General
6 cheat sheets.
Security Fundamentals
OS-level security primitives every operator should know: users and groups, file permissions, Linux capabilities, SUID/SGID, mandatory access control (SELinux, AppArmor), sandboxing concepts, least privilege, and encryption at rest vs in transit.
Processes
Process lifecycle on Unix: fork/exec/wait, PIDs, signals, zombies and orphans, parent/child trees, process groups, sessions, controlling terminals, and a tour of Linux cgroups.
Networking Stack
How packets actually move: the OSI and TCP/IP layer models, the BSD socket API, TCP vs UDP, the three-way handshake, MTU/MSS, NAT and port translation, basic IP routing, and the full DNS resolution flow.
Memory Management
How operating systems give every process its own address space: virtual memory and paging, swap, the OOM killer, mmap, copy-on-write, the page cache, allocator choices (glibc, jemalloc, mimalloc), and how to read memory counters in top, ps, and free.
Filesystems
Core filesystem concepts every operator should know: inodes, directory structure, hard vs symbolic links, journaling, copy-on-write, and a head-to-head of ext4, XFS, Btrfs, ZFS, APFS, and NTFS with mount options and pitfalls.
Operating Systems
Command-line references and tooling guides for Linux, Windows, z/OS, and macOS.