1. Java

https://www.javacjava.com/Tutorials.aspx

2. Perl

https://www.perl.org

3. mkdir -p {usr,sbin}/{share,opt} , root@debian:~# cp /etc/postfix/main.cf{,.backup}

4. Iptables Userguide

 – To remove iptables:

   # iptables -F

 – To print iptables rules:

 # iptables -L –line-numbers

Chain INPUT (policy ACCEPT)
num target prot opt source destination
1 f2b-postfix tcp — anywhere anywhere multiport dports http,https,smtp,submission,pop3,pop3s,imap2,imaps,sieve
2 ACCEPT all — anywhere anywhere state RELATED,ESTABLISHED
3 ACCEPT all — anywhere anywhere state RELATED,ESTABLISHED
4 ACCEPT tcp — anywhere anywhere tcp dpt:http state NEW
5 ACCEPT tcp — anywhere anywhere tcp dpt:https state NEW
6 ACCEPT tcp — anywhere anywhere tcp dpt:2205 state NEW
7 ACCEPT tcp — anywhere anywhere tcp dpt:smtp state NEW
8 DROP tcp — anywhere anywhere tcp dpt:imap2 state NEW
9 ACCEPT tcp — anywhere anywhere tcp dpt:submission state NEW
10 DROP icmp — anywhere anywhere icmp echo-request

#iptables -D <rule_name> <line_number>

# iptables -D INPUT 10 (to remove line number 10 – DROP icmp — anywhere anyway icmp echo-request)

5. https://stackoverflow.com/questions/4331092/finding-all-combinations-of-javascript-array-values

6. https://www.cyberciti.biz/faq/install-kvm-server-debian-linux-9-headless-server/

How to install LaTeX extension:

https://docs.fedoraproject.org/en-US/neurofedora/latex/