Interesting sites

regular expressions
https://regex101.com/
https://www.debuggex.com/
http://www.rexv.org/
http://regexr.com/
http://www.regextester.com/
http://www.regexpal.com/
http://www.rubular.com/
http://regex.larsolavtorvik.com/
http://www.nregex.com/
https://extendsclass.com/regex-tester.html (this site offers online tools for developers, including a regex writing help service.)

linux commands
http://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/
http://www.cs.ait.ac.th/~on/O/oreilly/unix/lrnunix/ch03_03.htm
https://www.debian.org/doc/manuals/securing-debian-manual/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/administration/guide/recommandations-de-securite-relatives-a-un-systeme-gnulinux/
https://www.alsacreations.com/tuto/lire/622-Securite-firewall-iptables.html 
https://fr-wiki.ikoula.com/fr/S%C3%A9curiser_sa_machine_Debian
https://docs.ovh.com/fr/dedicated/securiser-un-serveur-dedie/
https://documentation.online.net/fr/dedicated-server/tutorials/security/configure-firewall
https://documentation.online.net/fr/dedicated-server/tutorials/security/install-configure-fail2ban
https://documentation.online.net/fr/dedicated-server/tutorials/security/install-configure-logwatch
https://debian-handbook.info/browse/fr-FR/stable/
https://wiki.debian.org/fr/DontBreakDebian
https://www.debian-fr.org/t/documentation-debian-pour-les-nouveaux-et-les-autres/71023

git commands
http://rogerdudler.github.io/git-guide/index.html

git flow commands
http://danielkummer.github.io/git-flow-cheatsheet/index.fr_FR.html

open source software directories
https://framasoft.org/rubrique2.html
https://sill.etalab.gouv.fr
https://comptoir-du-libre.org/fr/softwares
http://enacit.epfl.ch/logiciel-libre/
https://sourceforge.net/
https://prism-break.org/

 
find alternatives to a software
https://alternativeto.net/

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/