Linux Commands

Master the Command Line Interface

systembeginner

96%+

Server Market

of top websites run Linux

500+

Distributions

Ubuntu, Fedora, Debian, etc.

1991

Created

by Linus Torvalds

Free

Open Source

GPL-2.0 license

What is Linux?

Linux is an open-source Unix-like operating system kernel created by Linus Torvalds in 1991. The command-line interface (CLI) is the primary way to interact with Linux systems, especially servers. Commands are small programs that perform specific tasks - you can combine them using pipes and redirects to build powerful workflows. The shell (bash, zsh, etc.) interprets your commands and communicates with the kernel to execute them.

Think of Linux as a universal remote for your computer

Instead of clicking through menus and windows, you type precise commands that tell the computer exactly what to do. Like a universal remote controls any TV, Linux commands work across servers, containers, cloud instances, and your laptop. Once you learn the commands, you can control any Linux system in the world the same way.

Key Features

Everything is a File

Devices, processes, and system info are all represented as files you can read and manipulate.

Powerful Pipelines

Chain commands together with pipes (|) to process data in sophisticated ways.

Process Control

Run, monitor, schedule, and manage any process with fine-grained control.

Remote Access

Manage servers anywhere via SSH. Same commands work locally or remotely.

When to Use

Managing servers, containers, and cloud instances
Automating repetitive tasks with shell scripts
Processing and analyzing log files and text data
DevOps, CI/CD pipelines, and infrastructure management
System administration and troubleshooting
Software development and build processes
Network configuration and diagnostics
Security auditing and forensics

When Not to Use

Simple tasks where a GUI is faster (e.g., casual photo editing)
When you need real-time collaboration (use collaborative tools)
For users who need accessibility features better served by GUIs
Tasks requiring visual design (use design software)
When a platform-specific tool is more appropriate (e.g., Windows admin)

Prerequisites

Access to a Linux system (physical, VM, WSL, or cloud)
Basic understanding of file systems and directories
A terminal emulator application

Installation

windows(WSL)

wsl --install

Windows Subsystem for Linux

windows(WSL Ubuntu)

wsl --install -d Ubuntu

Install Ubuntu specifically

macos(Terminal)

# Built-in, use Terminal.app

macOS is Unix-based

linux(Terminal)

# Already installed

Open your terminal emulator

docker(Docker)

docker run -it ubuntu:latest bash

Practice in a container

Verify installation: echo $SHELL && uname -a

Quick Start Steps

Open your terminal

Launch Terminal (Linux/macOS), Windows Terminal with WSL, or connect via SSH

# Press Ctrl+Alt+T on Linux, or open Terminal app

Explore your location

See where you are and what files exist

pwd                    # Print working directory
ls -la                 # List all files with details
cd ~                   # Go to home directory

Create and manipulate files

Practice basic file operations

mkdir practice         # Create a directory
cd practice
touch myfile.txt       # Create empty file
echo "Hello Linux" > myfile.txt
cat myfile.txt         # View file contents

Learn to get help

Every command has built-in documentation

man ls                 # Full manual for ls
ls --help              # Quick help
which python           # Find command location

ls -la

List all files with permissions and details

cd <directory>

Change to specified directory

pwd

Print current working directory

cp <src> <dest>

Copy files or directories (-r for recursive)

mv <src> <dest>

Move or rename files and directories

rm <file>

Remove files (-r for directories, -f to force)

mkdir -p <dir>

Create directory (and parents if needed)

cat <file>

Display file contents

grep <pattern> <file>

Search for patterns in files

find <path> -name <pattern>

Find files by name or criteria

chmod <mode> <file>

Change file permissions

sudo <command>

Execute command as superuser

ps aux

List all running processes

kill <pid>

Terminate a process by ID

man <command>

Display manual page for a command

Commands by Category

Pro Tips15

Master tab completion

workflow

Press Tab to autocomplete commands, paths, and filenames. Double-tap Tab to see all options when multiple matches exist.

Search command history with Ctrl+R

workflow

Press Ctrl+R and start typing to search through your command history. Press Ctrl+R again to cycle through matches.

