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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.

Linux Journalctl Command Cheat Sheet

Wed, 15 Apr 2026 06:39:17 +0000

I recently needed to free up disk space by removing old system logs, so I collected the command I used and other useful journalctl commands for future reference.

Disk usage & vacuum (remove old logs)

Show journal disk usage:

 
 BASH
 
sudo journalctl --disk-usage

Remove archived journal files until total size is below X:

 
 BASH
 
sudo journalctl --vacuum-size=500M

Remove archived journal files older than a time:

 
 BASH
 
sudo journalctl --vacuum-time=3d

Remove archived journal files, keeping only the most recent N files:

Ultimate Vi Shortcuts Cheat Sheet

Tue, 07 Apr 2026 20:43:00 +0000

Big picture of Vim commands

Normal mode (default)

 
 ├── Navigation (h, j, k, l, G)
 ├── Editing (d, c, x)
 ├── Insert triggers (i, a, o, O, A)

Insert mode → typing text
Visual mode → selecting text
Command mode (:) → commands like :w, :q, :10,20d

 
 TEXT
 
G

G → go to last line

2. Editing / insertion commands

o, O, A : These change the file and switch you to insert mode.

How to Fix the Apostrophe (') Key Not Working in VirtualBox Ubuntu

Tue, 07 Apr 2026 07:32:28 +0530

If you’re running Ubuntu inside Oracle VM VirtualBox and notice that pressing the apostrophe (’) or tildae (`) key doesn’t show anything until you hit space, you’re not alone.

This issue is caused by “dead keys” in keyboard layouts, where keys are designed to combine with others to create accented characters.

In this guide, I’ll show you two simple fixes to make your apostrophe key work.

Root cause

What you’re seeing is a keyboard layout / “dead key” behavior, not a VirtualBox bug.

Linux Commands Cheat Sheet (Most Used Commands)

Fri, 03 Apr 2026 07:32:28 +0530

Disk Usage / File Analysis
AWS S3 commands

Disk Usage / File Analysis

Find out space taken by files older than 5 days

 
 BASH
 
find . -type f -mtime +5 -print0 | du --files0-from=- -ch | grep total

Keep last 100d fies

 
 BASH
 
ls -t | sed -e '1,100d' | xargs -d '\n' rm -f --

 
 BASH
 
truncate -s 50M catalina.out

 
 BASH
 
ls -d */

Find out Year month of files

 
 BASH
 
stat -c '%y' * | awk '{print $1}' | xargs -I{} date -d {} "+%b %Y" | sort -u

List all files other than `.gz`

If you want this to do recursively:

Grep Failing on Hyphen Patterns? Try This Fix

Fri, 05 Dec 2025 18:48:38 +0530

Today I was searching for --debug in a log file.

So, I ran grep "--debug" file.log.

To my utter surprise, instead of seeing a match, I got this error:

grep output


grep: unrecognized option '--debug'
Usage: grep [OPTION]... PATTERNS [FILE]...

I didn’t expect to see any error because I had quoted the pattern ("--debug").

After few minutes of searching, I found the fix:

grep -- --debug file.log

This is where the humble double hyphen -- steps in.

11 Essential OpenSSH Components

Sat, 29 Nov 2025 20:05:20 +0530

You might have SSHed into servers hundreds of times.

You might have used ssh-add to load keys into your agent.

You might have used scp to copy files back and forth between your machine and server.

But have you ever paused and looked at the bigger picture?

All these commands come from one place — the OpenSSH suite. It’s much more than just ssh and scp.

OpenSSH components

OpenSSH has 11 major components.

10 Steps to Secure a New Linux VM

Wed, 22 Oct 2025 07:09:42 +0000

When you launch a new Linux VM, the level of security you really need depends on your use case. That said, there are some basic steps you can take right after provisioning a new VM, to improve it’s baseline security. Following these steps won’t make your server invincible, but it will make it safe and reduce exposure to common attacks.

1. Update your system

Newly deployed images can contain outdated packages with known vulnerabilities. Start by updating everything:

4 Ways to Watch Logs Live in Linux

Wed, 15 Oct 2025 12:49:55 +0530

1. `tail -f`

Use when:

You want to quickly stream a log file.
You want to perform short, one-off checks on a single log file.

Availibility: available on all Linux/Unix systems.

Example:

 
 BASH
 
tail -f /var/log/syslog

2. `tail -F`

Some services (like Nginx, Apache) rotate logs i.e. they stop writing logs to an old file and instead write to a new file (named sequentially). tail -f stops following when that happens, but tail -F automatically switches to the new file. In other words, it handles log rotation gracefully.

Differnece Between X Window System and a Desktop Environment

Tue, 14 Jan 2025 11:31:46 +0000

The X Window System and a desktop environment are related but serve different purposes in a graphical user interface (GUI) on Linux.

