mirror of https://github.com/docker/docs.git
66 lines
2.2 KiB
Markdown
66 lines
2.2 KiB
Markdown
---
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title: Interpolation
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description: Learn about interpolation
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keywords: compose, compose specification, interpolation, compose file reference
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---
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Values in a Compose file can be set by variables and interpolated at runtime. Compose files use a Bash-like
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syntax `${VARIABLE}`.
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Both `$VARIABLE` and `${VARIABLE}` syntax is supported. Default values can be defined inline using typical shell syntax:
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- `${VARIABLE:-default}` evaluates to `default` if `VARIABLE` is unset or
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empty in the environment.
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- `${VARIABLE-default}` evaluates to `default` only if `VARIABLE` is unset
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in the environment.
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Similarly, the following syntax allows you to specify mandatory variables:
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- `${VARIABLE:?err}` exits with an error message containing `err` if
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`VARIABLE` is unset or empty in the environment.
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- `${VARIABLE?err}` exits with an error message containing `err` only if
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`VARIABLE` is unset in the environment.
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Interpolation can also be nested:
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- `${VARIABLE:-${FOO}}`
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- `${VARIABLE?$FOO}`
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- `${VARIABLE:-${FOO:-default}}`
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Other extended shell-style features, such as `${VARIABLE/foo/bar}`, are not
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supported by Compose.
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You can use a `$$` (double-dollar sign) when your configuration needs a literal
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dollar sign. This also prevents Compose from interpolating a value, so a `$$`
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allows you to refer to environment variables that you don't want processed by
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Compose.
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```yml
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web:
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build: .
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command: "$$VAR_NOT_INTERPOLATED_BY_COMPOSE"
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```
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If Compose can't resolve a substituted variable and no default value is defined, it displays a warning and substitutes the variable with an empty string.
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As any values in a Compose file can be interpolated with variable substitution, including compact string notation
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for complex elements, interpolation is applied before a merge on a per-file basis.
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Interpolation applies only to YAML values, not to keys. For the few places where keys are actually arbitrary
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user-defined strings, such as [labels](05-services.md#labels) or [environment](05-services.md#environment), an alternate equal sign syntax
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must be used for interpolation to apply. For example:
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```yml
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services:
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foo:
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labels:
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"$VAR_NOT_INTERPOLATED_BY_COMPOSE": "BAR"
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```
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```yml
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services:
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foo:
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labels:
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- "$VAR_INTERPOLATED_BY_COMPOSE=BAR"
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```
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