Running multiple servers in a single Bash script#

I spotted this script that starts the opendream appication running both a Python uvicorn server and a npm run start script and it intrigued me - was it this easy to have a single Bash script run two servers? They were both started in the background with &, like this:

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#!/bin/bash
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python3 -m venv venv
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source venv/bin/activate
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pip install --upgrade pip
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pip install -r requirements.txt
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uvicorn opendream.server:app --reload &
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cd webapp/opendream-ui && npm install && npm run start &
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wait

But… this didn’t quite work. Hitting Ctrl+C quit the script, but it left the two servers running in the background. I’d like the servers to stop when the wrapping Bash script is terminated.

Is there a way to do that? I asked GPT-4 and it showed me this:

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#!/bin/bash
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# Function to be executed upon receiving SIGINT
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cleanup() {
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    echo "Caught SIGINT. Cleaning up..."
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    kill $server_pid1 $server_pid2  # Terminates both server processes
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    exit
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}
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# Set up the trap
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trap cleanup SIGINT
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# Start the Uvicorn server in the background
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uvicorn opendream.server:app --reload &
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server_pid1=$!  # Get the process ID of the last backgrounded command
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# Start the npm server in the background
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cd webapp/opendream-ui && npm install && npm run start &
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server_pid2=$!  # Get the process ID of the last backgrounded command
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# Wait indefinitely. The cleanup function will handle interruption and cleanup.
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wait

And sure enough, that works perfectly!

There are a couple of tricks here, explained by ChatGPT’s comments. Each server is started using & and then has its PID assigned to a variable using $! to access the PID.

A cleanup() Bash function is defined which runs kill against both of those stored PIDs.

Then the trap mechanism is used to cause that cleanup() function to be triggered on SIGINT, which is the signal that is fired when you hit Ctrl+C:

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trap cleanup SIGINT

So now I have a pattern for any time I want to write a Bash script that starts multiple background processe and terminates them when the script itself exits.