Foundations¶

What Linux is, how its layers fit together, and how to identify any machine before working on it.

Revision Card¶

Fact	Value
Linux	A kernel (Torvalds, 1991, GPL version 2); a distribution adds the userland
Layers	Hardware, kernel, system calls, C library, shell and applications
Kernel space vs user space	Privileged shared address space vs isolated unprivileged processes
System call	Only way into the kernel; failures return `-1` and an `errno` name
Kernel design	Monolithic with loadable modules
PID 1 and PID 2	`systemd` (user space) and `kthreadd` (kernel threads)
Families	Red Hat (`rpm`, `dnf`) and Debian (`dpkg`, `apt`)
Rebuilds and upstream	Rocky and Alma rebuild RHEL; CentOS Stream is ahead of RHEL
Support	RHEL 10 years; Ubuntu LTS 5 years standard
Containers	Own userland, shared host kernel
Memory	Read `available`, not `free`

Task	Command
Distribution and version	`cat /etc/os-release`
Kernel release and build	`uname -r`, `cat /proc/version`
CPU, memory, disks	`lscpu`, `free -h`, `lsblk`
PCI devices and hardware tree	`lspci`, `sudo lshw -short`
Firmware data	`sudo dmidecode -t system`
VM or bare metal	`systemd-detect-virt`
Owning package of a file	`rpm -qf <file>`, `dpkg -S <file>`
Trace system calls	`strace -e trace=openat <command>`
Shared library dependencies	`ldd <binary>`

File	Covers	Track	Weight
What Is Linux	History, GPL and copyleft, where Linux runs	Core	Low
Kernel vs OS vs Distro	Kernel, userland, shell, distribution; GNU/Linux; backported versions	Core	Med
Distributions	Families, release models, architecture names	Core	Low
Linux vs Windows	Security mechanisms, why servers run Linux	Core	Low
Architecture	Layers, kernel and user space, system calls, C library, kernel threads, monolithic design	Core	High
System Information	`os-release`, `uname`, `lscpu`, `free`, `lsblk`, `lspci`, `lshw`, `dmidecode`, uptime	Core	Med