Operating System Basics
An operating system (OS) is the software between the hardware and your applications. It manages CPU time, memory, storage, devices, and user accounts.
Major families:
- Windows — Microsoft. File system: NTFS. Paths use backslashes: C:\Users\jane\file.txt.
- macOS — Apple, Unix-based. File system: APFS.
- Linux — open-source, Unix-like. File system: ext4 (most common). Paths use forward slashes: /home/jane/file.txt. Distributions include Ubuntu, Red Hat, Debian, Kali.
- Mobile — Android (Linux-based), iOS (Unix-based).
Absolute vs. relative path. Absolute starts at the root (C:\…, /…). Relative starts at the current working directory (./reports/q3.txt).
Linux command line
The CT often gives a task ("list files in the current directory", "change permissions") and asks for the right command.
| Command | What it does |
|---|---|
ls |
List files in current directory |
cd <dir> |
Change directory |
pwd |
Print working directory |
cp <src> <dst> |
Copy a file |
mv <src> <dst> |
Move or rename |
rm <file> |
Delete a file (rm -r for directories) |
mkdir <dir> |
Make directory |
rmdir <dir> |
Remove empty directory |
cat <file> |
Show file contents |
grep <pattern> <file> |
Search inside a file |
find <path> -name <pat> |
Search for files by name |
chmod <mode> <file> |
Change permissions |
chown <user> <file> |
Change owner |
ps |
List running processes |
kill <pid> |
Terminate a process |
man <cmd> |
Show the manual page |
sudo <cmd> |
Run a command as root (administrator) |
Linux file permissions. Each file has three permission groups — owner, group, others — and three permissions — read (r), write (w), execute (x). ls -l displays them as rwxr-xr--. Numerically: r=4, w=2, x=1. chmod 755 file = owner rwx (7), group rx (5), others rx (5).
Windows command line and shell
| Windows | Linux equivalent |
|---|---|
dir |
ls |
cd |
cd |
copy |
cp |
move |
mv |
del |
rm |
md / mkdir |
mkdir |
type |
cat |
findstr |
grep |
tasklist |
ps |
taskkill |
kill |
Windows ships with both Command Prompt (cmd) and PowerShell (newer, scriptable). Task Manager shows processes and CPU/memory use.
Processes and services
- Process — a running program with its own memory space and PID (process ID).
- Thread — a unit of execution inside a process. Threads share their process's memory.
- Service (Windows) / daemon (Linux) — a long-running background program (web server, print spooler, antivirus).
- Booting: power on → firmware (BIOS/UEFI) runs POST (Power-On Self-Test) → bootloader → kernel → init → login.