Ever needed to test firewalls but didn't have netcat, telnet or FTP?

Enter /dev/tcp, your new best friend. /dev/tcp/(hostname)/(port) is a virtual device that bash can use to talk to TCP ports

First, exec sets up a redirect for /dev/tcp/$server/$host to file descriptor 5.

Then, as per some excellent feedback from @flatcap, we launch a redirect from file descriptor 5 to STDOUT (implied by

Finally, when the second cat dies (the connection is closed), we clean up the file descriptor with 'exec 5>&-'.

This one-liner opens a connection on a port to a server and lets you read and write to it from the terminal.

Example test

replace time.nist.gov with 127.0.0.1

replace 13 with 22

You should see a response from your SSH server:

SSH-2.0-OpenSSH_6.6.1p1 Ubuntu-2ubuntu2

Typing TEST should net you a Protocol Mismatch:

TEST

Protocol mismatch.

It can be used for FTP, HTTP, NTP, or netcat listening on a port (makes for a simple chat client!)

Replace /tcp/ with /udp/ to use UDP instead.