linux_introduction/essential/readme.md

Essential Linux

Introduction to Linux

Where does Linux originate from?

• The spiritual mother of Linux is Unix.
• POSIX is not an OS in itself but a standard.
  • Which OS's are POSIX compliant

Who is behind this project?

• Who is Richard Stallmann?
• Who is Linus Torvalds?

What is the deal with 'GNU-slash-Linux'?

• GNU/Linux controversy
  • What is a kernel
  • What does GNU bring to the table?
  • What's the deal with userspace and kernelspace?
    • userspace
    • kernelspace

Where can you get some Linux?

• You probably already have a Linux computer running at home!

Debian

• Reasons to choose Debian.
• Why is Debian called the universal operating system?
  • comparison of CPU architectures
  • download Debian

How to create a virtual machine

In order to run virtual machines, or VM's, we need a host program. One of the most popular ones out there is called virtualbox. It should be installed on your machine but in case it's not you can download it here. If you expand the section below you'll see a step by step walk-through of a VM creation in virtualbox with some notes on each step with best practice pointers.

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Nice!

1. Name your machine and select linux if it's not chosen automatically.
2. Set the amount of RAM for your machine. This can be anything between an absolute minimum and your physical machine's limit. You can't magically use more RAM than physically available!
3. You need a virtual hard drive to store the OS and all your data to.
4. The format doesn't matter that much. I always go for the default one.
5. Most of the time you'll want dynamic size meaning the disk file will only take up as much space as it needs. For example, if you set a size of 50GB for your disk and your OS plus personal files take up 12GB, the actual space this image takes op on the physical disk of your host will be about 12GB.
6. We'll soon discover how much space a basic Linux installation takes up but for now I would recommend the following.
   • with a graphical environment: 20GB
   • without a graphical environment: 10GB (minimal/server/headless install)
7. By default your machine will have only one CPU core but you can add more via the settings. This is something you can change whenever you need more processing power but the same rule as with the RAM applies, it's not a magical way to add resources to your machine.
8. Last but not least we need to insert the installation disk into the virtual machine and boot it up!

Breakdown of the Debian installation

All right, enough talking, let's get started! I'll install a full blown and modern graphical Debian machine and I would like you to not do it yourself but take notes on each step so you'll be able to reference your notes later when I ask you to install a machine yourself. Below is a gain a step by step walk through with some of my tips but a more details guide can be found in the Debian Administrator Handbook. This book is a real bible of information and I highly advise you to read through it.

My most important notes would be the following.

• Read each section carefully and you'll know what to do.
• Don't set a root password but use sudo instead.
• Don't install a graphical environment if you don't need one. It's a lot easier to add one later than to remove it.
• Don't forget to install grub at the end otherwise your installation will not be able to boot. You can recover from this mistake with supergrub2 but that's for an other day.

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Exercise

OK, now it's up to you! Please install a Debian machine with the default graphical environment. Once installed, take some time out to explore the system. Which software is installed by default? How can you tweak the system settings such as language, keyboard, networking? Can you install extra software?

Quick look at an idle system

You can inspect the system by running the gnome task manager. It lists all your running processes, sorted by CPU percentage, in the first tab which can give you a good idea of what's happening on your system. Note that an idle system uses almost no resources! Plus, a fresh install, with quite a few essential programs such as a browser, music player, text editor, etc only takes up about 5GB of disk space! This is mind blowing compared to a windows installation. How is this possible? That's the magic of a good package manager!

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Package managers

What is a package manager

Graphical installation

Modern Linux has come a long way and it's now quite usable without any command line knowledge. We can install extra software from the graphical environment if we want.

Let's look for vlc, a popular open source video player. When we look for it from activities we see a reference to it via the software program. If we click on it we'll be take to a sort of app or play store where we can choose to install or uninstall programs. Because adding and removing software from our machine is quite invasive, we need to prove we have the right to do so. A prompt will pop up where you'll need to input your password. If your password does not work, you probably set a root password and you should use that one instead.

