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Lab — Build a Deployment Script

Lab Overview

Purpose: Create a production-shaped deploy script for a static app artefact.

Scenario: A demo site artefact must be released to /var/tmp/rebash-app (lab) with rollback.

Expected outcome: A working script under ~/rebash-lab-shell with clear exit codes, stderr logging, and validation steps you can re-run.

This is a lab, not a tutorial

Apply Shell Scripting skills. Prefer small verified steps over rewriting everything at once.

Business Scenario

You implement download/stage/activate/healthcheck/rollback with flock and dry-run.

Learning Objectives

By the end of this lab, you will be able to:

  • Implement staged deploy directories
  • Support --dry-run
  • Health-check after activate
  • Rollback to previous release on failure
  • Prevent overlapping deploys with flock

Prerequisites

Knowledge

Software

Tool Notes
Bash required
curl or local artefact lab
flock util-linux

Estimated cost: £0.

Environment

Linux host; deploy only under lab paths.

Initial State

mkdir -p ~/rebash-lab-shell/deploy/{releases,shared,logs}
cd ~/rebash-lab-shell/deploy
mkdir -p artefact
echo 'ok' > artefact/index.html
echo 'ok' > artefact/healthz

Lab Tasks

Task 1 — Layout

  • releases/<id>/ — immutable release trees
  • current — symlink to active release
  • shared/ — persistent config

Task 2 — deploy.sh

Steps: lock → stage → activate symlink → health check (file exists / curl local) → unlock.

On health failure: relink previous release.

Task 3 — Dry-run

--dry-run prints steps without changing current.

Task 4 — Validate

./deploy.sh --source ./artefact --dry-run
./deploy.sh --source ./artefact
readlink current

Validation

  • First deploy creates current symlink
  • Failed health check rolls back
  • Concurrent deploy blocked by flock
  • Dry-run makes no changes

Troubleshooting

Symptom Possible cause Resolution
Broken symlink Activate order wrong Create new link then mv -Tf
Rollback target missing First deploy Fail closed with clear error

Cleanup

rm -rf ~/rebash-lab-shell

Stretch Goals

  • Add a --dry-run mode that prints actions without changing the system
  • Emit a machine-readable RESULT status=... line on stdout for CI
  • Schedule the script with cron or a systemd timer

Production Discussion

In production, wrap scripts with lock files, structured logging, explicit PATH, and documented exit codes. Prefer configuration files over hard-coded hosts and thresholds. Never embed secrets in scripts — use environment variables or a secrets manager.

Best Practices

  • Use #!/usr/bin/env bash and set -euo pipefail
  • Quote every expansion that may contain spaces
  • Log diagnostics to stderr; keep stdout for data
  • Prefer absolute paths in scheduled jobs
  • Validate inputs before destructive actions

Common Mistakes

Mistake Why it happens Correct approach
Unquoted paths Habit from interactive shell Always "$var"
Missing pipefail Default Bash pipeline behaviour set -o pipefail
Interactive-only PATH Cron/systemd minimal env Set PATH= at top
Skipping dry-run Time pressure Default to dry-run for risky ops

Success Criteria

  • Script runs under Bash with strict mode
  • Validation and failure paths are tested
  • Exit codes are documented
  • Cleanup leaves no lab artefacts (or documents what remains)

Reflection Questions

  1. What would break if this ran under /bin/sh (dash) instead of Bash?
  2. How would you make the script idempotent?
  3. How would you secure credentials and host inventories?
  4. How would you observe failures in production?

Interview Connection

Interviewers often ask about quoting, exit codes, cron environment differences, and how you prevent overlapping jobs. Be ready to walk through a small script and explain failure modes.