Fly.io (flyctl) CLI

Operate Fly.io apps safely and repeatably with flyctl.

Defaults / safety

• Prefer read-only commands first: fly status, fly logs, fly config show, fly releases, fly secrets list.
• Do not edit/modify Fly.io apps, machines, secrets, volumes, or databases without your human’s explicit approval.
  • Read-only actions are OK without approval.
  • Destructive actions (destroy/drop) always require explicit approval.
• When debugging builds, capture the exact error output and determine whether it’s a:
  • build/packaging issue (Dockerfile, Gemfile.lock platforms, assets precompile)
  • runtime issue (secrets, DB, migrations)
  • platform issue (regions, machines, health checks)

Quick start (typical deploy)

From the app repo directory:

1. Confirm which app you’re targeting

• fly app list
• fly status -a \x3Capp>
• Check fly.toml for app = "..."

2. Validate / inspect (read-only)

• fly status -a \x3Capp>
• fly logs -a \x3Capp>
• fly config show -a \x3Capp>

(Deploys are in High-risk operations below and require explicit user approval.)

Debugging deploy/build failures

Common checks

• fly deploy --verbose (more build logs)
• If using Dockerfile builds: verify Dockerfile Ruby/version and Gemfile.lock platforms match your builder OS/arch.

Rails + Docker + native gems (nokogiri, pg, etc.)

Symptoms: Bundler can’t find a platform gem like nokogiri-…-x86_64-linux during build.

Fix pattern:

• Ensure Gemfile.lock includes the Linux platform used by Fly’s builder (usually x86_64-linux).
  • Example: bundle lock --add-platform x86_64-linux
• Ensure Dockerfile’s Ruby version matches .ruby-version.

(See references/rails-docker-builds.md.)

Logs & config (read-only)

• Stream logs:
  • fly logs -a \x3Capp>
• Show config:
  • fly config show -a \x3Capp>
• List secrets (names only):
  • fly secrets list -a \x3Capp>

High-risk operations (ask first)

These commands can execute arbitrary code on servers or mutate production state. Only run them when the user explicitly asks you to.

• Deploy:
  • fly deploy / fly deploy --remote-only
• SSH exec / console:
  • fly ssh console -a \x3Capp> -C "\x3Ccommand>"
• Secrets changes:
  • fly secrets set -a \x3Capp> KEY=value

See references/safety.md.

Fly Postgres basics

Identify the Postgres app

• fly postgres list

Attach Postgres to an app

• fly postgres attach \x3Cpg-app> -a \x3Capp>

Create a database inside the cluster

• fly postgres db create \x3Cdb_name> -a \x3Cpg-app>
• fly postgres db list -a \x3Cpg-app>

Connect (psql)

• fly postgres connect -a \x3Cpg-app>

GitHub Actions deploys / previews

• For production CD: use Fly’s GitHub Action (superfly/flyctl-actions/setup-flyctl) and run flyctl deploy.
• For PR previews:
  • Prefer one preview app per PR and one database per PR inside a shared Fly Postgres cluster.
  • Automate create/deploy/comment on PR; destroy on close.

(See references/github-actions.md.)

Bundled resources

• references/safety.md: safety rules (read-only by default; ask before mutating state).
• references/rails-docker-builds.md: Rails/Docker/Fly build failure patterns + fixes.
• references/github-actions.md: Fly deploy + preview workflows.
• scripts/fly_app_from_toml.sh: tiny helper to print the Fly app name from fly.toml (shell-only; no ruby).