grep Command: Practical Examples & Usage Guide

# Basic syntax
grep "pattern" filename

# Search in multiple files
grep "pattern" file1.txt file2.txt

# Search from stdin (pipe)
cat file.txt | grep "pattern"
echo "Hello World" | grep "World"

# Case-insensitive search
grep -i "error" application.log

# Recursive search with line numbers
grep -rn "TODO" ./src/

# Match whole words only (won't match "errors" or "error_code")
grep -w "error" logfile.txt

# Count matches in each file
grep -c "import" *.js

# Show 3 lines before and 2 lines after each match
grep -B 3 -A 2 "Exception" error.log

# Show 5 lines of context around each match
grep -C 5 "CRITICAL" application.log

# Extract only the matching portions (useful with regex)
grep -oE "[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}" contacts.txt

# Quiet mode for scripts (check if pattern exists)
if grep -q "error" logfile.txt; then
    echo "Errors found!"
fi

# Search only in JavaScript files
grep -r --include="*.js" "useState" ./src/

# Search in multiple file types
grep -r --include="*.ts" --include="*.tsx" "interface" ./src/

# Exclude node_modules and build directories
grep -r --exclude-dir=node_modules --exclude-dir=dist "TODO" .

# Skip minified and map files
grep -r --exclude="*.min.js" --exclude="*.map" "function" ./src/

# Lines starting with "import"
grep "^import" app.js

# Lines ending with semicolon
grep ";$" code.js

# Match IP addresses
grep -E "([0-9]{1,3}\.){3}[0-9]{1,3}" access.log

# Match email addresses
grep -E "[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}" contacts.txt

# Match URLs
grep -E "https?://[a-zA-Z0-9./?=_-]+" links.txt

# Match phone numbers (US format)
grep -E "\(?[0-9]{3}\)?[-. ]?[0-9]{3}[-. ]?[0-9]{4}" contacts.txt

# Find all TODO comments in JavaScript/TypeScript files
grep -rn --include="*.js" --include="*.ts" --include="*.tsx" "TODO" ./src/

# Find function definitions
grep -rn "function.*(" --include="*.js" ./src/

# Find React component definitions
grep -rn "const.*=.*:.*FC\|function.*Component" --include="*.tsx" ./src/

# Find all imports from a specific package
grep -rn "from 'react'" --include="*.tsx" ./src/

# Find unused exports (files that export but aren't imported anywhere)
for file in $(grep -rl "export " --include="*.ts" ./src/); do
    name=$(basename "$file" .ts)
    if ! grep -rq "from '.*$name'" --include="*.ts" ./src/; then
        echo "Potentially unused: $file"
    fi
done

# Find console.log statements (to remove before production)
grep -rn "console\.log" --include="*.js" --include="*.ts" ./src/

# Find all errors in log files
grep -i "error\|exception\|fail" application.log

# Find errors with timestamp context
grep -B 1 -A 2 "ERROR" application.log

# Count errors per day
grep -o "^[0-9]\{4\}-[0-9]\{2\}-[0-9]\{2\}.*ERROR" app.log | cut -d' ' -f1 | sort | uniq -c

# Find slow API requests (>1000ms)
grep -E "duration=[0-9]{4,}ms" access.log

# Extract unique IP addresses from access logs
grep -oE "([0-9]{1,3}\.){3}[0-9]{1,3}" access.log | sort -u

# Find 5xx errors
grep -E "HTTP/[0-9.]+ 5[0-9]{2}" access.log

# Find requests by user agent
grep -i "curl\|wget\|python-requests" access.log

# Find all environment variable references
grep -rh "process\.env\." --include="*.js" --include="*.ts" ./src/ | sort -u

# Find hardcoded credentials (security audit)
grep -rniE "password\s*=\s*['"][^'"]+['"]" --include="*.js" --include="*.env" .

# Find API endpoints
grep -rn "api/\|/api/" --include="*.js" --include="*.ts" ./src/

# Find all URLs in config files
grep -hE "https?://[^"'\s]+" *.json *.yaml *.yml 2>/dev/null

# Find deprecated features
grep -rn "@deprecated" --include="*.ts" ./src/

# Search for multiple patterns (OR)
grep -E "error|warning|critical" log.txt

# Search for patterns that must all appear (AND - same line)
grep "error.*database\|database.*error" log.txt

# Search for pattern NOT followed by another pattern
grep "error" log.txt | grep -v "handled"

# Find lines with exactly 3 fields (tab-separated)
grep -E "^[^\t]+\t[^\t]+\t[^\t]+$" data.tsv

# Find duplicate consecutive words
grep -E "\b(\w+)\s+\1\b" document.txt

# Find files containing pattern A but not pattern B
grep -l "import React" --include="*.tsx" ./src/ | xargs grep -L "useState"

# Combine grep with find for complex searches
find . -name "*.js" -mtime -7 -exec grep -l "TODO" {} \;