Lesson 01 · Getting around

What Linux is

Linux is a kernel: the core program that runs a computer and shares its hardware between every other program. Add a shell, commands and apps and you get a whole operating system, sold or given away as distributions such as Ubuntu, Debian and Fedora. Then follow your first command, uname -a, from the terminal through the shell to the kernel and back, and watch ls make the same trip.

9 min11 steps5 questionsBeginner

Fig 1 · Interactive

Loading figure…

or ← →

Check yourself

5 quick questions

Pick an answer to see why. Your score saves with your progress.

01What is Linux, strictly speaking?

Linux is a kernel. Together with a shell, commands and apps it makes an operating system; ready-made bundles of these are called distributions.

02Which of these is a Linux distribution?

Ubuntu bundles the Linux kernel with tools and apps, ready to install. Debian and Fedora are others. Python is a language and bash is a shell (one of the tools inside a distribution).

03Why can't the ls program read the disk by itself?

Only the kernel, in kernel space, drives the hardware. ls asks it with a system call (getdents64) to read the folder, and the kernel hands back the names.

04You type uname -a and press Enter. Which program reads the line and starts uname?

The terminal hands your line to the shell. The shell splits it into the program name and its option, then asks the kernel to start that program.

05Which of these machines run the Linux kernel?

The same kernel runs on very different machines. Android phones use the Linux kernel, most web servers and cloud computers run it, and on the November 2017 TOP500 list every one of the 500 fastest supercomputers ran Linux.

Key terms

Glossary

Kernel: The core program of an operating system. It starts programs and is the only one allowed to drive the hardware: CPU, memory and disks.
Linux: A kernel, written from scratch by Linus Torvalds with help from people across the internet, and published as open source.
Distribution: A ready-made operating system: the Linux kernel plus a shell, commands and apps. Ubuntu, Debian and Fedora are examples.
Terminal: A window for text: you type commands into it and it shows what the programs print back.
Shell: The program that reads your commands and starts other programs. bash is the default on Ubuntu, Debian and Fedora.
User space: Where all ordinary programs run, the shell included. Code here cannot touch the hardware directly.
System call: A request a program makes to the kernel, for example to read a folder (getdents64) or to print text (write).
Open source: Software whose code anyone may read, change and share. Linux uses the GNU General Public License, version 2 (GPL).

Fine print

Notes and sources

The uname answer on the card is an example from an Ubuntu 24.04 computer named lab. The middle (the kernel's build number and date) is cut to "…". Your machine prints its own name and version.
Strictly, the terminal and the shell also talk through the kernel (a pseudo-terminal), and the shell waits for the program to finish before it shows the next prompt. The figure shows the main trip only. Also, on Linux the fork() wrapper in the C library (glibc) actually makes the clone system call.
Many of the everyday commands, uname and ls included, come from the GNU project. That is why uname ends with GNU/Linux, and why some people call the whole system GNU/Linux.
The kernel keeps recently used folders in memory, so ls often gets its answer without waiting for the disk.
TOP500 ranks the 500 fastest supercomputers twice a year, in June and November. The November 2017 list was the first on which every system ran Linux.

Quiz Unlocks after the last step.