Ctrl+R then type 'git' to find recent git commands

Use history shortcuts

workflow

!! repeats the last command, !$ gives the last argument. sudo !! is incredibly useful when you forgot sudo.

sudo !! to run last command with sudo

Retyping long commands manually

Use xargs for bulk operations

performance

Pipe results to xargs to run commands on multiple items. Use -P for parallel execution.

find . -name '*.log' | xargs -P 4 gzip

Always quote variables in scripts

gotcha

Unquoted variables break on filenames with spaces. Always use "$variable" instead of $variable.

rm "$file"

rm $file

Use set -e in scripts for safety

best-practice

set -e makes scripts exit on first error. Combine with set -u (undefined var error) and set -o pipefail.

set -euo pipefail at script start

Understand stdout vs stderr

gotcha

1 is stdout, 2 is stderr. Redirect both: command > file 2>&1. Suppress errors: command 2>/dev/null

find / -name foo 2>/dev/null

Use less instead of cat for large files

workflow

cat dumps everything to terminal. less lets you scroll, search (/pattern), and navigate efficiently.

less /var/log/syslog

cat /var/log/syslog

Test destructive commands with echo first

best-practice

Before running rm or other dangerous commands, prepend echo to see what would happen.

echo rm *.log first, then rm *.log

Prefer rsync over cp for large transfers

performance

rsync shows progress, can resume interrupted transfers, and only copies changed files on subsequent runs.

rsync -avh --progress source/ dest/

cp -r source/ dest/

Create aliases for frequent commands

automation

Add aliases to ~/.bashrc or ~/.zshrc for commands you use often. Example: alias ll='ls -la'

alias update='sudo apt update && sudo apt upgrade'

Use screen or tmux for persistent sessions

workflow

Terminal multiplexers keep sessions alive when you disconnect. Essential for long-running tasks on remote servers.

tmux new -s mysession for persistent work

Learn essential Ctrl shortcuts

workflow

Ctrl+A (start of line), Ctrl+E (end), Ctrl+K (delete to end), Ctrl+U (delete to start), Ctrl+W (delete word), Ctrl+L (clear screen)

Check if command exists in scripts

best-practice

Use 'command -v' or 'which' to verify a command is available before using it in scripts.

command -v docker &>/dev/null || { echo "Docker not found"; exit 1; }

Use LC_ALL=C for faster text processing

performance

Setting LC_ALL=C tells grep/sort to use ASCII-only matching, which is much faster for large files.

LC_ALL=C grep 'pattern' largefile.txt

Key Facts10

In Linux, everything is a file

Devices (/dev/sda), processes (/proc/1234), system info (/sys) are all represented as files. This unified interface simplifies system interaction.

Permission bits: 4=read, 2=write, 1=execute

These add up for octal notation: 7=rwx, 6=rw-, 5=r-x, 4=r--, 0=---. Three digits represent user, group, others.

Exit code 0 means success, non-zero means failure

Check with $? after a command. Scripts should exit with appropriate codes. Use 'set -e' to fail on errors.

SIGTERM (15) is graceful stop, SIGKILL (9) is force kill

Always try SIGTERM first (kill PID). SIGKILL doesn't let processes clean up. SIGHUP (1) often reloads config.

File descriptors: 0=stdin, 1=stdout, 2=stderr

Redirect: > (stdout to file), 2> (stderr to file), &> (both), >> (append), < (stdin from file).

Files starting with . are hidden by default

Use ls -a to see them. Config files like .bashrc, .gitignore follow this convention.

Root (UID 0) has unlimited system access

Use sudo for privileged operations instead of logging in as root. This provides audit trails and limits exposure.

Symlinks point to paths, hardlinks point to inodes

Symlinks can cross filesystems and link to directories. Hardlinks share the same data blocks on disk.

Process states: R (running), S (sleeping), D (disk wait), Z (zombie)

Zombies are finished processes waiting for parent to read exit status. Too many zombies indicate a parent process issue.

PATH determines where shell looks for commands

Commands are found by searching directories in PATH order. Add custom paths: export PATH=$PATH:/custom/bin

Interview & Exam Practice7

A server is running out of disk space. You need to quickly identify what's consuming the most space.

What commands would you use to diagnose and resolve this?

Users report the application is slow. You SSH into the server and need to identify what's consuming resources.

What's your troubleshooting approach?

A web application can't write to its log directory. You see 'Permission denied' errors. The directory exists and is owned by root.

How do you fix this properly?

After deploying an update, the nginx service fails to start. systemctl status shows it's 'failed'.

What's the best approach to diagnose and fix this?

You have a large CSV file and need to extract all rows where the third column value is greater than 100, then sum those values.

Which tool(s) would you use?

You need to run a long backup script on a remote server, but you must disconnect SSH before it completes.

How do you ensure the process continues after disconnecting?

You're trying to start a web server but port 8080 is already in use. You need to find and stop whatever is using it.

What's the correct sequence of commands?

Related Tools

dockerkubernetesansibleterraformgit

Resources

Official Documentation Source Code (GitHub)Download / Install

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