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