Health check a server with 'nohup $(cmd) &'

While working on a project with EdgeDB¹ and FastAPI², I wanted to perform health checks against the FastAPI server in the GitHub CI. This would notify me about the working state of the application. The idea is to:

Run the server in the background.
Run the commands against the server that’ll denote that the app is in a working state.
Perform cleanup.
Exit with code 0 if the check is successful, else exit with code 1.

The following shell script demonstrates a similar workflow with a Python HTTP server. This script:

Runs a Python web server in the background.
Makes an HTTP request against the server and checks if it returns HTTP 200 (OK). If the request fails or the server isn’t ready then waits for a second and makes the request again, and keeps retrying for the next 20 times before giving up.
Performs cleanups and kills the Python processes.
Exit with code 0 if the request is successful, else exit with code 1.

#!/bin/bash

set -euo pipefail

# Run the Python server in the background.
nohup python3 -m http.server 5000 >> /dev/null &

# Give the server enough time to be ready before accepting requests.
c=20
while [[ $c != 0 ]]
do
    # Run the healthcheck.
    if [[ $(curl -I http://localhost:5000/ 2>&1) =~ "200 OK" ]]; then
        echo "Health check passed!"

        # ...do additional cleanups if required.
        pkill -9 -i python
        exit 0
    fi
    ((c--))
    echo "Server isn't ready. Retrying..." $c
    sleep 1
done

echo "Health check failed!"
# ...do additional cleanups if required.
pkill -9 -i python
exit 1

The nohup before the python3 -m http.server 5000 makes sure that the SIGHUP signal can’t reach the server and shut down the process. The ampersand & after the command runs the process in the background. Afterward, the script starts making requests to the http://localhost:5000/ URL in a loop. If the server returns HTTP 200, the health check is considered successful. This will break the loop and the script will be terminated with exit 0 status. If the server doesn’t return HTTP 200 or isn’t ready yet, the script will keep retrying 20 times with a 1 second interval between each subsequent request before giving up. A failed health check will cause the script to terminate with exit 1 status.

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