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• Syntax. Use ${VAR} to reference an environment variable VAR. The variable's value is resolved from the combined environment, which includes variables defined in the host and the container image, the later taking precedence.
• Scope. Variable expansion is supported across all EDF parameters. This includes EDF’s parameters like mounts, workdir, image, etc. For example, ${SCRATCH} can be used in mounts to reference a directory path.
• Undefined Variables. Referencing an undefined variable results in an error. To safely handle undefined variables, you can use the syntax ${VAR:-}, which evaluates to an empty string if VAR is undefined.
• Preventing Expansion. To prevent expansion, use double dollar signs $$. For example, $$$$${VAR} will render as the literal string ${VAR}.
• Limitations:
• Variables defined within the [env] EDF table cannot reference other entries from [env] tables in the same or other EDF files (e.g. the ones entered as base environments) . Therefore, only environment variables from the host or image environment can be referenced.
• Environment Variable Resolution Order. The environment variables in containers are resolved set based on the following order:
1. TOML env: Variable values as defined in EDF’s [env].
2. Container Image: Variables defined in the container image's environment take precedence.
3. Host Environment: Environment variables defined in the host system.

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