Using "Exit Traps" for More Robust Bash Scripts

Bash scripts can be made more reliable by using "exit traps" to ensure that necessary cleanup operations are always performed, even when something unexpected goes wrong. Bash provides a pseudo-signal called EXIT that can be trapped, allowing commands or functions to execute when the script exits for any reason.

To use exit traps, simply define a "finish" function and place any code that needs to be run in it. For example, a common use case is creating a temporary scratch directory and deleting it after. Intermediate or temporary files can then be manipulated in the scratch directory without worry.

Without exit traps, cleanup operations can become messy and unreliable. For example, removing a scratch directory without the trap can lead to errors and leftover files.

Exit traps can also be useful for keeping services up, even in the face of runtime errors. For example, a script that temporarily stops a server for maintenance can ensure that it starts again at the end, regardless of any errors that may occur.

Here's an example of using exit traps to stop and restart a MongoDB server on an Ubuntu machine:

#!/bin/bash

finish () {
  # Restart MongoDB
  sudo service mongodb start
}

# Stop MongoDB
sudo service mongodb stop

# Do maintenance tasks here

# Ensure MongoDB is restarted even if there is an error
trap finish EXIT

By using exit traps, Bash scripts can be made more reliable and robust, ensuring that necessary cleanup operations are always performed and services are kept up, no matter what.