Command Line Breakdown - Ports in Use on Ubuntu Server

There are plenty of material out in the web-o-sphere to determine what is using which network port.

I thought I'd take a command I recently used and break it down in order to help newer Linux Admins/Users understand the command and structure.

I used: sudo lsof -i -P -n | grep LISTEN

We'll break this down but first the output looks like this:

sshd     3420     root    3u  IPv4  15383      0t0  TCP *:22 (LISTEN)
sshd     3420     root    4u  IPv6  15392      0t0  TCP *:22 (LISTEN)
node     3456    user01  16u  IPv4  15723      0t0  TCP 127.0.0.1:2368 (LISTEN)
nginx   19676     root    7u  IPv4 323846      0t0  TCP *:80 (LISTEN)
nginx   19676     root    8u  IPv6 323847      0t0  TCP *:80 (LISTEN)
[...]

I wanted to know which ports were bound on this server. In the example output above, I can see that 22, 2368 and 80 are all being used as a listening port -- it's waiting for an inbound network connection.

**sudo** lsof -i -P -n | grep LISTEN

The sudo command which you've probably used before is "super user do"; it takes the user you are logged in as and elevates you to "root" type user permissions for a short amount of time.

sudo **lsof** -i -P -n | grep LISTEN

lsof is the program we are running which simply returns a list of open files.

Linux treats network connections, as well as others, as open files.

sudo lsof **-i -P -n** | grep LISTEN

The -i -P -n are the flags we are passing to lsof; this is probably familiar to you from other CLI work. Linux programs tend to have two types of flags: - and --. In our case, the flags used break down as:

  • -i: Requests a list of IP sockets.
  • -P: Don't resolve the port names, instead list the port number. Note the capital "P"; some programs in Linux use case sensitive flags which mean different things.
  • -n: Don't resolve the DNS (I.E. Domain). In my case, theOpenSourceU.org.

sudo lsof -i -P -n **|** grep LISTEN

The pipe (|) warrants a section of it's own as you'll find in various Linux commands and such. I feel it's important to understand at a high-level.

It takes the output of the left operation (lsof -i ...) and gives it to the program on the right -- in this case grep which allows us to use grep as a filter which in itself is common but not the only purpose.

The output of lsof is never actually seen; instead, it's given to grep because of the | and we only see the output of that.

sudo lsof -i -P -n | **grep LISTEN**

The grep is a regular expression tool which searches input for the given pattern effectively altering the output. In this case, the pattern is simply LISTEN. The output is given on the console.

Frank Villasenor

Frank Villasenor

Owner and principal author of this site. Professional Engineering Lead, Software Engineer and Architect working from the Chicagoland area for healthcare SaaS company.
Chicago, IL