how to assign terminal variable from previous command's output

Question

I'm very very new to the terminal script world. Here's what I want to do:

1) Find out the process that's using a given port (8000 in this case)
2) Kill that process

Pretty simple. I can do it manually using:

lsof -i tcp:8000 -- get the PID of what's using the port
kill -9 $PID -- terminate the app using the port

For reference, here's exactly what gets returned when using lsof -i tcp:8000

COMMAND   PID       USER   FD   TYPE             DEVICE SIZE/OFF NODE NAME
php     94735     MyUser    5u  IPv6 0x9fbd127eb623aacf      0t0  TCP localhost:irdmi (LISTEN)

Here's my problem: how do I capture the PID value from lsof -i tcp:8000 so that I can use that variable for the next command? I know how to create variable that I assign... just not ones that are dynamically made.

Answer 1

The thing you’re looking for is called command substitution. It lets you treat the output of a command as input to the shell.

For example:

$ mydate="$(date)"
$ echo "${mydate}"
Mon 24 Feb 2014 22:45:24 MST

It’s also possible to use `backticks` instead of the dollar sign and parentheses, but most shell style guides recommend avoiding that.

In your case, you probably want to do something like this:

$ PID="$(lsof -i tcp:8000 | grep TCP | awk '{print $2}')"
$ kill $PID