the_information_nexus/tech_docs/linux/grep.md

The Grep Mastery Guide: From Basic Search to Advanced Pattern Matching

Table of Contents

1. Grep Fundamentals
2. Pattern Matching
3. Context Control
4. File Operations
5. Output Control
6. Performance Optimization
7. Advanced Techniques
8. Real-World Recipes
9. Alternatives & Complements

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1. Grep Fundamentals

Core Syntax

grep [OPTIONS] PATTERN [FILE...]

Essential Modes

grep "text" file.txt       # Basic search
grep -E "regex" file.txt   # Extended regex (ERE)
grep -F "string" file.txt  # Fixed string (no regex)
grep -P "pattern" file.txt # Perl regex (GNU only)

Common Options

-i          # Case insensitive
-v          # Invert match
-w          # Whole words only
-n          # Show line numbers
-c          # Count matches
-l          # Show matching files only

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2. Pattern Matching

Regex Basics

grep "^start" file.txt     # Lines starting with
grep "end$" file.txt      # Lines ending with
grep "a.b" file.txt       # Any char between
grep "a.*b" file.txt      # Anything between

Character Classes

grep "[[:digit:]]" file.txt  # Any digit
grep "[A-Za-z]" file.txt     # Any letter
grep "[^0-9]" file.txt       # Non-digits

Quantifiers

grep -E "a{3}" file.txt    # Exactly 3 a's
grep -E "a{2,4}" file.txt  # 2 to 4 a's
grep -E "a+" file.txt      # One or more
grep -E "a*" file.txt      # Zero or more

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3. Context Control

Show Surrounding Lines

grep -A 2 "error" log.txt  # 2 lines After
grep -B 3 "warning" log.txt # 3 lines Before
grep -C 1 "critical" log.txt # 1 line Context

Match Grouping

grep -E "(error|warning)" file.txt  # OR condition
grep -E "(fatal).*\1" file.txt      # Backreference

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4. File Operations

Recursive Search

grep -r "function" /src/     # Search directories
grep -R --include="*.py" "def" . # Python files only

Binary Files

grep -a "text" binary.file  # Treat binary as text
grep -I "pattern" *         # Skip binary files

Multiple Files

grep "pattern" *.log        # All log files
grep "pattern" file1 file2  # Specific files

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5. Output Control

Formatting

grep -H "text" file.txt     # Show filename
grep -h "text" *.log        # Hide filename
grep -o "pattern" file.txt  # Only matching part

Color Highlighting

grep --color=auto "error" log.txt
export GREP_COLORS='mt=01;31:sl=:cx=:fn=35:ln=32:bn=32:se=36'

Null-Terminated Output

grep -l -Z "pattern" *.txt | xargs -0 rm  # Safe for pipes

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6. Performance Optimization

Fast Searches

grep -F "static string" largefile.txt  # Fixed string faster
LC_ALL=C grep "pattern" file.txt       # ASCII speedup

Parallel Grep

find . -type f | parallel -j+0 grep "pattern" {}

Exclude Directories

grep -r --exclude-dir={node_modules,.git} "function" .

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7. Advanced Techniques

Multiple Patterns

grep -e "error" -e "warning" log.txt  # Multiple patterns
grep -f patterns.txt file.txt         # Patterns from file

Inverse Matching

grep -v "success" log.txt | grep "error"  # Errors without success

Process Substitution

diff <(grep "A" file1) <(grep "B" file2)

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8. Real-World Recipes

Log Analysis

# Top 10 frequent error messages
grep -o "ERROR: .*" app.log | sort | uniq -c | sort -nr | head

Code Refactoring

# Find all function calls needing update
grep -rnw '/src/' -e 'deprecated_function'

CSV Processing

# Extract column 3 where column 1 matches
awk -F, '$1 ~ /pattern/{print $3}' data.csv | grep "value"

Network Diagnostics

# Find all unique IPs hitting error pages
grep " 404 " access.log | grep -oE "\b([0-9]{1,3}\.){3}[0-9]{1,3}\b" | sort -u

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9. Alternatives & Complements

When Not to Use Grep

Task	Better Tool
JSON processing	jq
Columnar data	awk
Complex regex	perl/rg
File content replacement	sed

Enhanced Grep Variants

rg (ripgrep)       # Faster, .gitignore aware
ack                # Perl-powered, for coders
ag (silver searcher) # Similar to rg

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Pro Tips

• Always quote patterns to prevent shell expansion
• Combine with xargs for bulk operations:

  grep -l "old_version" *.js | xargs sed -i 's/old_version/new_version/g'
  

• Use -- to indicate end of options:

  grep -- "-v" file.txt  # Search for literal "-v"
  

Further Learning

• man grep - Official documentation
• regex101.com - Interactive regex tester
• ripgrep - Modern grep alternative

Need a specific grep solution? Describe your search challenge!