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For finding out which ports of the machine are being opening by which services, I used:

netstat -tulpn

I checked the man page for netstat command, but I found nothing about this option. What's the meaning of the -tulpn option?

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    Did you read man netstat? – pa4080 Mar 16 '20 at 10:01
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    Read the man page manpages.ubuntu.com/manpages/netstat.8.html , one option at a time (-t, -u, -l, -p, -n) – FedonKadifeli Mar 16 '20 at 10:01
  • You can also visit Netstat command in Linux, which has snaps showing what you'll get as output for different param. – Novice Mar 16 '20 at 10:07
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    @FedonKadifeli Thank you. I thought that it's an individual option, not mixed. Please add an answer then I accept and upvote. – M. Rostami Mar 16 '20 at 10:07
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    @M.Rostami Not all Linux / UNIX utilities follow this pattern, but a general rule of thumb is that the long form of an option starts with a double dash (e.g., --tcp or --udp), and the short form starts with a single dash (e.g, -t or -u.) And as you have just discovered, some commands allow the short form options to be combined. There are plenty of exceptions, though! Off the top of my head I can think of tar which accepts options with or without the dash (e.g., tar xvzf foo.tgz is the same as tar -xvzf foo.tgz), so always check info or man pages if in doubt. – hotwebmatter Mar 17 '20 at 3:30
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As answered in https://serverfault.com/questions/387935/whats-the-difference-betwen-the-single-dash-and-double-dash-flags-on-shell-comm, in a Linux command line;

A single hyphen can be followed by multiple single-character flags.

A double hyphen prefixes a single multi-character option.

If you look at netstat man page, you will see that (Note that, netstat -tulpn is equivalent to netstat -t -u -l -p -n):

--tcp|-t

--udp|-u

-l, --listening
   Show only listening sockets.  (These are omitted by default.)

-p, --program
   Show the PID and name of the program to which each socket belongs.

--numeric, -n
   Show numerical addresses instead of trying to determine symbolic host, port or user names.

So, your command is equivalent to the following long form also:

netstat --tcp --udp --listening --program --numeric
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In addition to man netstat you can type info netstat to get a shorter summary and longer explanation:

NETSTAT(8)                      Linux Programmer's Manual                      NETSTAT(8)

NAME
       netstat  -  Print  network connections, routing tables, interface statistics, mas‐
       querade connections, and multicast memberships

SYNOPSIS
       netstat [address_family_options] [--tcp|-t] [--udp|-u] [--raw|-w] [--listening|-l]
       [--all|-a]  [--numeric|-n]  [--numeric-hosts]  [--numeric-ports] [--numeric-users]
       [--symbolic|-N] [--extend|-e[--extend|-e]]  [--timers|-o]  [--program|-p]  [--ver‐
       bose|-v] [--continuous|-c]

For -t -u -l -p -n above you see --tcp, --udp, --listen, --program and --numeric without having to scroll.

Scrolling down you can see verbose explanations.

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Looks like you were looking for the man page for netstat(8).

Linux.die.net has man pages for seemingly all Linux tools. See below the man page for netstat(8) which should answer your question.

https://linux.die.net/man/8/netstat

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    It is better to point to Ubuntu man pages: manpages.ubuntu.com/manpages/netstat.8.html – FedonKadifeli Mar 16 '20 at 10:03
  • Indeed it probably is, but I still answered the question. – aa2397 Mar 16 '20 at 10:04
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    It is not about who wins the right answer tag, it is about helping community by doing so. – Novice Mar 16 '20 at 10:10
  • As I mentioned, I checked the man page for netstat command. However, thank you for your response. – M. Rostami Mar 16 '20 at 10:41

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