Blogs on law@deb

Simple vim tricks that no one talks about

Fri, 22 May 2026 12:11:42 +0000

Escape to normal mode

Many people are still using esc key to escape to normal mode in vim. Some even remap capslock to esc, or remap jj or jk to escape to vim mode to make it easier to escape to normal mode.

In fact, you can just use ctrl+c or ctrl+[ to escape to normal mode. These are accessible, and are the same across different machines, so you won’t build the muscle memories that don’t work without your configuration.

Exit vim

Most of the tutorials teach you to exit vim using :wq command, That’s too many key strokes. The real way to save and exit is ZZ, and exit without saving is ZQ.

Use pass to manage passwords

Sun, 10 May 2026 12:11:31 +0000

Stop using the same password across different services, use a password manager instead. Here is one of the most suckless password manager, pass (This exist on most distro’s software repo!).

Initial setup

Create a gpg key and setup a passphrase, which serve as the master password.

gpg --full-generate-key

Then initialise the pass directory.

pass init "key"

Add passwords

Use the following command to add passwords, it can be in a subdirectory,

pass insert email/example@gmail.com

then type in the password twice. Or use pass to generate password of 20 characters using the following command.

pass insert email/example@proton.me 20

To create multi-line passwords.

pass insert -m account/vpn

Then insert multi-line password, and hit ctrl+d to exit.

List passwords

To view a list of created passwords, just use the command pass, it will give the following output.

Password Store
├── account
│   └── vpn
└── email
 ├── example@gmail.com
 └── example@proton.me

View passwords

To show the password in the terminal, use the following command and type in the master password.

pass show email/example@gmail.com

Or directly copy to clipboard without displaying in terminal.

pass show -c email/example@gmail.com

Change master password

gpg --edit-key $(cat .password-store/.gpg-id)

Enter the command passwd, enter current password in the prompt, then enter new password. Lastly, save it using the save command.

Simple editing using ImageMagick

Sun, 19 Apr 2026 16:32:35 +0000

Resize an image

Resize the image using percentage.

convert image.png -resize 60% resized.png

Or define the dimension explicitly.

convert image.png -resize 640x360 resized.png

Flip an image

Mirror an image left and right.

convert image.png -flop image_mirrored.png

Or flip an image upside down.

convert image.png -flip image_upside_down.png

Crop an image

Crop image from top or bottom, where 60 is the number of pixels.

convert image.png -gravity North -chop 0x60 cropped.png
convert image.png -gravity South -chop 0x60 cropped.png

Or crop image from left or right.

convert image.png -gravity East -chop 60x0 cropped.png
convert image.png -gravity West -chop 60x0 cropped.png

Crop from centre of the image.

convert image.png -gravity center -crop 640x360+0+0 cropped.png

Or crop the image with offset from the top left corner, with the first number after the dimension being the horizontal offset, and the second number being the vertical offset.

convert image.png -crop 640x360+390+248 cropped.png

Image gallery

Put 2 images side by side.

convert left.png right.png +append combined.png

Create a 4 images gallery.

convert \( top_left.png top_right.png +append \) \( bottom_left.png bottom_right.png +append \) -append combined.png

Remove metadata

Remove the metadata in place.

mogrify -strip image.png

Check metadata

Check for Properties from the output of the following command.

identify -verbose image.png

Download YouTube Contents

Sun, 05 Apr 2026 11:40:21 +0000

Create python virtual environment

python3 -m venv yt
source yt/bin/activate

Install yt-dlp

pip3 install yt-dlp

Show available formats

yt-dlp -F [link]

Download the format

yt-dlp -f [id] [link]

For example:

yt-dlp -f 242 https://www.youtube.com/watch?v=example-link

Download audio only

yt-dlp -x --audio-quality 0 [link]

Use FFmpeg to Edit Videos

Thu, 02 Apr 2026 08:22:18 +0000

Connect videos

Run the following command to connect multiple videos together.

echo "file 'video1.mp4'" > video_list.txt
echo "file 'video2.mp4'" >> video_list.txt
echo "file 'video3.mp4'" >> video_list.txt
ffmpeg -f concat -safe 0 -i video_list.txt -c copy output.mp4

This is extremely fast and there is no quality loss, because there is no transcoding.