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Command line installation

waldek@hellodebian:~$ htop
-bash: htop: command not found
waldek@hellodebian:~$ apt install htop
E: Could not open lock file /var/lib/dpkg/lock-frontend - open (13: Permission denied)
E: Unable to acquire the dpkg frontend lock (/var/lib/dpkg/lock-frontend), are you root?
waldek@hellodebian:~$ sudo apt install htop
[sudo] password for waldek: 
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
Suggested packages:
  lm-sensors strace
The following NEW packages will be installed:
  htop
0 upgraded, 1 newly installed, 0 to remove and 0 not upgraded.
Need to get 127 kB of archives.
After this operation, 328 kB of additional disk space will be used.
Get:1 http://deb.debian.org/debian bullseye/main amd64 htop amd64 3.0.5-7 [127 kB]
Fetched 127 kB in 0s (2,030 kB/s)
Selecting previously unselected package htop.
(Reading database ... 141359 files and directories currently installed.)
Preparing to unpack .../htop_3.0.5-7_amd64.deb ...
Unpacking htop (3.0.5-7) ...
Setting up htop (3.0.5-7) ...
Processing triggers for mailcap (3.69) ...
Processing triggers for desktop-file-utils (0.26-1) ...
Processing triggers for hicolor-icon-theme (0.17-2) ...
Processing triggers for gnome-menus (3.36.0-1) ...
Processing triggers for man-db (2.9.4-2) ...
waldek@hellodebian:~$ 

The menu bar at the bottom shows you can press F10 to quit. This does not work because of a shortcut of gnome-terminal! You can deactivate this shortcut in the preferences though. But pressing q also exits the program.

Adding a secondary desktop environment

During the initial installation we where offered a choice of desktop environments to install. This menu was an actual program called tasksel we can run again to add different ones! As it's a program that can heavily modify the system, we need administrator privileges.

waldek@hellodebian:~$ sudo tasksel
[sudo] password for waldek: 

Changing the runlevel

The first process started by the kernel can be discover by htop and is /usr/sbin/init. This is rather vague but we can find out a bit more about this program via ls which we'll get into more detail later. For now just have a look at the output of the following commmand.

waldek@hellodebian:~$ ls -l /usr/sbin/init 
lrwxrwxrwx 1 root root 20 Jul 13  2021 /usr/sbin/init -> /lib/systemd/systemd
waldek@hellodebian:~$ 

Now, what is this systemd? And more importantly, how do we interact with it? For now just accept that the main program to communicate with systemd is systemctl. A quick look at the man pages gives us the following.

SYSTEMCTL(1)                                                         systemctl                                                         SYSTEMCTL(1)

NAME
       systemctl - Control the systemd system and service manager

SYNOPSIS
       systemctl [OPTIONS...] COMMAND [UNIT...]

DESCRIPTION
       systemctl may be used to introspect and control the state of the "systemd" system and service manager. Please refer to systemd(1) for an
       introduction into the basic concepts and functionality this tool manages.

waldek@hellodebian:~$ sudo systemctl set-default multi-user.target
[sudo] password for waldek: 
Removed /etc/systemd/system/default.target.
Created symlink /etc/systemd/system/default.target → /lib/systemd/system/multi-user.target.
waldek@hellodebian:~$ sudo reboot now

After the reboot you'll be confronted with the following screen. Don't stress, your machine is not broken! It's just running in a more minimal mode. You can log in just as with the graphical login window but you'll only have a command line to interact with the machine. A quick look at htop tells us not a lot of stuff is running, just the bare minimum.

To undo this change and regain the graphical interface again you can run the following commands.

waldek@hellodebian:~$ sudo systemctl set-default graphical.target
[sudo] password for waldek: 
Removed /etc/systemd/system/default.target.
Created symlink /etc/systemd/system/default.target → /lib/systemd/system/graphical.target.
waldek@hellodebian:~$ sudo reboot now

Exercise

Please create a second virtual machine and install a minimal Debian. Minimal means no graphical environment. I advise you to not set a root password.

Once this machine is up and running, install a few programs. The only one we saw up until now is htop but maybe try out bmon, elinks and/or ranger. Once these are installed have a look at the running services and programs via htop and compare it to both your graphical installation and your graphical install but running in multi-user.target.

Guest additions

Let's make our user experience a bit nicer. Virtual machines can integrate with the host machine more fluently when you install the guest additions in the VM. It's a three step process.

1. install the required packages to build the guest additions in you VM
2. insert the guest additions CD into your VM
3. run the correct script from the CD

The dependencies can be installed as follows. First you update your package list, next you install three packages.

