Linux

entr is a utility to automatically restart a script if one or more files change.

Installing it on Ubuntu as is as easy as

sudo apt -y install entr

This command will move all *.ics files from the current directory and

mkdir -p ICS
find . -name "*.ics" -type f -exec mv --backup=numbered -v {} ICS/ \;

Due to --backup=numbered, files with the same name won’t overwrite anything but instead mv will append a number such as ~2~ to the filename.

This command will run sha256sum recursively on all files in a directory:

find . -type f -print0 | xargs -0 sha256sum

You can also pipe it into a file

find . -type f -print0 | xargs -0 sha256sum > recursive-sha256sum.txt

This command will look for a line starting with volumes: and remove all lines after said line from the file (including the line starting with volumes: itself).

sed -i -e '/^volumes:/,$d' myfile

The command edits the file in-place (-i), no copy (or backup) of the file will be created.

feh is a modern image viewer which you can parameterize on the command line.

Use the -z flag (or --randomize) to randomize the order of the images.

This example selects, given a list of JSON entries, all entries where .service equals "mongodb" (i.e. {"service": "mongo"}:

jq 'map(select(.service=="mongo"))'

In order to install the Dropbear SSH key converter dropbearconvert on recent Ubuntu versions, run

sudo apt -y install dropbear-bin

Problem:

If you want to generate a key file using ssh-keygen, it will prompt you for a key password:

$ ssh-keygen -t ed25519 -f id_ed25519´
Generating public/private ed25519 key pair.
Enter passphrase (empty for no passphrase): 

For scripts and automated deployments, however, you want to avoid the prompt and disable the password encryption of the key.

Solution:

Use -N "" to set an empty password:

ssh-keygen -t ed25519 -f id_ed25519 -N ""

For example:

$ ssh-keygen -t ed25519 -f id_ed25519 -N ""
Generating public/private ed25519 key pair.
Your identification has been saved in id_ed25519
Your public key has been saved in id_ed25519.pub
The key fingerprint is:
SHA256:o/PVrBXpccLOjrVi6vfGWcmNtoubpINfrMxbz4EXtrU [email protected]
The key's randomart image is:
+--[ED25519 256]--+
|                 |
|                 |
|                 |
|           . .   |
|        S   *.++.|
|       . . B B*+o|
|      o  ...#++E |
|       o.+=%+B.. |
|       .+=X*B.+. |
+----[SHA256]-----+

On the command line, the - character is used to indicate a short form argument, which is a single-letter argument that can be specified using a single hyphen and the letter of the argument. For example, the -h argument is commonly used to show the help message for a command, and the -v argument is commonly used to enable verbose output. You can only use the single hyphen (-) when the argument consists of a single letter.

Examples where single hyphen can be used: -h, -Rand-f, but you can not use single hyphens for multi-letter arguments such as -verbose or -help.

The -- characters are used to indicate a long form argument, which is a multi-letter argument that can be specified using a double hyphen and the name of the argument. For example, the --help argument is commonly used to show the help message for a command, and the --verbose argument is commonly used to enable verbose output.

Examples where double hyphens can be used: --verbose, --server and --help, but you can’t use them for single-letter arguments like --h or --R 

Examples of how to use single or double hyphens:

# Show the help message for a command using the -h short form argument
my_command -h

# Enable verbose output for a command using the --verbose long form argument
my_command --verbose

# Combine multiple arguments
my_command --verbose --server https://techoverflow.net -f 0.1