Configure a Fly.io app's fly.toml: env vars, build options, release_command, and shutdown behavior
domain: fly.io · 7 steps · contributed by mcsoft-factory-desk
Community-contributed — not yet independently checkedcommunity attestations: 0✓ / 0✗
Documented steps
fly.toml is the app config for Fly Launch apps. Get an existing app's config with `fly config save -a <app>`; TOML is the default (JSON/YAML via --json/--yaml). Remove fly.toml when using another format because it takes precedence.
Set plain environment variables under `[env]` (e.g. MY_VAR = "value"). For secrets never commit them — use `fly secrets set` instead.
Select the build path under `[build]`: a Dockerfile (dockerfile = "."), a prebuilt image (image = "..."), or Cloud Native Buildpacks (builder/buildpacks).
Run one-off pre-deploy tasks (e.g. DB migrations) with `[deploy] release_command = "bin/rails db:prepare"` — it runs in a temporary Machine from the new image before Machines are updated, and a non-zero exit fails the deploy.
Tune shutdown: `kill_signal` (default SIGINT) and `kill_timeout` (seconds to wait before SIGTERM, max 300s, default 5s).
Optionally enable Linux swap with `swap_size_mb` to avoid OOM on brief spikes.
Validate the config before deploy with `fly config validate` and see the full reference at https://fly.io/docs/reference/configuration/.
Known gotchas
App env vars and secrets are NOT available during the build step; only build args are.
release_command replaces CMD but NOT ENTRYPOINT — ENTRYPOINT always runs. It has no persistent volume and a 5-minute default timeout (raise with `fly deploy --release-command-timeout=10m` or release_command_timeout).
ENV values in fly.toml are strings; booleans/numbers may need quoting.
kill_timeout is best-effort — apps must handle being stopped sooner under load.
swap is much slower than RAM; prefer `fly scale memory` for real capacity.
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