Linux: Find Port Number Used by Process with CAT and ECHO

In this post, I'll show how to find the port number used by a process when you know its PID, using two native Linux commands: cat and echo in bash.

1. Pick a Random PID

List the contents of /proc to see running process IDs:

ls /proc

Example output:

1   33   asound   cgroups   consoles   devices   driver   ...

2. Print Process TCP Connection Info

Display TCP connections for a specific process by replacing $PID with your process ID:

cat /proc/$PID/net/tcp

For example, with PID 33:

cat /proc/33/net/tcp

Sample output:

sl  local_address rem_address   st tx_queue rx_queue tr tm->when retrnsmt   uid  timeout inode
0: 00000000:0050 00000000:0000 0A 00000000:00000000 00:00000000 00000000     0        0 2078133 1 000000002283b263 100 0 0 10 0

Look for the local_address column. The port number is the value after the colon (:), here it's 0050.

3. Convert the Port Number from Hex to Decimal

The port number (0050) is in hexadecimal. Convert it to decimal with:

echo $((16#0050))

Output:

80

Now you know how to find which port a process is using by its PID!