Linux¶

Change hostname¶

Change permanently with

sudo hostnamectl set-hostname your-new-hostname

Check

hostnamectl

Reboot is required to show it in the prompt

Message Of The Day (MOTD)¶

Add a new message¶

sudo vi /etc/update-motd.d/99-custom

Write something like that:

#!/bin/bash
echo """
SOME TEXT or ASCII Art
"""

Change permissions

sudo chmod 755 /etc/update-motd.d/99-custom

Logout login to check

APT packet research¶

When you want to execute a command, but you don't remind the name of the packet who contain this command you can try:

Simply run the command¶

On Ubuntu, it will help you with the package to install:

$ traceroute
Command 'traceroute' not found, but can be installed with:
sudo apt install inetutils-traceroute  # version 2:2.6-1ubuntu3, or
sudo apt install traceroute            # version 1:2.1.6-1

Search using apt-file¶

Install apt-file

sudo apt install apt-file

Update list

sudo apt-file update

If you know exactly the path you can try:
Example for the dig command

apt-file find /usr/bin/dig -F

-F will search for this exact pattern

else you can try:

apt-file find '/dig$' -x

That will search all packets that contain a script named dig -x specify that pattern is a regex.

Example output:

apt-file find -x /dig$
    apparmor: /etc/apparmor.d/dig
    bind9-dnsutils: /usr/bin/dig
    epic4: /usr/share/epic4/script/dig
    epic4-help: /usr/share/epic4/help/8_Scripts/dig

Here we can guess that to correct one is bind9-dnsutils

Journalctl¶

A CLI tool to manage logs.

To see all logs

journalctl

To see only 20 last logs

journalctl -n 20

To follow logs output

journalctl -f

To see kernel logs only

journalctl -k

To see specific process logs (example for ssh service)

journalctl -u ssh.service

To filter by time:

journalctl --since "2023-10-27 10:00:00" --until "2023-10-27 11:00:00"

To filter by log level:

journalctl -p 1 -b

-b: To see error only until last reboot
-p 1: To see log level 1 and above

There are 8 log levels, from the most critical to the most informative

Log level	Name
0	`emerg`
1	`alert`
2	`crit`
3	`err`
4	`warning`
5	`notice`
6	`info`
7	`debug`

Misc¶

tldr¶

Get useful command usage example

Install

sudo snap install tldr

Get tar usage example:

tldr tar

sudoedit¶

To edit a root file use sudoedit instead of sudu vi. It allows you to still have your user .vimrc config.

Files and folders size¶

Get size of a folder

du -sh a_folder/

Get files and folders size with a TUI

Install

sudo apt install ncdu

Use (press q to quit)

ncdu .