Docker and DevOps - Docker Compose stacks
Docker Compose is where people lose the most time over details that look minor: a volume that keeps an old password, a depends_on that does not wait, a database port left open by accident. The examples library covers the stacks people set up most often locally (Postgres, Redis, Nginx, Node, WordPress, Prometheus + Grafana), each with its healthcheck, its .env.example and the real error it avoids, and links to the generator to get the compose.yaml ready-made.
Linux chmod permissions
POSIX permissions (read, write and execute for owner, group and others) plus special bits (setuid, setgid, sticky) sit at the centre of access control on Unix systems. The guide covers the 3- and 4-digit octal notation, the full symbolic notation (relative ops, copying between audiences) and frequent cases on web directories, SSH keys and setuid binaries.
Umask in Linux - default permissions
umask defines the POSIX permissions that newly created files and directories receive, subtracting bits from the system maximum (666 for files, 777 for directories). The guide explains the formula, why new files do not get execute by default, the differences between 022, 027, 077, 002 and 000, and how umask complements chmod rather than replacing it.
CIDR subnetting for IPv4
CIDR is the modern way to describe IP networks, replacing the old class A/B/C system. Reading a /24, a /30 or a /32 is essential when configuring firewalls, VPCs, ACLs or static routing. The guide covers masks, usable hosts, edge cases like /31 (RFC 3021) and /32, plus basic FLSM and VLSM subnetting.
MAC addresses, OUI and vendor
A MAC address identifies a network interface on the local link, not a device on the internet. The guide breaks down its structure (OUI + extension), when the first OUI reveals the vendor and when it does not (locally administered, virtual MAC, OUI snapshot), what multicast/broadcast mean at the MAC layer and why a MAC never geolocates.
JSON and YAML - when to use each
JSON and YAML are often pitched as interchangeable, but they solve different problems: JSON shines in APIs and programmatic payloads, YAML in human-edited config. The guide covers when to pick each, common errors (trailing commas, indentation, implicit YAML 1.1 booleans), big-number gotchas and why validation belongs before deploy.
Unix/POSIX cron expressions
Crontab and related schedulers are often misunderstood around one subtle rule: when day-of-month and day-of-week are both restricted, many Unix/POSIX implementations combine them with OR, not AND. The guide includes practical examples by frequency, common shortcuts and the POSIX gotcha spelled out.
Hash functions, checksums and integrity
Hash functions turn any input into a fixed-length output, useful to check that a file has not been corrupted or modified, to identify stable pieces of content, and as a building block for digital signatures. The guide covers the difference between a hash and a checksum, when to pick SHA-256 over MD5, how to verify an official download step by step, and why no plain hash is safe for storing passwords.
JWT: header, payload, signature and common pitfalls
JSON Web Tokens are base64url over JSON with an attached signature; decoding them only reads their content, it does not prove authenticity. The guide covers the three parts (header, payload, signature), the differences between HS256, RS256 and ES256, what to do about expiration and nbf, why `alg: none` still causes incidents, and where to store the token in the browser without exposing it to XSS.
ToolsOps