Grep Cheat Sheet

Essential grep commands for text searching and pattern matching in files.

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Table of Contents

• Basic Commands
• Common Options
• Regular Expressions
• Extended Grep (egrep)
• Fixed Grep (fgrep)
• Recursive Searching
• Context Lines
• Multiple Patterns
• Output Formatting
• Practical Examples
• Advanced Usage

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Basic Commands

Command	Description	Example
grep "pattern" file	Search for pattern in file	grep "error" log.txt
grep "pattern" file1 file2	Search in multiple files	grep "TODO" *.py
grep "pattern" *	Search in all files in directory	grep "config" *
cat file | grep "pattern"	Search in piped input	cat log.txt | grep "warning"
command | grep "pattern"	Filter command output	ps aux | grep "nginx"

Common Options

Option	Description	Example
-i	Case insensitive search	grep -i "error" log.txt
-v	Invert match (exclude lines)	grep -v "debug" log.txt
-n	Show line numbers	grep -n "error" log.txt
-l	Show only filenames with matches	grep -l "TODO" *.py
-L	Show only filenames without matches	grep -L "TODO" *.py
-c	Count matching lines	grep -c "error" log.txt
-w	Match whole words only	grep -w "cat" file.txt
-x	Match whole lines only	grep -x "exact line" file.txt
-q	Quiet mode (no output)	grep -q "pattern" file && echo "found"
-s	Suppress error messages	grep -s "pattern" nonexistent.txt
-H	Always print filename	grep -H "pattern" file.txt
-h	Never print filename	grep -h "pattern" *.txt
--color	Colorize matches	grep --color "error" log.txt

Regular Expressions

Pattern	Description	Example
.	Any single character	grep "a.b" file.txt (matches "aab", "acb")
*	Zero or more of preceding	grep "ab*" file.txt (matches "a", "ab", "abb")
^	Start of line	grep "^Error" log.txt
$	End of line	grep "done$" log.txt
[abc]	Any character in brackets	grep "[aeiou]" file.txt
[^abc]	Any character not in brackets	grep "[^0-9]" file.txt
[a-z]	Character range	grep "[a-zA-Z]" file.txt
\	Escape special characters	grep "\." file.txt (literal dot)
\<	Start of word	grep "\<cat" file.txt
\>	End of word	grep "cat\>" file.txt
\b	Word boundary	grep "\bcat\b" file.txt

Extended Grep (egrep)

Extended regex patterns (use grep -E or egrep):

Pattern	Description	Example
+	One or more of preceding	grep -E "ab+" file.txt
?	Zero or one of preceding	grep -E "colou?r" file.txt
{n}	Exactly n occurrences	grep -E "[0-9]{3}" file.txt
{n,}	n or more occurrences	grep -E "[0-9]{3,}" file.txt
{n,m}	Between n and m occurrences	grep -E "[0-9]{3,5}" file.txt
|	OR operator	grep -E "cat|dog" file.txt
()	Grouping	grep -E "(cat|dog)s?" file.txt

Fixed Grep (fgrep)

Search for literal strings (use grep -F or fgrep):

Command	Description	Example
grep -F "string" file	Literal string search	grep -F ".*" file.txt
fgrep "string" file	Same as grep -F	fgrep "[ERROR]" log.txt
grep -F -f patterns.txt file	Multiple literal patterns from file	grep -F -f patterns.txt data.txt

Recursive Searching

Command	Description	Example
grep -r "pattern" directory/	Recursive search in directory	grep -r "TODO" src/
grep -R "pattern" directory/	Recursive with symlinks	grep -R "config" /etc/
grep -r --include="*.py" "pattern" .	Include specific file types	grep -r --include="*.py" "import" .
grep -r --exclude="*.log" "pattern" .	Exclude specific file types	grep -r --exclude="*.log" "error" .
grep -r --exclude-dir=".git" "pattern" .	Exclude directories	grep -r --exclude-dir=".git" "TODO" .