• build-essential
• dkms
• linux-headers-$(uname -r)

The third package name is a bit weird looking no? This is a bit of command line kung fu. The actual name of the package is as follows.

waldek@hellodebian:~$ echo linux-headers-$(uname -r)
linux-headers-5.10.0-11-amd64
waldek@hellodebian:~$ 

The $(uname -r) is a call to a program called uname with the option -r. We'll go more into detail on this later.

waldek@hellodebian:~$ sudo apt update
Hit:1 http://deb.debian.org/debian bullseye InRelease
Hit:2 http://deb.debian.org/debian bullseye-updates InRelease
Hit:3 http://security.debian.org/debian-security bullseye-security InRelease
Reading package lists... Done                 
Building dependency tree... Done
Reading state information... Done
All packages are up to date.
waldek@hellodebian:~$ sudo apt install build-essential dkms linux-headers-$(uname -r)
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
The following additional packages will be installed:
  binutils binutils-common binutils-x86-64-linux-gnu dctrl-tools dpkg-dev fakeroot g++ g++-10 gcc gcc-10 libalgorithm-diff-perl
  libalgorithm-diff-xs-perl libalgorithm-merge-perl libasan6 libatomic1 libbinutils libc-dev-bin libc-devtools libc6-dev libcc1-0 libcrypt-dev
  libctf-nobfd0 libctf0 libfakeroot libgcc-10-dev libitm1 liblsan0 libnsl-dev libstdc++-10-dev libtirpc-dev libtsan0 libubsan1
  linux-compiler-gcc-10-x86 linux-headers-5.10.0-11-common linux-headers-amd64 linux-kbuild-5.10 linux-libc-dev make manpages-dev patch
Suggested packages:
  binutils-doc debtags debian-keyring g++-multilib g++-10-multilib gcc-10-doc gcc-multilib autoconf automake libtool flex bison gdb gcc-doc
  gcc-10-multilib gcc-10-locales glibc-doc libstdc++-10-doc make-doc ed diffutils-doc
The following NEW packages will be installed:
  binutils binutils-common binutils-x86-64-linux-gnu build-essential dctrl-tools dkms dpkg-dev fakeroot g++ g++-10 gcc gcc-10 libalgorithm-diff-perl
  libalgorithm-diff-xs-perl libalgorithm-merge-perl libasan6 libatomic1 libbinutils libc-dev-bin libc-devtools libc6-dev libcc1-0 libcrypt-dev
  libctf-nobfd0 libctf0 libfakeroot libgcc-10-dev libitm1 liblsan0 libnsl-dev libstdc++-10-dev libtirpc-dev libtsan0 libubsan1
  linux-compiler-gcc-10-x86 linux-headers-5.10.0-11-amd64 linux-headers-5.10.0-11-common linux-headers-amd64 linux-kbuild-5.10 linux-libc-dev make
  manpages-dev patch
0 upgraded, 43 newly installed, 0 to remove and 0 not upgraded.
Need to get 62.0 MB of archives.
After this operation, 258 MB of additional disk space will be used.
Do you want to continue? [Y/n] y

Once this is done we can insert the CD. You can verify the disk's content via the files explorer in gnome.

Now, open a terminal and run the following.

waldek@hellodebian:~$ cd /media/cdrom0/
waldek@hellodebian:/media/cdrom0$ ls
AUTORUN.INF  NT3x          TRANS.TBL                          VBoxLinuxAdditions.run          VBoxWindowsAdditions.exe
autorun.sh   OS2           VBoxDarwinAdditions.pkg            VBoxSolarisAdditions.pkg        VBoxWindowsAdditions-x86.exe
cert         runasroot.sh  VBoxDarwinAdditionsUninstall.tool  VBoxWindowsAdditions-amd64.exe
waldek@hellodebian:/media/cdrom0$ sudo bash VBoxLinuxAdditions.run 
Verifying archive integrity... All good.
Uncompressing VirtualBox 6.1.28 Guest Additions for Linux........
VirtualBox Guest Additions installer
Copying additional installer modules ...
Installing additional modules ...
VirtualBox Guest Additions: Starting.
VirtualBox Guest Additions: Building the VirtualBox Guest Additions kernel 
modules.  This may take a while.
VirtualBox Guest Additions: To build modules for other installed kernels, run
VirtualBox Guest Additions:   /sbin/rcvboxadd quicksetup <version>
VirtualBox Guest Additions: or
VirtualBox Guest Additions:   /sbin/rcvboxadd quicksetup all
VirtualBox Guest Additions: Building the modules for kernel 5.10.0-11-amd64.
update-initramfs: Generating /boot/initrd.img-5.10.0-11-amd64
VirtualBox Guest Additions: Running kernel modules will not be replaced until 
the system is restarted
waldek@hellodebian:/media/cdrom0$ sudo reboot now

Once the machine is rebooted you'll can resize the window and the screen resolution will adapt automatically! You can also enable copy/paste and drag and drop between your host and VM now.

