alias finderr='grep -r "error" .'
For complex patterns, consider using egrep (equivalent to grep -E)