If you wish to see which ports are open on your Linux system, perhaps to check your configuration, you can use the nmap tool. It's a powerful tool, but we'll focus on just this simple task.
If you don't have nmap, first install it. For example, on Ubuntu just run sudo apt-get install nmap. On Fedora it should be sudo yum install nmap. On Arch it should be sudo pacman -Sy nmap.
Once you've got nmap just run this simple command. Note that we're running it with superuser privileges (sudo), which is necessary.
$ nmap localhost
Your output may look something like this:
Starting Nmap 6.40 ( http://nmap.org ) at 2014-11-26 23:56 CET Nmap scan report for localhost (127.0.0.1) Host is up (0.0089s latency). Other addresses for localhost (not scanned): 127.0.0.1 Not shown: 994 closed ports PORT STATE SERVICE 21/tcp open ftp 22/tcp open ssh 53/tcp open domain 80/tcp open http 443/tcp open https 3306/tcp open mysql
So it shows you the open port numbers and the service that is using each. The above is pretty standard stuff. If you don't see what you expected you should check your configuration.
If you'd like to do more with nmap you can explore the nmap built in documentation by running man nmap, which contains a breadth of information.
See Also
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