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Showing content from https://docs.docker.com/reference/compose-file/interpolation/ below:

Interpolation | Docker Docs

Values in a Compose file can be set by variables and interpolated at runtime. Compose files use a Bash-like syntax ${VARIABLE}. Both $VARIABLE and ${VARIABLE} syntax is supported.

For braced expressions, the following formats are supported:

• Direct substitution
  • ${VAR} -> value of VAR
• Default value
  • ${VAR:-default} -> value of VAR if set and non-empty, otherwise default
  • ${VAR-default} -> value of VAR if set, otherwise default
• Required value
  • ${VAR:?error} -> value of VAR if set and non-empty, otherwise exit with error
  • ${VAR?error} -> value of VAR if set, otherwise exit with error
• Alternative value
  • ${VAR:+replacement} -> replacement if VAR is set and non-empty, otherwise empty
  • ${VAR+replacement} -> replacement if VAR is set, otherwise empty

Interpolation can also be nested:

• ${VARIABLE:-${FOO}}
• ${VARIABLE?$FOO}
• ${VARIABLE:-${FOO:-default}}

Other extended shell-style features, such as ${VARIABLE/foo/bar}, are not supported by Compose.

Compose processes any string following a $ sign as long as it makes it a valid variable definition - either an alphanumeric name ([_a-zA-Z][_a-zA-Z0-9]*) or a braced string starting with ${. In other circumstances, it will be preserved without attempting to interpolate a value.

You can use a $$ (double-dollar sign) when your configuration needs a literal dollar sign. This also prevents Compose from interpolating a value, so a $$ allows you to refer to environment variables that you don't want processed by Compose.

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.

As any values in a Compose file can be interpolated with variable substitution, including compact string notation for complex elements, interpolation is applied before a merge on a per-file basis.

Interpolation applies only to YAML values, not to keys. For the few places where keys are actually arbitrary user-defined strings, such as labels or environment, an alternate equal sign syntax must be used for interpolation to apply. For example:

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