111 lines
3.4 KiB
Markdown
111 lines
3.4 KiB
Markdown
# string searching using grep in Linux
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## 1. Basic Usage:
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The fundamental syntax of grep is:
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- `grep [options] pattern [file...]`
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Example:
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- `grep "error" logfile.txt` # This searches for the word "error" in logfile.txt.
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## 2. Case Sensitivity:
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- By default, grep IS case-sensitive.
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- Use `-i` for case-insensitive search:
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- `grep -i "eRRor" logfile.txt` # This will return all occurances of "error" in any case sensitivity. Error, erRor, ERROR, etc...
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## 3. Regular Expressions:
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grep supports regular expressions for powerful pattern matching.
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- . (dot): Matches any single character
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- *: Matches zero or more of the preceding character
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- ^: Matches the start of a line
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- $: Matches the end of a line
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- [ ]: Matches any single character in brackets
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Example:
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- `grep "^Error" logfile.txt` # Lines starting with "Error"
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- `grep "failed$" logfile.txt` # Lines ending with "failed"
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- `grep "t[ae]st" logfile.txt` # Matches "test" or "tast"
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## 4. Extended Regular Expressions:
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Use -E option or egrep command for extended regex:
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- `grep -E "Error|Warning" logfile.txt` # Searches for "Error" or "Warning" in "logfile.txt"
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- `egrep "Error|Warning" logfile.txt` # Same as above
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## 5. Inverting the Match:
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-v option inverts the match, showing lines that don't match:
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- `grep -v "success" logfile.txt` # Match any line that does not contain the work "success" in logfile.txt
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## 6. Displaying Line Numbers:
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`-n` option shows line numbers:
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- `grep -n "error" logfile.txt`
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## 7. Counting Matches:
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`-c` option counts the number of matching lines:
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- `grep -c "error" logfile.txt`
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## 8. Showing Context:
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- `-A n`: Shows n lines after the match
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- `-B n`: Shows n lines before the match
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- `-C n`: Shows n lines before and after the match
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Example:
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- `grep -C 2 "critical error" logfile.txt` # Match "critical error" inside "logfile.txt" and show 2 lines above/below each occurance.
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## 9. Recursive Search:
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`-r` option searches recursively through directories:
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- `grep -r "TODO" /path/to/project`
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## 10. Matching Whole Words:
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`-w` option matches whole words only:
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- `grep -w "log" logfile.txt`
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## 11. Displaying Filename:
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- `-H`: Always print filename
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- `-h`: Never print filename
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- `grep -H "error" *.log`
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## 12. Quiet Mode:
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`-q` option suppresses output, useful in scripts:
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```
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if grep -q "error" logfile.txt; then
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echo "Errors found"
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fi
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```
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## 13. Using grep with Pipes:
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grep works well with pipes for filtering output:
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- `ps aux | grep "nginx"`
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## 14. Multiple Patterns:
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Use `-e` option for multiple patterns:
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- `grep -e "error" -e "warning" -e "critical" logfile.txt`
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## 15. Reading Patterns from a File:
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`-f` option reads patterns from a file:
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`grep -f patterns.txt logfile.txt`
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## 16. Binary Files:
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`-a` Process binary files as text
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- \-\-binary-files=without-match: Assume binary files don't match
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`grep -a "string" binary_file`
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## 17. Colorizing Output:
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`--color` option highlights matches:
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- `grep --color "error" logfile.txt`
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## 18. Excluding Files:
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`--exclude` option excludes files from the search:
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- `grep "TODO" --exclude="*.o" -r .`
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## 19. Including Only Certain Files:
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`--include` option includes only specified files:
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- `grep "function" --include="*.c" -r .`
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## 20. Null-Separated Output:
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`-Z` option outputs a zero byte after filename:
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- `grep -lZ "error" *.log | xargs -0 rm`
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