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Practical Grep: Filtering Errors and Patterns in Log Files

Tue, 30 Jun 2026 11:41:54 +0000

Introduction

This walkthrough covers using grep to search and filter log files for errors, specific requests, and multi-pattern matches.

Tasks

Find all ERROR messages in system.log and count how many there are.
Find all lines containing POST requests in access.log.
Find all lines in application.log that contain both WARNING and query (case-insensitive).
Search for all user authentications (containing User authenticated) across all log files.

Requirements

Use grep for all searches (combining with other commands like wc where needed).
Save each task’s output to its own file: task1_output.txt, task2_output.txt, task3_output.txt, task4_output.txt.
Original log files must not be modified.

Solution

Task 1

 
 BASH
 
grep -c "ERROR" system.log > task1_output.txt

grep -c counts the number of matching lines instead of printing them. Redirecting the output saves the count to task1_output.txt.