Interesting sites
regular expressions
https://regex101.com/
https://www.debuggex.com/
https://regexr.com/
https://www.regextester.com/
https://www.regexpal.com/
https://extendsclass.com/regex-tester.html (this site offers online tools for developers, including a regex writing help service.)
linux commands
https://juliend.github.io/linux-cheatsheet/
https://crontab.guru/
https://chmod-calculator.com/
https://chmodcommand.com/
https://wintelguy.com/umask-calc.pl/
https://www.webune.com/forums/umask-calculator.html/
https://www.cs.ait.ac.th/.../ch03_03.htm
https://www.debian.org/.../index.fr.html
https://wiki.debian-fr.xyz/Securite
https://www.remipoignon.fr/securiser-son-serveur-linux/
https://www.remipoignon.fr/authentification-ssh-par-cle-privee/
https://www.ssi.gouv.fr/...systeme-gnulinux/
https://www.alsacreations.com...iptables.html
https://fr-wiki.ikoula.com/...Debian
https://docs.ovh.com/...serveur-dedie/
https://documentation.online.net/fr/dedicated-server/tutorials/security/configure-firewall
https://documentation.online.net/...install-configure-fail2ban
https://documentation.online.net/...install-configure-logwatch
https://debian-handbook.info/browse/fr-FR/stable/
https://wiki.debian.org/fr/DontBreakDebian
https://www.debian-fr.org/.../71023
security
https://blog.peterruppel.de/ed25519-for-ssh/
https://www.gniibe.org/.../keygen-25519.html
https://carmagnole.ovh/...-ed25519/
https://asecuritysite.com/ecc/openpgp01
https://metacode.biz/openpgp/openpgp4fpr
git commands
https://rogerdudler.github.io/git-guide/index.html
git flow commands
https://danielkummer.github.io/git-flow-cheatsheet/index.fr_FR.html
open source software directories
https://framasoft.org
https://sill.etalab.gouv.fr
https://comptoir-du-libre.org/fr/softwares
https://enacit.epfl.ch/logiciel-libre/
https://sourceforge.net/
https://european-alternatives.eu/
https://prism-break.org/
images and photos under the Creative Commons Zero license
https://openclipart.org/
https://www.pexels.com/
rosettacode : to compare the syntax of different programming languages
https://rosettacode.org
scriptol : history, presentation and comparison of programming languages
https://www.scriptol.fr/
https://www.scriptol.fr/programmation/langages-populaires.php
developpez.com : news site in the field of application development
https://www.developpez.com/
awesome
https://github.com/sindresorhus/awesome
awesome-python
https://github.com/vinta/awesome-python
awesome-dotnet
https://github.com/quozd/awesome-dotnet
awesome-R
https://github.com/qinwf/awesome-R
awesome-go
https://github.com/avelino/awesome-go
awesome-go-storage
https://github.com/gostor/awesome-go-storage
Go
https://www.golang-book.com/books/intro
https://gobyexample.com
https://go.developpez.com/tutoriels/go-par-l-exemple/
https://www.programming-books.io/essential/go/
Free online IIR design tool
http://t-filter.engineerjs.com/