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The grep command is used for searching text and patterns in files and command output. It supports regular expressions, case-insensitive search, recursive search, and more.
Basic grep Usage
| Command | Description |
|---|---|
| grep “word” file.txt | Search for “word” in file.txt. |
| grep -i “word” file.txt | Case-insensitive search. |
| grep -w “word” file.txt | Match whole word only. |
| grep -n “word” file.txt | Show line numbers with matches. |
| grep -c “word” file.txt | Count occurrences of “word”. |
| grep -v “word” file.txt | Show lines that do NOT contain “word”. |
Example: Case-insensitive search for “error” in a log file
grep -i "error" /var/log/syslog
Searching in Multiple Files
| Command | Description |
|---|---|
| grep “word” file1.txt file2.txt | Search in multiple files. |
| grep -l “word” *.txt | Show only filenames containing “word”. |
| grep -r “word” /path/to/dir | Recursively search inside all files in a directory. |
| grep -Rl “word” /path/to/dir | List filenames with matches (no content). |
Example: Recursively find “TODO” comments in code
grep -r "TODO" ~/projects/
Using Regular Expressions
| Command | Description |
|---|---|
| grep “^word” file.txt | Find lines starting with “word”. |
| grep “word$” file.txt | Find lines ending with “word”. |
| grep “[0-9]” file.txt | Find lines containing numbers. |
| grep “error.*warning” file.txt | Match “error” followed by “warning”. |
| `grep -E “error | fail” file.txt` |
Example: Find lines that contain “log” at the beginning
grep "^log" logfile.txt
Filtering Command Output
| Command | Description |
|---|---|
| `ps aux | grep “firefox”` |
| `ls -l | grep “^d”` |
| `df -h | grep “/dev/sd”` |
| `cat file.txt | grep -v “^#”` |
Example: Find all processes related to “apache”
ps aux | grep "apache"
Grep with Context (Surrounding Lines)
| Command | Description |
|---|---|
| grep -A 3 “error” file.txt | Show 3 lines after each match. |
| grep -B 3 “error” file.txt | Show 3 lines before each match. |
| grep -C 3 “error” file.txt | Show 3 lines before & after match. |
Example: Show errors with 2 lines of surrounding context
grep -C 2 "error" logfile.txt
Advanced Grep Techniques
| Command | Description |
|---|---|
| grep -o “[0-9]\+” file.txt | Extract only numbers from a file. |
| `grep -E “ERROR | WARN” logfile.txt` |
| grep –color=auto “pattern” file.txt | Highlight matched text. |
| grep -P “\d{3}-\d{2}-\d{4}” file.txt | Find social security numbers (-P for Perl regex). |
Example: Extract email addresses from a file
grep -E "[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}" emails.txt
Saving and Redirecting Grep Output
| Command | Description |
|---|---|
| grep “error” logfile.txt > errors.txt | Save results to a file. |
| grep “error” logfile.txt >> errors.txt | Append results to a file. |
| `grep “error” logfile.txt | tee errors.txt` |
Example: Save all warning messages to “warnings.log”
grep "WARNING" /var/log/syslog > warnings.log
Summary
| Feature | Command Example |
|---|---|
| Basic Search | grep “word” file.txt |
| Case Insensitive | grep -i “word” file.txt |
| Recursive Search | grep -r “word” /path/to/dir |
| Show Line Numbers | grep -n “word” file.txt |
| Search Multiple Patterns | `grep -E “error |
| Show Before/After Context | grep -C 3 “error” file.txt |
| Extract Numbers | grep -o “[0-9]\+” file.txt |
This Linux grep Cheat Sheet will help you find text efficiently in files, logs, and command output.