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
  

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
egrep "Error|Warning" logfile.txt

5. Inverting the Match:

-v option inverts the match, showing lines that don't match:

grep -v "success" 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

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
```

This guide covers many of grep's powerful features. Remember to consult the man pages (man grep) for more detailed information and additional options.