Linux Miscellaneous Thingies

Screenshots

• sudo apt install gnome-screenshot
• Open keyboard shortcuts -> add custom shortcuts
• I use ctrl-alt-4 to enable gnome-screenshot -a

Window Focus

On my Ubuntu 22.04 system, when trying to download webpages / images from Chrome / Firefox, I noticed that the download dialog box is not focused in the first place. After searching for auto-focusing, I came up with a sub-optimal solution: auto-shift to window that the mouse currently hovers on.

1. sudo apt install gnome-tweaks
2. Press the Super (Windows) key and type Tweaks, then press Enter.
3. Choose focus on hover,
   • If you don’t want to see the other window that’s out of focus already, choose “Raise Window When Focused”
4. Press Alt + F2, type r, and press Enter. This restarts the GNOME Shell without logging out.

File Differences

• diff shows which files are different and which files show in both places

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diff -qr filter_projects/ Fun_Projects/filter_projects/
Only in filter_projects/: build_project.sh
Files filter_projects/face_tracker/face_tracker.cpp and Fun_Projects/filter_projects/face_tracker/face_tracker.cpp differ
Files filter_projects/face_tracker/face_tracker.hpp and Fun_Projects/filter_projects/face_tracker/face_tracker.hpp differ

• If in a git repo, see when the directory was last modified:

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git log -1 --format="%ci" -- filter_projects/
2022-03-31 12:07:13 -0500

• -1: Limits the output to the most recent commit.
• --format="%ci": Formats the output to display the commit date in ISO format.

File Formats

• AppImage: a linux file format that’s directly runnable without unzipping, debian instalation, etc. This might require libfuse to run: sudo apt install libfuse
  • libfuse is a library that uses FUSE (Filesystem in Userspace). We can create and manage file systems in the user space instead of the kernel space.
    • It can create a virtual filesystem, like AppImage, where it’s mounted as a virtual filesystem.
    • It works with SSHFS, mounting remote file systems via SSH

System Commands

• hostname: host machine name in “USERNAME@HOSTNAME”

Hot keys

• If to swap the function of Fn when hitting F2 and mute, Fn lock is the way to go. Just do Esc + Fn

Hardware Interfaces

gpiochip Interface

The gpiochip interface in Linux represents a GPIO (General Purpose Input/Output) controller. It allows users to interact with GPIO pins on embedded systems, SoCs, or microcontrollers.

• The active-high state means that a logical 1 corresponds to a high voltage level.
• Some GPIOs are configured as inputs, while others are outputs.
• gpioinfo to check such info
• ttyACMx is for serial-USB interface. ttySx is for native Hardware-Serial interface

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/dev/ttyACM0  (Main USB Serial - SERIAL_DEBUG)

Linux Device Tree

Linux Device Tree is a hardware description of the layout of hardware components in embedded linux systems. (Buses, peripherals, memory). With it, we don’t need to modify the kernel source code.

• Device tree source (DTS) files define the config, and are compiled into device tree blobs (DTB), which the kernel load at boot.
• A device tree snippet:

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pinctrl_uart3: uart3grp {
    fsl,pins = <
        MX8MP_IOMUXC_UART3_RXD__UART3_DCE_RX  0x40
        MX8MP_IOMUXC_UART3_TXD__UART3_DCE_TX  0x40
    >;
};

• Many embedded processors have multiplexed I/O pins that are muxed (multiplexed) to a specific functionailty, like I2C, SPI, UART, GPIO
  • MX8MP_IOMUXC_UART3_RXD__UART3_DCE_RX configures UART3
  • MX8MP_IOMUXC_UART3_TXD__UART3_DCE_TX: configures UART3 TX
  • 0x40 is a pad control register configuration for electrical characteristics like pull-up/down, drive strength, etc.
  • UART2 and UART3 have the same pad control values. They don’t necessarily cause issues as long as they are mapped to different pins.
• To verify serial being successfully configured at runtime:

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    ls -l /dev/serial*
    dmesg | grep tty
    cat /proc/tty/drivers
  

  • check for ttyUSBX and ttymxcX
• To check pins at runtime: cat /sys/kernel/debug/pinctrl/*/pins | grep UART

dmesg (diagnostic message) prints kernel ring buffer that contains system logs for: boot process, hardware detection (UART, I2C, SPI, etc.), driver loading, kernel errors.

UART Setup

• UART communication is typically a point-to-point protocol, meaning only one device should transmit (TX) while another listens (RX). However, UART is Push-Pull (actively driving line High/Low), not open-drain (like I2C), this could result in electrical conflicts.
  • Indeterminate bus values (data corruption)
  • High current draw (power dissipation due to short circuits?), and possible hardware damage
• Setting a processor pin to GPIO is safer, because it doesn’t drive the bus. Instead it uses pull-up resistors or open-drain config.
  • When the GPIO pin is set to input, it has high impedance.

Tmux

• Ctrl + b, w: all windows
• Ctrl + b, n: next window, Ctrl + b, p: previous window, Ctrl + b, <number>: numbered window
• Ctrl + b, arrow
• Horizontal split: Ctrl + b, ", vertical split: Ctrl + b, %
• scroll up: c-b [

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tmux attach-session -t <WINDOW_NUM_OPTIONAL>

Checksum

Most zip tools have checksum check built in, even though checksum is not part of the zip format

ffmpeg

• cut a video from 8th second to 19th second:

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    ffmpeg -ss 8 -i ndt_sputnik2-2025-04-19_11.37.57.mp4 -to 19 -c copy cut_output2.mp4 
  

• Make the video 3x faster:

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    ffmpeg -i cut_output2.mp4 -filter:v "setpts=0.33*PTS" -an cut_output2x.mp4    
  

FireFox & Chrome & PDF Ops

• To switch tabs in Firefox, follow this page

• Firefox browser may have trouble actually saving PDFs. I’ve had cases where hitting the save button there does not actually save.

  • Chrome is safe
  • qoppa PDF viewer 2024 is actually great! No watermark, full PDF editor with signature, textbox, etc.
  • For merging pdfs pdfunite file1.pdf file2.pdf merged.pdf pdfunite omits some javascript fields. Use pdftk in1.pdf in2.pdf cat output merged.pdf
  • If a pdf is encrypted, one can use the “printer” option to create an unencrypted copy

Power Modes

Balanced (or Default):

• Ubuntu dynamically adjusts CPU frequency based on load, but still biases toward saving battery when idle.
  • A Hard Disk Drive rotates to access all bits on it
    • A bit is a small patch on HDD that has a magnetic field alignment

Power Saving:

• Lowers screen brightness more aggressively, spins down disks sooner, and keeps CPU frequencies lower overall.
• Useful if you’re on battery and want maximum runtime (e.g., traveling, remote work).
• You’ll notice longer battery life, but heavier tasks (e.g., compiling code, video encoding, gaming) will run noticeably slower.

Performance (or High Performance):

• Keeps CPU frequencies higher, avoids throttling, and keeps sensors/power management features (like autosuspend) more relaxed.
• Need to be plugged in through power cable because USB-C doesn’t have enough current

CPU Frequency “Governors”

The ter, throttling generally refers to intentionally slowing down a component—usually the CPU or GPU—in order to:

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