Linux: Package Management & Storage

Kernel

Core of the OS managing hardware, memory, processes, system calls — the layer between applications and physical hardware

System Call

Mechanism by which a user-space program requests a service from the kernel (e.g. open(), read(), write())

Shell

Command-line interpreter that takes typed commands and executes them (bash, zsh, sh)

Distro

A packaged version of Linux combining the kernel with a package manager, utilities, and config conventions

Boot Process order

BIOS/UEFI -> bootloader (GRUB) -> kernel loads -> systemd/init (PID 1) takes over -> services start

Process

An independent running program with its own memory space

Thread

Smallest sequence of instructions manageable independently by the OS scheduler

Process vs Thread

Process = isolated memory space

VM (Virtual Machine)

Virtualizes hardware, full separate OS/kernel per VM — heavier, stronger isolation

Container

Virtualizes at the OS level, shares host's kernel — lighter, faster startup

Hypervisor

Software layer that creates/manages VMs, allocates hardware resources across guest VMs

Hypervisor Type 1

Runs directly on physical hardware, no host OS underneath (e.g. ESXi, Hyper-V, Xen)

Hypervisor Type 2

Runs as an application on top of an existing host OS (e.g. VirtualBox, VMware Workstation/Fusion)

/ (root)

Where every file, folder, and device starts from — top of the filesystem hierarchy

/home

Individual user directories — personal files, user-specific config, SSH keys

/bin, /usr/bin

Standard everyday command-line programs (ls, cd, cp)

/etc

Configuration files for the system — hosts, account details, SSH config

/var

Variable data that changes constantly — logs, temp caches

/tmp

Temporary files, cleared on reboot

/opt

Optional/third-party software installs

/usr

User-installed programs/libraries

/root

Root user's home directory (distinct from / itself)

/dev

Device files — how Linux represents hardware as files

/proc

Virtual filesystem exposing running process and kernel info — not real files on disk

/mnt vs /media

/mnt = manual/temporary mount points (admin use)

Regular File (-)

Contains actual data blocks — text, source code, executables, images

Directory (d)

A file containing filenames and pointers to where those files live

Symbolic Link (l)

A pointer/shortcut to another file or directory PATH — breaks if the original is moved/deleted, can cross filesystems

Hard Link

A second directory entry pointing to the SAME inode/data as the original — not a shortcut, indistinguishable from the original, can't cross filesystems, survives original being deleted

Symbolic vs Hard Link

Symlink points to a path (breaks if target moves)

Inode

Metadata structure storing a file's permissions, owner, size, timestamps, and pointers to its data blocks — NOT the filename itself

Where filenames live

Filenames are stored in the directory entry, mapping a name to an inode number — not inside the inode

Block Device (b)

Hardware handling data read/write in fixed-size blocks, with system caching (e.g. disks)

Character Device (c)

Hardware handling data stream I/O one byte at a time, sequentially, unbuffered (e.g. keyboards, serial ports)

Named Pipe (p) / FIFO

Local communication channel letting two processes pass data directly, first-in-first-out

Network Socket (s)

Endpoint for bidirectional communication between processes, local or networked

Absolute Path

Complete, unique location starting from root (/)

Relative Path

Location defined based on the current working directory

pwd

Print current working directory

cd

Change directory

cd -

Return to the previous working directory

cd (no argument)

Return to the current user's home directory

ls -la

List all files/directories including hidden (dot) files, long format

ls -lh

Long format with human-readable file sizes (K/M/G)

find

Search filesystem by name, type, size, permissions, mod time

find -name

Search by filename pattern

find -type d/f

Filter results to directories or files only

find -mtime

Filter by modified time (days)

find -delete

Combine with a search to delete matching results

locate

Fast filename search via a pre-built database, not the live filesystem — may be stale if db not updated

updatedb

Manually refreshes the locate database

which / type

Show which binary/path a command resolves to, checked against PATH

cp -r

Copy a file or directory (recursive needed for directories)

mv

Move or rename a file/directory

rm -rf

Remove recursively and forcefully — dangerous combo, no confirmation, no undo

mkdir -p

Create a directory including any needed parent directories

rmdir

Remove an EMPTY directory only — fails if it has contents

touch

Create an empty file, or update its modification timestamp if it already exists

cat

Print a file's full contents to the terminal

stat filename

Show detailed metadata about a file — inode number, size, permissions, timestamps

df vs du (quick reference)

df = filesystem/partition-level free space