X Window System (X11): It is the foundational framework that provides the basic functionality for displaying graphical applications. X manages windows, handles input devices (like the mouse and keyboard), and allows graphical applications to display their content. However, X alone does not provide a full, user-friendly interface; it merely acts as the underlying protocol for graphical communication.

Understanding Bootloader With a Toy Robot Example

Tue, 14 Jan 2025 10:47:04 +0000

Imagine you have a toy robot that needs to be turned on before it can start moving and talking. When you press the power button, the robot goes through a quick check to make sure its batteries are working, its wheels can spin, and its voice box is ready. It makes sure all components are connected and are in a working state.

This initial check is like a bootloader. The bootloader performs similar checks on your computer or device. It ensures the system’s hardware (like memory, storage, and CPU) is functioning correctly before anything else happens.

Understanding the Kernel and Distribution With a Restaurant Example

Tue, 14 Jan 2025 09:28:28 +0000

Let’s use the example of a restaurant to illustrate the concepts of a kernel and a distribution.

Kernel: The Chef

Think of the kernel as the head chef in a restaurant. The chef is responsible for managing the kitchen, ensuring that all the ingredients are prepared correctly, and that the cooking processes are followed. The chef makes sure that everything runs smoothly in the kitchen, coordinating the cooking of different dishes and managing the kitchen staff. Without the chef, the kitchen would be chaotic, and the food wouldn’t be prepared properly.

Key Linux Terms Explained: Kernel, Bootloader, Filesystem & More

Tue, 14 Jan 2025 08:26:23 +0000

Before you begin to use Linux, you need to be aware of certain terms like: Kernel, Bootlader, Window system, service etc.

Kernel

Kernel is the glue between hardware and applications. It controls hardware and allows applications to make use of the hardware.

Examples: Linux kernel

Distribution

A Linux distribution (or distro) is essentially the same as an operating system (OS) in the context of Linux.

Linux distro includes not only the Linux kernel (the core part of the OS) but also additional software, tools, libraries, and user interfaces that together form a complete, functional system.

Shortcuts to Navigate in a Terminal

Wed, 13 Sep 2023 16:51:56 +0530

Like in any other text based program, linux terminals also support usage of arrow keys, backspace to navigate and edit text. But there are some shortcuts which can replace multiple use of arrow keys and simplify user experience. Following table lists some shortcuts.

Shortcut	Action
Ctrl+A	Jump to the beginning of the command line.
Ctrl+E	Jump to the end of the command line.
Ctrl+U	Clear from the cursor to the beginning of the command line.
Ctrl+K	Clear from the cursor to the end of the command line.
Ctrl+LeftArrow	Jump to the beginning of the previous word on the command line.
Ctrl+RightArrow	Jump to the end of the next word on the command line.
Ctrl+R	Search the history list of commands for a pattern.

More such shortcuts can be found in the bash man page.

One Line Command to Create an SSH Key

Thu, 30 Mar 2023 00:00:00 -0800

Of late, I have been creating a lot of SSH keys. I am tired of answering each question that one gets asked during such a process. So, I thought of looking up all the switches (options) for the ssh-keygen command so that I can specify everything at once.

Here is the command:

For bash:

 
 POWERSHELL
 
ssh-keygen -t ed25519 -b 4096 -C "you@mail.com for Gitlab" -N "" -f "/home/usr/.ssh/key"

For powershell:

 
 POWERSHELL
 
ssh-keygen -t ed25519 -b 4096 -C "you@mail.com for Gitlab" -N '""' -f "C:\Users\uname\.ssh\keyname"

Description

The above command generates an SSH key pair using the ed25519 algorithm with a 4096-bit key length, and saves the private and public keys with the specified name (and location).

How to Fix SSH Error: Could not open a connection to your authentication agent

Sat, 07 Jan 2023 07:32:28 +0530

The problem

Recently, I was trying to add a SSH key to the ssh-agent. So, I ran the following command:

 
 BASH
 
ssh-add ~/.ssh/<key_name>

This resulted in the following error:

 
 BASH
 
Could not open a connection to your authentication agent.

The solution

The error message is pretty clear. Since a connection to the ssh-agent could not be opened, first let’s check if the ssh-agent is running or not.

 
 BASH
 
ps -e | grep [s]sh-agent

If the ssh-agent is not running, you will not see any output. This is exactly what happened in my case. So, the solution is simple: you need to start the ssh-agent.

Equivalent of Linux Commands in Powershell

Wed, 26 Oct 2022 17:54:30 +0530

Being an avid linux user, I am used to the linux commands. But I often work on a windows machine, which is when I look for powershell equivalent of linux commands.

I created this post to keep a track of the commands that I use often.

Well-known linux commands and their powershell equivalents

The PowerShell commands for working with the file system are same as that of linux ones. You navigate around the file system with the cd (alias for Set-Location) command.