Remove audio

ffmpeg -i input.mp4 -an output.mp4

Trim videos

Trim video from 1:30 to 2:45.

ffmpeg -i input.mp4 -ss 00:01:30 -to 00:02:45 -c copy output.mp4

Trim video from 1:30, to 60s afterwards.

ffmpeg -i input.mp4 -ss 00:01:30 -t 60 -c copy output.mp4

Compress videos

Choose a crf value from 18 to 28, with larger number = more compression.

ffmpeg -i input.mp4 -vcodec libx265 -crf 28 output.mp4

Change frame rate to 24 fps.

ffmpeg -i input.mp4 -r 24 -c:a copy output.mp4

Resize video to 1280:720

ffmpeg -i input.mp4 -vf scale=1280:720 -c:a copy output.mp4

Stop Using PowerPoint

Thu, 26 Mar 2026 10:10:05 +0000

I have seen enough people going up on stage, logging in to their gmail account to open up their PowerPoint or Google slides, only to find out that opening it up on lecture hall computer completely messes things up. You need to learn latex beamer, it will save you a lot of headache, and it is not even that hard. By the end of this tutorial, you will be able to understand how to create this template slides.

Installation

On Debian based distros, install the full version with the following command.

sudo apt install texlive-full

This will install 8GB of packages! Or you may install only the most useful packages.

sudo apt install biber texlive texlive-science \
 texlive-bibtex-extra texlive-latex-extra \
 texlive-pictures texlive-publishers \
 texlive-fonts-extra

For other distros, you will need to look up the package name.

Your first latex slides

\documentclass[aspectratio=169]{beamer}
\usetheme{Madrid}

\title{Presentation Title}
\author{Your Name}
\institute{Your Institute}
\date{\today}

\begin{document}
\begin{frame}
\titlepage
\end{frame}
\end{document}

Here, copy your first latex slides, paste it in a file named beamer.tex. Madrid is just a theme I like, you may find different themes here. Then run the following command.

pdflatex beamer.tex

A file called beamer.pdf will be generated. To remove the footer and the icons, add the following code to the preamble

\setbeamertemplate{footline}{}
\setbeamertemplate{navigation symbols}{}

This is the most basic setup with latex beamer. Each pages are enclosed by the \begin{frame} and \end{frame} tag, like the title page. Now, add the following code to create another page.

Multilingual support

Let’s kick this off with multilingual support, if you only understand English, I am sorry for you bro, just skip to next section. You will need to install a Unicode supported typesetting engine, and a language pack. I am going to show how to add Chinese characters as an example.

sudo apt install texlive-xetex texlive-lang-chinese

Add the following code to the preamble.

\usepackage{xeCJK}
\setCJKmainfont{Droid Sans Fallback}

To find the available fonts on your system, run the following command.

fc-list :lang=zh

Then, you will be able to add Chinese characters anywhere!

\begin{frame}{Multilingual support 支持多種語言}

在 beamer 展示其他語言很容易啊。

It's easy to show other languages in beamer.

\end{frame}

To compile it to pdf, you will need to use xetex. Run the following command.

xelatex beamer.tex

If you are using Chinese characters to proceed, replace all of pdflatex command by xelatex in the reset of the tutorial.

Lists

\begin{frame}{Lists}

\begin{itemize}
\item Point 1
\item Point 2
\item Point 3
\end{itemize}

\begin{enumerate}
\item Number 1
\item Number 2
\item Number 3
\end{enumerate}

\end{frame}

You may specify the title of the page at the end of \begin{frame}. Create bullet points with itemize, and numbered list with enumerate.

Maths equations

\begin{frame}{Maths equations}

\begin{block}{Important equation}
$$ \int_1^e \frac{\mathrm{d}x}{x} = 1 $$
\end{block}

\begin{alertblock}{Even more important equation}
$$ e = \sum_{n=0}^\infty \frac{1}{n!} = 1 + \frac{1}{1} + \frac{1}{1 \cdot 2} + \frac{1}{1 \cdot 2 \cdot 3} + \cdots $$
\end{alertblock}

\end{frame}

In latex beamer, writing mathematics equations is automatically supported. You can just copy your equations from latex report to here! Enclose the equation with block or alertblock to make it looks important.

Pictures

\begin{frame}{Pictures}

\begin{figure}
\centering
\includegraphics[width=0.5\textwidth]{./latex.jpeg}
\caption{LaTeX logo}
\end{figure}

\begin{figure}
\centering
\includegraphics[trim={1.5cm 3cm 3cm 1.2cm}, clip, width=0.5\textwidth]{./latex.jpeg}
\caption{Cropped LaTeX logo}
\end{figure}

\end{frame}

To add a picture, use the above template and change the relative path of the picture. It is also useful to crop the picture in place, adding trim={1.5cm 3cm 3cm 1.2cm}, clip can crop the image, where the arguments specify cropping from left, bottom, right, and top respectively.

