Unix & Linux

Notes related to Unix-based operating systems.

1. Resources

• The Cyber Plumber’s Handbook - The definitive guide to Secure Shell (SSH) tunneling, port redirection, and bending traffic like a boss

2. Keeping some script/program alive

If you want to keep some script/program alive (e.g. a node.js script) you need to use crontab.

The command

crontab -e

opens a crontab jobs file in the default editor that contains all the cron jobs that are being run for current user.

You can add a reference to a bash script that runs desired script/program or normal reference to the script/program that needs to be run. You can set the time conditions confining the moments in which desired script/program will be run.

It is advised that the program is shut down before running it again.

3. ANSI Text Attributes

Esc[Value;...;Valuem

3.1. Text effects

Using \033[38;5;206m you’re accessing the 256-colour-palette. Replace 38 with 48 and you’ll be altering the background colour.

To get even more colours use \033[38;2;R;G;Bm to use full RGB spectrum by replacing R, G and B with values from range of 0-255.

Source

4. Customizing bash prompt

When configuring the bash prompt you would use ANSI escape sequences to enhance the text formatting to make it more visually appealing. However, inserting an ANSI escape sequence is a lot of characters that amount to special usage and no physical characters. That’s why we need to mark these sequences as zero-length to prevent the bash from being confused where does the actual prompt end.
Every time you insert an ANSI escape sequence wrap it inside \[«ANSI escape sequence»\].

Source

5. Redirecting stdout to stderr

To redirect stdout to stderr just add >&2 echo 'error at the end of the command you want to execute.

6. Downloading a whole website using wget

Using wget you can download a whole webpage with all the resources that are referenced through links on that webpage. It can be a very useful tool for archiving websites.

Executing

wget -rpl1 --tries=3 https://example.com

will download the webpage and all PDFs, images and other resources that appear on that webpage to a folder called example.com.

There is a possibility that above command will not work on first try. For example the HTML file containing the markup may have been downloaded, but the resources that appear on that webpage couldn’t have been downloaded. Then, using this website in HTML (or otherwise) form we can download the rest of the resources.

wget -rpl1 --tries=3 --base=https://example.com/subpage --force-html --relative -i example.com/subpage/index.html

Executing above command will download all resources that appear on this subpage. All relative links will be preceded with the URL provided with the --base option.

7. Permanently setting the DNS server

First, we need to install the resolvconf package:

sudo apt update
sudo apt install resolvconf

Now, let’s add the IP address of the desired DNS server.
Edit following file:

sudo nvim /etc/resolvconf/resolv.conf.d/head

adding line below to make 1.1.1.1 the DNS server:

nameserver 1.1.1.1

Source

8. Fixing broken packages (downloaded from an external PPA) by forcefully overwriting apt packages

Sometimes adding external PPAs results in broken packages and the infamous error:

E: Unmet dependencies. Try 'apt --fix-broken install' with no packages (or specify a solution).

One solution would be to remove that PPA

sudo add-apt-repository --remove ‹PPA›

and to forcefully overwrite the broken packages from software sources that were already present in the system.

sudo apt install -o Dpkg::Options::="--force-overwrite" --fix-broken

Source

9. Useful PDF tools

1. PDF to PNG: bash pdftoppm pdf.pdf result -png Source

2. Merge two PDFs side-by-side: bash pdfjam one.pdf two.pdf --nup 2x1 --landscape --outfile merged.pdf Source

10. Pass URL/ file argument to program defined by a .desktop file

Programs can accept links/ paths to remote/ local files as arguments. The .desktop files handles that by adding %u or %U at the end of the Exec parameter.

• %u — only one URL can be passed
• %U — multiple URLs can be passed

Usage example:

[Desktop Entry]
Name=Google Chrome
Exec=/usr/bin/google-chrome-stable %U
Type=Application
Categories=Network;WebBrowser;
# […]

Source: Desktop specification — The Exec key

11. Routing VLC audio output to a specific PulseAudio sink

To force VLC to play audio to a device different from the default one run it with temporarily set Pulse sink variable like so:

PULSE_SINK=$sink_id cvlc file.mp3

Source: Answer on SuperUser

12. Pretty printing JSON data

Example:

echo '{
    "items": [
        {
            "name": "One",
            "description": "the one and only",
            "labels": [
                "Not funny",
                "A label"
            ]
        },
        {
            "name": "Two",
            "description": "the sequel",
            "labels": [
                "Yup"
            ]
        }
    ]
}' \
| jq -j '.items[] | .name, " (", .description, ") [ ", (.labels | join(", ")), " ]\n"'

Sources: self, Answer on Unix StackExchange

13. Compressing images

convert -strip -interlace Plane -gaussian-blur 0.05 -quality 85% source.jpg result.jpg

Using convert from the imagemagick package you can cut down on size for image files.

Source: Answer on StackOverflow

14. Nvidia + Intel flickering

To resolve flickering for laptops with Nvidia+Intel graphics turn off PSR.

PSR is Intel’s Panel Self Refresh setting which caches some frames to display them later. This functionality may cause these flickering problems because of some memory leaking from the frame buffer, causing the display to show unsynced content.

To verify PSR is turned on run

cat /sys/kernel/debug/dri/0/i915_edp_psr_status

with root privileges.

If it is indeed turned on, modify /etc/default/grub:

-GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
+GRUB_CMDLINE_LINUX_DEFAULT="quiet splash i915.enable_psr=0"

and run

update-grub

with root privileges to turn this feature off and reboot the computer to apply the changes.

Source: Lj Miranda