The Ultimate Grep Command Guide

grep.md

🔍 The Ultimate Grep Command Guide

🚀 Basic Usage

Command	Description	Example
grep pattern file	🔎 Search for a pattern in a file	grep error logfile.txt
grep -i pattern file	🔠 Case-insensitive search	grep -i ERROR logfile.txt
grep -w word file	🔤 Match whole words only	grep -w error logfile.txt
grep -v pattern file	❌ Invert match (show lines that don't match)	grep -v success logfile.txt

📂 File and Directory Operations

Command	Description	Example
grep -r pattern directory	🗂️ Recursive search in directory	grep -r password /etc/
grep -l pattern files	📄 List files that match pattern	grep -l error *.log
grep -c pattern file	🔢 Count occurrences of pattern	grep -c error logfile.txt

🔣 Regular Expressions

Command	Description	Example
grep '^pattern' file	🎬 Match pattern at start of line	grep '^ERROR' logfile.txt
grep 'pattern$' file	🏁 Match pattern at end of line	grep 'FAILED$' logfile.txt
grep '[0-9]' file	🔢 Match any digit	grep '[0-9]' data.txt
grep 'error|warning' file	🔀 Match multiple patterns (OR)	grep 'error|warning' logfile.txt

🔧 Advanced Options

Option	Description	Example
-A n	🔽 Show n lines after match	grep -A 3 error logfile.txt
-B n	🔼 Show n lines before match	grep -B 2 error logfile.txt
-C n	🔼🔽 Show n lines before and after match	grep -C 1 error logfile.txt
-n	🔢 Show line numbers	grep -n error logfile.txt
-E	🔣 Use extended regular expressions	grep -E 'error|warning' logfile.txt

🎨 Output Formatting

Option	Description	Example
--color	🌈 Highlight matching text	grep --color error logfile.txt
-o	💡 Show only the matched parts of a line	grep -o 'ERROR.*' logfile.txt

🔗 Combining with Other Commands

Command	Description	Example
command | grep pattern	🚿 Pipe output of a command to grep	ls -l | grep '^d'
grep pattern file | wc -l	🔢 Count matching lines	grep error logfile.txt | wc -l

📊 Practical Examples

1. Search for IP addresses in a log file:

   grep -E '\b([0-9]{1,3}\.){3}[0-9]{1,3}\b' access.log

2. Find all files containing a specific string in a directory:

   grep -rnw '/path/to/directory' -e 'pattern'

3. Search for words starting with 'error' in a case-insensitive manner:

   grep -i '\berror\w*' logfile.txt

4. Extract email addresses from a file:

   grep -E '\b[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\.[A-Z|a-z]{2,}\b' file.txt

5. Find lines that don't start with a '#' (non-comments):

   grep -v '^#' config.txt

6. Search for a pattern in gzipped files:

   zgrep 'error' /var/log/syslog.*.gz

7. Find all PHP files containing 'function' but not 'deprecated':

   grep -l 'function' $(find . -name '*.php') | xargs grep -L 'deprecated'

🏆 Pro Tips

1. 🚀 Use fgrep (or grep -F) for literal string matching (faster than regular grep).
2. 🔣 Learn and use regular expressions to unleash the full power of grep.
3. 🎭 Use grep -P for Perl-compatible regular expressions (PCRE) in supported versions.
4. 📚 Combine grep with xargs for powerful file operations: grep -l pattern files | xargs command.
5. 🔒 Use grep -i cautiously with security-related searches to avoid case-based oversights.
6. 📊 For complex patterns, consider using awk or sed in combination with grep.
7. 🔍 Use grep -f patternfile to search for multiple patterns listed in a file.

🔧 Performance Tips

1. 🚄 Use LC_ALL=C grep for faster grep operations (uses byte comparison instead of character comparison).
2. 📂 When searching large directories, use find with -exec grep instead of grep -r for better performance.
3. 🎯 Use anchors (^ and $) when possible to reduce the search space.
4. 🔢 For simple string matching, fgrep (fixed string grep) is faster than standard grep.

🛠️ Troubleshooting

• If grep isn't finding a pattern you expect, check for hidden characters using cat -A on your file.
• For very large files, consider using split to break the file into smaller chunks, then grep those.
• If grep is slow on a large file, try using grep --mmap if available on your system.

Remember, grep is an incredibly powerful tool, but with great power comes great responsibility. Always double-check your patterns, especially when using grep with system files or in scripts. Happy grepping! 🔍📚