Structuring a Script (Functions, main() Pattern, Exit at the End)

One Long Script vs. Small Functions

Splitting logic into small, named functions -- each responsible for one clear task -- makes a script's overall structure obvious at a glance and each piece independently testable.

Example: One Long Script vs. Small Functions

#!/bin/bash
greet() {
    echo "Hello, $1"
}

farewell() {
    echo "Goodbye, $1"
}

greet "Ada"
farewell "Ada"

The main() Pattern

Defining a main function that calls the other functions in the right order, and invoking main "$@" as the very last line of the script, gives the whole script one clear, obvious entry point, similar to main in C or Python's if __name__ == "__main__".

Example: The main() Pattern

#!/bin/bash
setup() {
    echo "setting up..."
}

run_task() {
    echo "running task: $1"
}

cleanup() {
    echo "cleaning up..."
}

main() {
    setup
    run_task "$1"
    cleanup
}

main "demo-task"

Exiting Explicitly at the End

Ending main with an explicit exit using its own return status makes the script's final exit code intentional and easy to find, rather than relying on whatever the last command inside a deeply nested function happened to return.

Example: Exiting Explicitly at the End

#!/bin/bash
main() {
    echo "doing the real work"
    return 0
}

main "$@"
exit $?

Making a Script Sourceable for Testing

Guarding the main call so it only runs when the script is executed directly (not when it's sourced) lets other scripts or a test harness load your functions without automatically triggering the whole program.

Note: This BASH_SOURCE[0] == $0 guard is the bash equivalent of Python's if __name__ == "__main__":.

Example: Making a Script Sourceable for Testing

#!/bin/bash
greet() {
    echo "Hello, $1"
}

main() {
    greet "World"
}

if [[ "${BASH_SOURCE[0]}" == "${0}" ]]; then
    main "$@"
fi