Reference

To add references, create a file called citation.bib, and paste the bibtex citation in it, it can usually be found on google scholar under cite. Here is a sample.

@article{dummy,
 title={A Study on Generic Topics Drives Significant Findings},
 author={Doe, John and Smith, Jane and Others, Anonymous},
 journal={Journal of Generic Research},
 pages={1--10},
 year={2024},
 publisher={Generic Publishing Group}
}

Then add the following code to the preamble.

\usepackage[backend=biber, style=vancouver]{biblatex}
\addbibresource{citation.bib}

Add the following frame as the last slide.

\begin{frame}{References}
\printbibliography
\end{frame}

Lastly, cite this in the slides. Here is an example.

\item Point 1 \cite{dummy}

To render the references in the slides, run the following command to generate beamer.aux which allows you to inject references to it.

pdflatex beamer.tex

Then, run the following command to generate beamer.bbl.

biber beamer

Run pdflatex beamer.tex 2 more times to first generate the references page, and then the inline citation. Once you have beamer.aux, you will only need to first run biber beamer and then pdflatex beamer.tex to compile it.

Full sample

\documentclass[aspectratio=169]{beamer}
\usetheme{Madrid}
\setbeamertemplate{footline}{}
\setbeamertemplate{navigation symbols}{}
\usepackage[backend=biber, style=vancouver]{biblatex}
\addbibresource{citation.bib}

\title{Presentation Title}
\author{Your Name}
\institute{Your Institute}
\date{\today}

\begin{document}

\begin{frame}
\titlepage
\end{frame}


\begin{frame}{Lists}

\begin{itemize}
\item Point 1 \cite{dummy}
\item Point 2
\item Point 3
\end{itemize}

\begin{enumerate}
\item Number 1
\item Number 2
\item Number 3
\end{enumerate}

\end{frame}


\begin{frame}{Maths equations}

\begin{block}{Important equation}
$$ \int_1^e \frac{\mathrm{d}x}{x} = 1 $$
\end{block}

\begin{alertblock}{Even more important equation}
$$ e = \sum_{n=0}^\infty \frac{1}{n!} = 1 + \frac{1}{1} + \frac{1}{1 \cdot 2} + \frac{1}{1 \cdot 2 \cdot 3} + \cdots $$
\end{alertblock}

\end{frame}


\begin{frame}{Pictures}

\begin{figure}
\centering
\includegraphics[width=0.5\textwidth]{./latex.jpeg}
\caption{LaTeX logo}
\end{figure}

\begin{figure}
\centering
\includegraphics[trim={1.5cm 3cm 3cm 1.2cm}, clip, width=0.5\textwidth]{./latex.jpeg}
\caption{Cropped LaTeX logo}
\end{figure}

\end{frame}


\begin{frame}{References}
\printbibliography
\end{frame}

\end{document}

Creating Disposable VM with KVM

Sun, 15 Mar 2026 14:38:23 +0000

Use case

Every changes in disposable vm (including changes in the root directory) will be gone in the next boot. Therefore, it is very useful when you want to test new software (potentially buggy) and don’t want to mess with your actual computer.

Creating disposable vm

Template vm

First, create a vm normally, this serves as a base of the disposable vm. Install software you want to use, and configure them to your preference. The software and the configurations will be available every time you boot up the disposable vm and are immutable from the disposable vm.

Disposable vm

Open up virt manager, right click on the template vm, then click clone. In the pop-up window, uncheck cloning storage. After this, go to the edit menu, and click preferences. Enable XML editing in the pop-up window.

Open up the details page of the disposable vm in view menu. Under virtual disk, open XML tab, and add <transient/>. Voilà! This is it, you can now turn on the disposable vm and try all the fun stuff like sudo rm /usr/bin/rm without any consequences.

vm without internet connection

Sometimes, you may want to write some sensitive files to your computer, and you may not want the computer to connect to internet at all. This can also be easily achieved by removing the internet hardware of the vm. Now, when you turn on this vm, you literally can’t connect to internet in any way.

Updating disposable vm

Updating the disposable vm is very easy. Just update the template vm, all the changes will be available on the disposable vm. Remember to keep your footprint minimal in template vm, and do all the testings in the disposable vm.