Basic Linux

Linux is an open-source Unix-like kernel used across servers, cloud VMs, and containers.

DevOps engineers rely on Linux for automation, CI runners, and Kubernetes nodes.

Ubuntu and Debian dominate cloud images; RHEL and Amazon Linux power enterprise AWS workloads.

Choose a distro based on support contracts, package manager, and security update cadence.

The kernel manages hardware; user space runs shells, services, and your applications.

Understand runlevels (or systemd targets), processes, and the role of init systems.

Everything is a file: devices, sockets, and pipes live under a single directory tree rooted at /.

Key paths: /etc for config, /var for logs, /home for users, /tmp for ephemeral data.

ls lists directory contents; cd changes directory; pwd prints the working directory.

Use ls -la for permissions and hidden files; tab completion speeds daily work.

ls -la
cd /var/log
pwd

Permissions use rwx for user, group, and others. chmod and chown adjust access.

Principle of least privilege: avoid chmod 777 in production.

chmod 640 app.conf
chown deploy:deploy app.conf

Users map to UIDs in /etc/passwd; login shells and home directories define interactive access.

useradd and userdel manage accounts; passwd sets credentials.

sudo useradd -m -s /bin/bash devops
sudo passwd devops

Groups bundle permissions—add users with usermod -aG group user.

Debian/Ubuntu use apt; RHEL/Amazon Linux use yum or dnf.

Keep systems patched; pin versions in production images.

sudo apt update && sudo apt upgrade -y
sudo yum install -y nginx

Resolve conflicts with apt-cache or rpm -qa; prefer immutable images over live drift.