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Basic command and procedures

by Wintermute0110

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This section describes some very basic commands and procedures that you will use a lot when configuring your HTPC with this guide. If something is not explained in other sections of this guide have first a look here.

The Raspberry Pi Documentation project has some very informative tutorials specifically for the Pi but many things are also applicable for any Linux distribution. I recommand to have a look to introduction to the terminal and Linux commands.

Unprivileged user commands and root commands

Some commands must be run by an unprivileged user. In this case the prompt ends with the $ character.

wintermute@pc:~$ lsblk

Other commands must be run with root privileges. In this case the prompt ends with a # character.

root@pc:~# dd bs=4M conv=fdatasync status=progress if=path/to/input.iso of=/dev/sdd

You can also use sudo to run privileged commands. # sudo dd bs=4M ... is equivalent to the previous example (do not type the # character which indicates the command must be run by the root user).

In Ubuntu the root user does not have a password by default. Sometimes using sudo all the time is inconvenient, specially when you need to use many privileged commands in a row. To become the user root you can type $ sudo su. To exit the root session and return to the unprivileged user session type exit.

Configuration files

Lines starting with a # are comments and can be safely skipped. For example:

# File /home/kodi/.xinitrc

# Execute an Openbox window manager session.
# Openbox will execute the contents of /home/kodi/.config/openbox/autostart
exec openbox-session

is equivalent to:

exec openbox-session

In this guide all configuration files start with a line such as # File ... that shows where you have to place the configuration file.

Basic Linux keyboard shortcuts

By default the Linux system in your HTPC will start in text mode because the X server (graphical environment) is not installed by default. After introducing your username and password you can start typing commands in the text console. By default there are 6 text terminals named from tty1 to tty6. You can switch to a different text console with Alt + Fx, where x is a number from 1 to 6. Also you can use Alt + Right_arrow or Alt + Left_arrow to cycle from one text terminal to another.

When the X server is active you can press Control + Alt + Fx to return to the text console. The X server runs in terminal 7 so to go back to the X server, cycle over the text consoles until you reach the terminal 7.

Rebooting and powering off your HTPC

To reboot the system use the command reboot. To power off your HTPC use the command poweroff or press the power button. By default in Ubuntu Focal Fossa both commands can be run by normal users logged into the system.

Installing software and upgrading your system

In Ubuntu/Debian Linux software is managed using advanced packagin tool (APT).

# apt update
# apt search keyword1 keyword2 ...
# apt install package_name
# apt remove package_name

From time to time upgrade the software in your HTPC. apt update updates the local database of available packages. apt dist-upgrade performs the actual update of the software packages. apt autoremove and apt clean are cleanup commands to save disk space.

# apt update
# apt dist-upgrade
# apt autoremove
# apt clean

Terminal emulator in the X server

lxterminal is a lightweight terminal emulator with few dependencies. I recommend to use a font like Monospace o Noto Mono.

Editing files in the text console

Use nano in the text-mode console. Use Ctrl + X to exit nano and press y to save the file.

Editing files in the X server (graphical environment)

In the X server use mousepad, pluma, featherpad or geany. These text editors are simple have few dependencies as opposed to the editors of GNOME or KDE, which have many features and will install many packages you don’t really need in your HTPC. If in doubt I recommend to install mousepad.

mousepad, pluma and geany are based on the GTK library and featherpad uses Qt.

Managing files in the text console with Midnight Commander

Midnight Commander is a Norton Commander clone for Linux and it is very convenient to manage files. In some terminals in the X server the key F10 (required to exit Midnight Commander) is not available. In this case use F9 to bring up the menu and the browse to File and then Exit. To browse the menus in MC you can use the arrow keys.

apt install mc

Systemd services

Services are programs running in the background that carry out many tasks. For example, the session manager (the graphical program that greets you and prompts for use name and password) is a service. Most Linux distributions nowadays, including Ubuntu and Debian, use systemd to run and manage services.

# systemctl start display-manager.service
# systemctl stop display-manager.service
# systemctl restart display-manager.service
# systemctl reload display-manager.service

# systemctl enable display-manager.service
# systemctl disable display-manager.service

$ systemctl list-units
$ systemctl list-units --all
$ systemctl list-unit-files
$ systemctl cat display-manager.service
$ systemctl show display-manager.service

$ systemctl status display-manager.service
$ systemctl list-dependencies display-manager.service
$ systemctl list-dependencies --all display-manager.service

Targets are equivalent to runlevels. The system can be only on one runlevel at a given time.

