FrançaisGet started

Making a script robust

Bash's strict mode, trap for cleaning up, mktemp, errors on the error output with an exit status, idempotence and shellcheck: knowing where a script stops and what it leaves behind.

  • 30 cards
  • 5 topics
  • Free
Study this deck

The outline, topic by topic

Each topic is weighted the way the exam weighs it.

  1. Concepts
  2. Questions
  3. Sort and link
  4. Scenarios
  5. Understand

A few cards from the deck

  • Strict mode: what is it?

    Show the answer

    Three Bash settings that make a script fail instead of carrying on: set -e (stop on error), set -u (undefined variable) and set -o pipefail (failure in a pipe).

    Without it, cd /nope fails and the script goes on with rm -rf * in the wrong folder. But set -e ignores failures in a test (if, &&, ||): it does not replace checking for foreseeable cases.

  • What happens when this script runs?

    ```bash
    set -e
    cd /nope
    echo fini
    `

    • It stops on the cd error, status 1, without printing fini
    • It prints the cd error, then fini, and returns status 0
    • It prints fini with no error, since cd only changes folder
    • It asks for confirmation before carrying on with the script
    Show the answer

    It stops on the cd error, status 1, without printing fini

    cd /nope fails (“No such file or directory”); under set -e, any simple command's failure stops the script with that command's status, here 1. Without set -e, fini would be printed.

  • With set -u, reading a variable that was never defined stops the script with the message “unbound variable”; without it, Bash replaces it with ….

    Show the answer

    With set -u, reading a variable that was never defined stops the script with the message “unbound variable”; without it, Bash replaces it with an empty string.

    It is what catches the typo rm -rf "$dosier"/*: without -u, the empty variable gives /*. A deliberate default avoids the stop: ${1:-default}.

  • pipefail: what is it?

    Show the answer

    A setting that gives a pipe the status of the last command that failed, instead of that of its last command alone.

    false | true returns 0 by default: the failure of false goes unnoticed. With set -o pipefail, it returns 1, and set -e can then stop the script.

  • Every night, the /tmp folder grows by 2 GB until the disk is full. The processing script creates a file with tmp=$(mktemp) and deletes it on its last line, but sometimes stops on an error. Which fix is right?

    • Add trap 'rm -f "$tmp"' EXIT just after mktemp
    • Put the rm -f at the start of the script rather than the end
    • Replace mktemp with a fixed name in /tmp
    • Add set -u so the script stops earlier
    Show the answer

    Add trap 'rm -f "$tmp"' EXIT just after mktemp

    When the script stops before its last line, the final rm never runs and the file stays. The EXIT trap runs however the script ends. A rm at the start would delete a file that does not exist yet.

Study this deck