Context Lines

Option	Description	Example
-A n	Show n lines after match	grep -A 3 "error" log.txt
-B n	Show n lines before match	grep -B 2 "error" log.txt
-C n	Show n lines before and after	grep -C 2 "error" log.txt
--group-separator=SEP	Separator between match groups	grep -C 2 --group-separator="---" "error" log.txt

Multiple Patterns

Command	Description	Example
grep -e "pat1" -e "pat2" file	Multiple patterns	grep -e "error" -e "warning" log.txt
grep -f patterns.txt file	Patterns from file	grep -f search_terms.txt data.txt
grep "pat1|pat2" file	OR with basic regex	grep "error|warning" log.txt
grep -E "pat1|pat2" file	OR with extended regex	grep -E "cat|dog" animals.txt

Output Formatting

Option	Description	Example
-o	Show only matching part	grep -o "[0-9]\+" file.txt
-m n	Stop after n matches	grep -m 5 "pattern" file.txt
--line-buffered	Force line buffering	tail -f log.txt | grep --line-buffered "error"
-T	Make tabs line up	grep -T "pattern" file.txt
-Z	Null character after filename	grep -lZ "pattern" * | xargs -0 ls -l

Practical Examples

Log File Analysis

# Find all error messages
grep -i "error" /var/log/apache2/error.log

# Count different log levels
grep -c "ERROR" log.txt
grep -c "WARNING" log.txt
grep -c "INFO" log.txt

# Find recent errors with context
grep -A 5 -B 5 "$(date '+%Y-%m-%d')" /var/log/syslog | grep -i error

# Exclude debug messages
grep -v "DEBUG" application.log

Source Code Search

# Find TODO comments in Python files
grep -r --include="*.py" "TODO\|FIXME\|HACK" .

# Find function definitions
grep -n "^def " *.py

# Find imports
grep -r "^import\|^from.*import" . --include="*.py"

# Find SQL queries
grep -r -i "select\|insert\|update\|delete" . --include="*.py"

System Administration

# Find processes by name
ps aux | grep -v grep | grep nginx

# Find files modified today
find /path -type f -newermt "$(date '+%Y-%m-%d')" | grep -E "\.(log|conf)$"

# Check for failed login attempts
grep "Failed password" /var/log/auth.log

# Find large files in logs
find /var/log -name "*.log" -exec grep -l "size" {} \;

Network and Security

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

# Find suspicious activities
grep -i "hack\|attack\|malware\|virus" /var/log/security.log

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

# Find URLs
grep -E -o "https?://[^\s]+" file.txt

Advanced Usage

Combining with Other Commands

# Count unique occurrences
grep "pattern" file.txt | sort | uniq -c

# Find and replace (with sed)
grep -l "old_pattern" *.txt | xargs sed -i 's/old_pattern/new_pattern/g'

# Find files and show matches
find . -name "*.log" -exec grep -l "error" {} \;

# Use with xargs
grep -l "pattern" *.txt | xargs wc -l

Performance Tips

# Use fixed strings for better performance
grep -F "literal_string" large_file.txt

# Limit search to specific file types
grep -r --include="*.txt" "pattern" /large/directory/

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

# Use binary file detection
grep -I "pattern" * # Skip binary files

Complex Patterns

# Match lines with specific word counts
grep -E "^(\S+\s+){5,10}\S*$" file.txt

# Find lines that don't contain any digits
grep -v "[0-9]" file.txt

# Match lines starting with capital letter
grep "^[A-Z]" file.txt

# Find empty lines
grep "^$" file.txt

# Find lines with only whitespace
grep "^\s*$" file.txt

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Resources

• GNU Grep Manual
• Regular Expressions Info
• POSIX Regex Documentation

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Author

🤖 casjay: Github 🤖

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Originally compiled from various sources. Contributions welcome!