more screenshots and intro to systemd

essential/readme.md

@@ -49,7 +49,6 @@ Step by step walk-through of a VM creation in virtualbox with some notes on each
 ![virtualbox setup](../assets/2022-02-18-123011_1920x1080_scrot.png)
 ![virtualbox setup](../assets/2022-02-18-123047_1920x1080_scrot.png)
 ![virtualbox setup](../assets/2022-02-18-123121_1920x1080_scrot.png)
-![virtualbox setup](../assets/2022-02-18-123142_1920x1080_scrot.png)
 
 ## Breakdown of the Debian installation
 
@@ -102,19 +101,120 @@ 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
+
+![Debian installation](../assets/VirtualBox_debian_18_02_2022_12_53_40.png)
+![Debian installation](../assets/VirtualBox_debian_18_02_2022_12_54_33.png)
+![Debian installation](../assets/VirtualBox_debian_18_02_2022_12_54_41.png)
+
 ## Package managers
 
 * what is a [package manager](https://en.wikipedia.org/wiki/Package_manager)
 
+### Graphical installation
+
 ![Debian installation](../assets/VirtualBox_debian_18_02_2022_12_51_24.png)
 ![Debian installation](../assets/VirtualBox_debian_18_02_2022_12_52_06.png)
 ![Debian installation](../assets/VirtualBox_debian_18_02_2022_12_52_15.png)
 ![Debian installation](../assets/VirtualBox_debian_18_02_2022_12_52_36.png)
 ![Debian installation](../assets/VirtualBox_debian_18_02_2022_12_52_46.png)
 ![Debian installation](../assets/VirtualBox_debian_18_02_2022_12_52_53.png)
-![Debian installation](../assets/VirtualBox_debian_18_02_2022_12_53_40.png)
+
-![Debian installation](../assets/VirtualBox_debian_18_02_2022_12_54_33.png)
+### Command line installation
-![Debian installation](../assets/VirtualBox_debian_18_02_2022_12_54_41.png)
+
-
+```bash
+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:~$ 
+```
+
+![htop](../assets/VirtualBox_debian_18_02_2022_13_18_25.png)
+![htop](../assets/VirtualBox_debian_18_02_2022_13_18_51.png)
+![htop bug with gnome-terminal](../assets/VirtualBox_debian_18_02_2022_13_19_35.png)
+
+### Adding a secondary desktop environment
+
+![tasksel](../assets/VirtualBox_debian_18_02_2022_13_21_33.png)
+![tasksel](../assets/VirtualBox_debian_18_02_2022_13_22_33.png)
+![tasksel](../assets/VirtualBox_debian_18_02_2022_13_22_47.png)
+![tasksel](../assets/VirtualBox_debian_18_02_2022_13_23_57.png)
+
+### 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.
+
+```bash
+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](https://en.wikipedia.org/wiki/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.
+
+```bash
+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.
+
+```
+
+```bash
+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
+```
+
+![minimal runlevel](../assets/VirtualBox_debian_18_02_2022_13_40_00.png)
+![minimal runlevel](../assets/VirtualBox_debian_18_02_2022_13_40_11.png)
+
+```bash
+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
+```