$ systemctl list-unit-files --type=target
$ systemctl get-default
# systemctl set-default multi-user.target
$ systemctl list-dependencies multi-user.target

The multi-user.target is installed by default when you install Ubuntu server edition, the graphical.target must be configured before you can use it.

systemd places its configuration files in /etc/systemd/system/ and /lib/systemd/system/. The first directory can be changed by the system administrator (you!) but the latter should never be modified as it has the system defaults installed by packages. The configuration files in the first directory take precedence over the latter.

Systemd Essentials

Understanding Systemd Units and Unit Files

How To Use Journalctl to View and Manipulate Systemd Logs

systemd.special

systemd manpages

Passwordless SSH login

In this example your desktop computer is named laptop and your username in laptop is wintermute. You want to setup passwordless SSH connections to your HTPC machine named htpc with username kodi. In other words, you can connect from your laptop to your HTPC with $ ssh kodi@htpc and SSH will never ask for a password.

Step 1) Create a passwordless public-private key pair

wintermute@laptop:~$ ssh-keygen -t rsa -f ~/.ssh/wintermute-laptop-rsa -N ""

This command creates the private key in ~/.ssh/wintermute-laptop-rsa and the public key in ~/.ssh/wintermute-laptop-rsa.pub. The private key file looks like:

$ cat ~/.ssh/wintermute-laptop-rsa
-----BEGIN RSA PRIVATE KEY-----
MIIEpAIBAAKCAQEAsmU24mTyq564/AuXuzcIr5E8YXkDqJPXSzCcfLX1IqGYGhKn
...
P+7kPvjL3BqZS2UJwoJcMAoVnc0a24F3IuGKTpDLgQr1HDV7vtCJXA==
-----END RSA PRIVATE KEY-----

The public key wintermute-laptop-rsa.pub contains one very long line of text and looks like:

ssh-rsa AAAAB...fYa7 wintermute@laptop

Step 2) Add the private key into the default SSH file for private keys

wintermute@laptop:~$ cd ~/.ssh/
wintermute@laptop:~/.ssh$ cat wintermute-laptop-rsa >> id_rsa
wintermute@laptop:~/.ssh$ chmod 600 id_rsa

Step 3) Add the public key into the authorized_keys file into the remote host

wintermute@laptop:~/.ssh$ ssh kodi@htpc mkdir -p ~/.ssh
wintermute@laptop:~/.ssh$ cat wintermute-laptop-rsa.pub | ssh kodi@htpc "cat - >> ~/.ssh/authorized_keys"

The first command creates the directory ~/.ssh/ in the remote host htpc. You can safely skip this command if the directory already exists.

Step 4) Test your setup

You should be able to SSH to your HTPC without being asked for a password.

wintermute@laptop:~$ ssh kodi@htpc
Welcome to Ubuntu 14.04 LTS - XBMCbuntu (GNU/Linux 3.13.0-29-generic x86_64)

Last login: Sat Jul  5 01:16:52 2014 from 192.168.11.3
NUC:~$

If you get into trouble check that the authorized_keys file in the remote host has the correct permissions and ownership. For a user named kodi it should be:

$ ls -l ~/.ssh/authorized_keys
kodi@nuc:~$ ls -l ~/.ssh/authorized_keys 
-rw------- 1 kodi kodi 1489 Dec 12  2015 /home/kodi/.ssh/authorized_keys

If permissions (-rw-------) and/or ownership (kodi kodi) are wrong, changed them with the following commands:

$ chown kodi:kodi ~/.ssh/authorized_keys
$ chmod 600 ~/.ssh/authorized_keys

Step 5) Connect to more computers

You can use your public/private key to connect to more than one computer. Just copy the public key into the authorized_keys of as many hosts as you want.

Kernel modules

To show a list of currently loaded modules:

$ lsmod

To show information about a module, including the module parameters:

# modinfo i915

To list the options that are set for a currently loaded module use systool. systool is in the sysfsutils package.

# systool -v -m i915

ArchLinux wiki: kernel modules