Introduction to the command line

We'll be using a few new words to reference the command line such as shell, bash and terminal through out the course. They all pretty much mean the same thing but with some small, and not so important, differences between them. Essentially a command line is a textual interface for humans to operate a computer. What is very important to understand is that textual commands and graphical actions operate on the same computer. For example, if you create a file via the command line, it will show up in you file explorer and vice versa. The graphical and textual interfaces are just different representations of the same machine.

Now open up a terminal and you'll see the following.

waldek@hellodebian:~$ 

This is what we call a prompt. It's not much but it's our window into the computer. As with most things in life you can question it's who, where, what and when. This information is actually embedded in the prompt. Let's break it down.

• waldek is who I am on this computer
• hellodebian is the what, as in what computer I'm operating on
• ~ is where I am located on this computer

What about the when then? Let's type in date and see what happens.

waldek@hellodebian:~$ date
Fri 18 Feb 2022 03:46:59 PM CET
waldek@hellodebian:~$ 

There we see our when! But this miniscule operation illustrates us the fundamental operation of a command line!

1. we have a prompt where we can run a program
2. the program runs and outputs it's information on the terminal
3. once the program finishes we can run an other program

I'm deliberately saying program here but is date really a program? It's a bit basic no? Well, it is a program and most commands you'll type into your terminal are actually programs. We can illustrate this as follows.

waldek@hellodebian:~$ vlc
VLC media player 3.0.16 Vetinari (revision 3.0.13-8-g41878ff4f2)
[0000559c46ee95b0] main libvlc: Running vlc with the default interface. Use 'cvlc' to use vlc without interface.
[0000559c46f89790] main playlist: playlist is empty

Vlc is now running and the terminal is blocked meaning we can't run other programs or commands in it. If you try to run the date command again, or ls or htop, it won't work! Try it out if you don't believe me. But what happens if you close vlc? The commands you typed get executed! This is an illustration of the sequential nature of a command line.

Now, I don't think we have sufficiently proven that date is a full blown program so let's dig a bit deeper.

waldek@hellodebian:~$ which vlc
/usr/bin/vlc
waldek@hellodebian:~$ which date
/usr/bin/date
waldek@hellodebian:~$ which which
/usr/bin/which

There is quite bit to unpack in the example above. First, what on earth is which? Well, it's also a program and it's sole purpose in life is to tell you where a program is located on your system. Because which by itself does not make a lot of sense it needs what we call an argument. Here the argument is the name of the program we want to know it's location of. The existence of arguments is the second big thing we discovered here. The third new thing we can observe here is what we call paths, meaning locations on the system. For example, vlc is a binary program located in a folder called bin which is located in a folder called usr which is at the root of your system. If this sounds complicated, don't worry, we'll go into detail a bit later.

Now that we know where some of our programs are located, let's find out what they are. The methodology is the same as with which but we'll use an other program called file who's purpose in life is to tell more about the content of a certain file. Logically, file needs an argument and this argument is the path to the file you want to inspect.

waldek@hellodebian:~$ file /usr/bin/vlc
/usr/bin/vlc: ELF 64-bit LSB pie executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, BuildID[sha1]=51c40f8234213415771b3a344cab25a140543f8a, for GNU/Linux 3.2.0, stripped
waldek@hellodebian:~$ file /usr/bin/date
/usr/bin/date: ELF 64-bit LSB pie executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, BuildID[sha1]=b740f054aaef6a85aff024858c914c7eae70a6a5, for GNU/Linux 3.2.0, stripped
waldek@hellodebian:~$ file /usr/bin/which
/usr/bin/which: POSIX shell script, ASCII text executable
waldek@hellodebian:~$ file $(which which) # this is some command line kung fu...
/usr/bin/which: POSIX shell script, ASCII text executable
waldek@hellodebian:~$ 

Here we learn that both vlc and date are executables, compiled for an x86-64 system. I would say they are both created equally no? Both are actual programs. But what about which? It's also an executable but not compiled, it's a POSIX shell script. So not all programs are created equally?

Compiled vs interpreted

Executing a file, or program, means you take this file and tell the computer it needs to execute the actions that are stored in the file. Compiled programs contain actual instructions the computer understands out of the box. This means that programs that are compiled are always compiled for a specify architecture which in our case is x86-64. On a Raspberry Pi this would be armhf or arm64.

root and / and /root are not the same thing