grep.md

Ubuntu Grep Best Practices Cheatsheet

Basic Syntax

grep [options] pattern [file...]

Essential Options

Option	Description	Example
-i	Case insensitive search	grep -i "error" log.txt
-r or -R	Recursive search	grep -r "TODO" /path/to/dir
-n	Show line numbers	grep -n "pattern" file.txt
-w	Match whole words only	grep -w "class" *.java
-l	Show only filenames	grep -l "pattern" *.txt
-c	Count matches	grep -c "error" log.txt
-v	Invert match (show non-matching)	grep -v "^#" config.txt
-E	Extended regex support	`grep -E "word1

Best Practices

1. Using Context Options

# Show 3 lines before match
grep -B 3 "error" log.txt

# Show 3 lines after match
grep -A 3 "error" log.txt

# Show 3 lines before and after
grep -C 3 "error" log.txt

2. Color Output

# Enable color output
grep --color=auto "pattern" file.txt

# Add to ~/.bashrc for permanent color:
export GREP_OPTIONS='--color=auto'

3. Excluding Directories

# Exclude specific directories
grep -r "pattern" --exclude-dir={.git,node_modules,vendor} .

# Exclude by pattern
grep -r "pattern" --exclude-dir=".*" .

4. File Type Filtering

# Search only specific file types
grep -r "pattern" --include="*.php" .

# Multiple file types
grep -r "pattern" --include="*.{js,jsx,ts,tsx}" .

5. Advanced Pattern Matching

Regular Expressions

# Match start of line
grep "^start" file.txt

# Match end of line
grep "end$" file.txt

# Match any single character
grep "h.t" file.txt

# Match zero or more occurrences
grep "wo*rd" file.txt

# Match specific number of occurrences
grep "o\{2\}" file.txt

Extended Regular Expressions (-E or egrep)

# Match either pattern
grep -E "pattern1|pattern2" file.txt

# Match one or more occurrences
grep -E "o+" file.txt

# Match zero or one occurrence
grep -E "colou?r" file.txt

6. Performance Optimization

For Large Files

# Stop after first match
grep -m 1 "pattern" large_file.txt

# Use binary grep for faster search
grep -a "pattern" binary_file

# Count matches without printing
grep -c "pattern" large_file.txt

For Multiple Files

# Process files in parallel
parallel "grep 'pattern' {}" ::: *.txt

# Use find with grep
find . -type f -exec grep "pattern" {} +

7. Error Handling

# Suppress error messages
grep -s "pattern" /path/to/files/*

# Show only error messages
grep -q "pattern" file.txt 2>&1 >/dev/null

8. Common Use Cases

Finding IP Addresses

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

Finding Email Addresses

grep -E "[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\.[A-Za-z]{2,}" file.txt

Finding URLs

grep -E "https?://[A-Za-z0-9./]+" file.txt

Tips for Daily Use

1. Use ripgrep (rg) for Better Performance

   • Install: sudo apt install ripgrep
   • Usage: rg "pattern"

2. Create Aliases for Common Operations

   # Add to ~/.bashrc
   alias grepjs='grep -r --include="*.js"'
   alias grepphp='grep -r --include="*.php"'
   alias greperr='grep -i -r "error|warning|critical"'

3. Use Process Substitution

   # Search in command output
   grep "pattern" <(command)

4. Combine with Other Commands

   # Find and grep
   find . -type f -name "*.log" -exec grep "error" {} +
   
   # Pipe with grep
   ps aux | grep "[n]ginx"

Common Pitfalls to Avoid

1. Don't use grep without -r for directory searches
2. Avoid using GREP_OPTIONS (deprecated)
3. Remember to escape special characters in patterns
4. Don't forget to use quotes around patterns with spaces
5. Be careful with -R vs -r (symbolic links)

Debugging Tips

1. Use -v to see what's being excluded
2. Use --debug to see detailed matching information
3. Use -h to suppress filenames in multi-file searches
4. Add -n to show line numbers for easier reference