LWM-Linux/06 - Linux File Operations/String Matching (Inside files using grep).md

# string searching using grep in Linux

## 1. Basic Usage:
   The fundamental syntax of grep is:
   - `grep [options] pattern [file...]`
   
   Example:
   - `grep "error" logfile.txt` # This searches for the word "error" in logfile.txt.

## 2. Case Sensitivity:
   - By default, grep IS case-sensitive.
   - Use `-i` for case-insensitive search:
     - `grep -i "eRRor" logfile.txt` # This will return all occurances of "error" in any case sensitivity. Error, erRor, ERROR, etc...

## 3. Regular Expressions:
   grep supports regular expressions for powerful pattern matching.
   
   - . (dot): Matches any single character
   - *: Matches zero or more of the preceding character
   - ^: Matches the start of a line
   - $: Matches the end of a line
   - [ ]: Matches any single character in brackets
   
   Example:
   - `grep "^Error" logfile.txt`  # Lines starting with "Error"
   - `grep "failed$" logfile.txt`  # Lines ending with "failed" 
   - `grep "t[ae]st" logfile.txt`  # Matches "test" or "tast"
   
## 4. Extended Regular Expressions:
   Use -E option or egrep command for extended regex:
   - `grep -E "Error|Warning" logfile.txt` # Searches for "Error" or "Warning" in "logfile.txt"
   - `egrep "Error|Warning" logfile.txt` # Same as above
   

## 5. Inverting the Match:
   -v option inverts the match, showing lines that don't match:
   - `grep -v "success" logfile.txt` # Match any line that does not contain the work "success" in logfile.txt

## 6. Displaying Line Numbers:
   `-n` option shows line numbers:
   - `grep -n "error" logfile.txt`

## 7. Counting Matches:
   `-c` option counts the number of matching lines:
   - `grep -c "error" logfile.txt`
   

## 8. Showing Context:
   - `-A n`: Shows n lines after the match
   - `-B n`: Shows n lines before the match
   - `-C n`: Shows n lines before and after the match
   
   Example:
   - `grep -C 2 "critical error" logfile.txt` # Match "critical error" inside "logfile.txt" and show 2 lines above/below each occurance.

## 9. Recursive Search:
   `-r` option searches recursively through directories:
   - `grep -r "TODO" /path/to/project`

## 10. Matching Whole Words:
   `-w` option matches whole words only:
   - `grep -w "log" logfile.txt`

## 11. Displaying Filename:
   - `-H`: Always print filename
   - `-h`: Never print filename
   - `grep -H "error" *.log`

## 12. Quiet Mode:
   `-q` option suppresses output, useful in scripts:
   ```
    if grep -q "error" logfile.txt; then
    echo "Errors found"
    fi
   ```

## 13. Using grep with Pipes:
   grep works well with pipes for filtering output:
   - `ps aux | grep "nginx"`

## 14. Multiple Patterns:
    Use `-e` option for multiple patterns:
    - `grep -e "error" -e "warning" -e "critical" logfile.txt`

## 15. Reading Patterns from a File:
    `-f` option reads patterns from a file:
    `grep -f patterns.txt logfile.txt`

## 16. Binary Files:
    `-a` Process binary files as text
   - \-\-binary-files=without-match: Assume binary files don't match
    
    `grep -a "string" binary_file`

## 17. Colorizing Output:
   `--color` option highlights matches:
   - `grep --color "error" logfile.txt`

## 18. Excluding Files:
   `--exclude` option excludes files from the search:
   - `grep "TODO" --exclude="*.o" -r .`

## 19. Including Only Certain Files:
   `--include` option includes only specified files:
   - `grep "function" --include="*.c" -r .`

## 20. Null-Separated Output:
   `-Z` option outputs a zero byte after filename:
   - `grep -lZ "error" *.log | xargs -0 rm`