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Environment Forwarding

Read this when you are:

  • adding a new environment variable that a remote command needs to see;
  • debugging "why is $CI empty inside crabbox run?";
  • loading tunable values from a local profile file instead of flags;
  • reviewing a change that loosens or tightens the env allowlist.

crabbox run does not forward your whole local environment to the remote command. Forwarding is opt-in and name-based: an allowlist of variable names decides what may cross over, and Crabbox forwards only the allowlisted names that are actually present locally (or supplied by a profile file).

Why an allowlist

Agent and CI environments are full of tokens, credentials, terminal paths, and vendor-specific debug flags. Forwarding everything would leak secrets to remote runners, introduce drift between local and CI runs, and make it impossible to reason about what actually affects a remote command. The allowlist makes the contract explicit: the repo decides what counts as input to a remote command, and the value is auditable in config.

The allowlist

The allowlist is a list of variable names (not values). It is assembled from these sources, highest precedence last:

  1. Built-in default: CI and NODE_OPTIONS are always on the list.
  2. The repo allowlist in config under env.allow (replaces the default).
  3. The CRABBOX_ENV_ALLOW environment variable (replaces the list).
  4. --allow-env flags on crabbox run (appends to the list).

Config

env:
  allow:
    - CI
    - NODE_OPTIONS
    - PROJECT_*

Setting env.allow replaces the built-in default. Profile-scoped allowlists are also supported through the profile's own env.allow.

CRABBOX_ENV_ALLOW

A comma-separated override that replaces the configured list for that run:

CRABBOX_ENV_ALLOW='CI,NODE_OPTIONS,PROJECT_*' crabbox run -- pnpm test

Use it for one-off experiments; persistent allowances belong in env.allow.

--allow-env

Repeatable (or comma-separated) flag that appends names to the resolved allowlist for a single run:

crabbox run --allow-env PYTEST_ADDOPTS --allow-env 'VITEST_*' -- pytest

Matching rules

  • Entries are names, not values.
  • A trailing * is a prefix wildcard (PROJECT_* matches PROJECT_FOO and PROJECT_BAR); inline wildcards such as PROJECT_*_DEBUG are not supported, and a bare * is ignored.
  • Otherwise the match is exact and case-sensitive.
  • Empty entries are ignored.
  • Names must be valid shell identifiers ([A-Za-z_][A-Za-z0-9_]*); anything else is dropped during forwarding.

What gets forwarded

For each allowlisted name, Crabbox checks whether the variable is set locally. If it is, the variable is forwarded to the remote command with the same name and value. If it is not set, nothing is forwarded — Crabbox does not invent values. Variables that match the allowlist but are unset locally simply do not appear in the forwarded set.

Inline-allowlisted variables (those resolved from the local environment) are passed through the provider's remote-command transport. SSH-backed providers pass them as part of the SSH command itself, with quoting and escaping handled automatically so values containing shell metacharacters pass through safely. Delegated-run providers may use a provider-specific transport instead; for example, Docker Sandbox writes the selected values to a temporary local env file and passes only that file path to sbx exec --env-file so the values do not appear in local process arguments.

Profiles: --env-from-profile

--env-from-profile <file> loads variable values from a local .env-style file. Only names that also match the allowlist are kept, so the profile is a value source, not a second allowlist. The flag is repeatable; when files set the same key, later files win.

crabbox run \
  --allow-env 'PROJECT_*' \
  --env-from-profile .env.local \
  -- pnpm test

The profile parser is intentionally conservative:

  • KEY=value lines, with an optional leading export ;
  • # comments (at line start or after whitespace) and blank lines are ignored;
  • single- and double-quoted values are unquoted (\" and \\ honored inside double quotes);
  • bare values must be a single token (no unquoted spaces);
  • values containing command substitution ($(...) or backticks) are skipped;
  • entries with invalid variable names are skipped.

Profile values are not inlined into the SSH command. Instead they are written to a per-run file at .crabbox/env/<run-or-lease-id>.env inside the workdir (chmod 600), which the remote command sources before it runs. The file is removed after the run, and a one-line probe reports which names landed:

env profile remote=.crabbox/env/cbx_ab12cd34ef56.env vars=PROJECT_FOO=set,PROJECT_BAR=set

Reusable helpers: --env-helper

--env-helper <name> persists the selected profile env as a reusable remote helper instead of deleting it after the run. It requires profile values (--env-from-profile selected by the allowlist) and cannot be combined with --sync-only. The name must be a simple identifier — no path separators.

It writes two files under .crabbox/env/ in the workdir: the profile <name>.env (chmod 600) and an executable wrapper <name> (chmod 700) that sources the profile and execs whatever command you pass it:

crabbox run --allow-env 'PROJECT_*' --env-from-profile .env.local \
  --env-helper ci -- true
# later, on the box:
./.crabbox/env/ci pnpm test

--env-helper is not supported on native Windows targets yet.

Capability-injected env

A small set of variables is injected by Crabbox when the matching capability is requested. These are not part of the allowlist — Crabbox owns them — and they take precedence over allowlisted values when names overlap:

CRABBOX_DESKTOP=1         when --desktop
DISPLAY=:99               when --desktop (X11; omitted on Wayland)
CRABBOX_BROWSER=1         when --browser
BROWSER=<path>            when --browser, after probing the box
CHROME_BIN=<path>         when --browser, after probing the box

On Wayland/GNOME desktops the probe contributes WAYLAND_DISPLAY (and friends) instead of DISPLAY. Because capability env wins on overlap, allowlisting BROWSER will not override a --browser probe result for that run; drop the capability flag if you need to supply your own value.

Secrets

Do not put secrets in the allowlist even when forwarding seems convenient. Crabbox forwards whatever it finds locally, so a secret in the allowlist leaks on every run of every contributor who has it set. Secrets belong in:

  • coordinator secret injection for brokered provider credentials;
  • the operator's credential store for short-lived tokens;
  • a per-runner image bake when the secret should be on every lease;
  • post-bootstrap secret injection in repo-owned setup (devcontainer, bin/setup).

As a guardrail, Crabbox treats names containing KEY, TOKEN, SECRET, PASSWORD, PASS, CREDENTIAL, or AUTH as secret-shaped: in summaries and probes their values are replaced with secret=true plus a length, never the value itself. This redacts diagnostics; it does not stop forwarding, so keep secrets out of the allowlist in the first place.

Inspecting forwarding

Crabbox prints a one-line forwarding summary whenever you explicitly engage forwarding — that is, when CRABBOX_ENV_ALLOW is set, or --allow-env / --env-from-profile is used:

env forwarding provider=hetzner behavior=forwarded vars=CI=set,PROJECT_FOO=set,PROJECT_BAR=set

If nothing matched, the line reads matched=none allow=<list>. Secret-shaped names render as NAME=set len=<n> secret=true. This line is the source of truth for "what did the remote command actually see" — variables on the allowlist but unset locally never appear.

Examples

A repo allowlist tuning common test knobs:

env:
  allow:
    - CI                    # mark a remote command as CI-driven
    - NODE_OPTIONS          # adjust Node memory in test suites
    - PYTEST_ADDOPTS        # tune pytest flags from the local env
    - PROJECT_*             # repo's own debug knobs
    - VITEST_*              # let agents override vitest config
    - DEBUG                 # `debug` package selector

Things you usually do not allowlist:

HOME, USER, PATH, SHELL    the runner already has its own
SSH_*                       leaks SSH agent state
GITHUB_TOKEN                use Actions hydration or runner setup
AWS_*                       use IAM roles or an instance profile
*_API_KEY, *_TOKEN          use a secret manager

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