Linux basics and commands: shell syntax query:

www.linux6.com www.tutorialspoint.com/linux_admin…

Regular expressions:

www.regexr.com/

Cron test:

cron.schlitt.info/

Git learning:

git-scm.com/book/zh/v2

SQL command query:

www.tutorialspoint.com/sql/sql-ope…

Javascript/CSS/HTML learning:

www.w3school.com.cn/index.html

Json online formatting: Image resizing:

www.sojson.com/

Universal Programming 3 Swordsman:

google

Stackoverflow

GitHub