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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.

Docker Basic Commands

Mon, 01 Jun 2026 12:33:56 +0000

Task 1

What is the version of the Docker Server Engine running on the host?

Check Docker version

$ docker version

Client:
Version: 25.0.5
API version: 1.44

Server:
Engine:
Version: 25.0.5
API version: 1.44

Answer: 25.0.5

Task 2

How many containers are currently running on this host?

List running containers

$ docker ps

CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES

Answer: 0

Task 3

How many images are available on this host?

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.

Why git add Wildcard Won’t Stage Your Nested Files (And How to Fix It)

Sun, 26 Oct 2025 14:54:48 +0530

I recently ran into an issue while working with Git. I had a bunch of Markdown files spread across multiple nested folders and wanted to stage them all at once. So I ran:

 
 BASH
 
git add *.md

But only the .md files in the current directory got added (to the staging area). The .md files in the nested directories didn’t get added.

The root casue: the * wildcard isn’t processed by Git, but by the shell. So the shell was only matching them in the current folder. What a revelation!

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.

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.

Command to Get a Count of Python Dependencies

Sat, 22 Apr 2023 00:00:00 +0530

From the root of your project run:

 
 BASH
 
pip freeze --all | wc -l

This will return a number which indicates the total no. of dependencies of your Python project.

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).

View Files Changed in Git Commits

Thu, 16 Mar 2023 00:00:00 +0530

Introduction

I recently got into a situation, where changes made to a file was committed, but I was not sure which commit captured those changes. So, I wanted to know the files changed in past commits. One of the easiest way to do this is to simply check the history of your repo on GitHub or GitLab etc. But I didn’t push my local commits, since I wanted to ensure that things are perfect before I push. I found the following commands to be useful in this regard.

How to Exclude Files When Using git add

Tue, 22 Nov 2022 23:33:25 +0530

Problem

Sometimes you want to add all files to git except some files or folders.

Solution

You can use the :(exclude) syntax to exclude files or folders from the git add command. The :(exclude) pathspec modifier is a special syntax that allows you to exclude files or folders. A pathspec is a way to specify files in Git. It is a string that describes a set of files. You can use wildcards to match multiple files. For example, *.html matches all html files. **/*.html matches all html files in all subfolders.

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.

CLI on Nerdsid.com

Practical Grep: Filtering Errors and Patterns in Log Files

Introduction

Tasks

Requirements

Solution

Task 1

Docker Basic Commands

Task 1

Task 2

Task 3

Linux Commands Cheat Sheet (Most Used Commands)

Table of Contents

Disk Usage / File Analysis

Find out space taken by files older than 5 days

Keep last 100d fies

Find out Year month of files

List all files other than .gz

Grep Failing on Hyphen Patterns? Try This Fix

Why git add Wildcard Won’t Stage Your Nested Files (And How to Fix It)

4 Ways to Watch Logs Live in Linux

1. tail -f

2. tail -F

Shortcuts to Navigate in a Terminal

Command to Get a Count of Python Dependencies

One Line Command to Create an SSH Key

Description

View Files Changed in Git Commits

Introduction

How to Exclude Files When Using git add

Problem

Solution

Equivalent of Linux Commands in Powershell

Well-known linux commands and their powershell equivalents

List all files other than `.gz`

1. `tail -f`

2. `tail -F`