{ { < include "compose/interpolation.md" > } }

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.

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.

web:
  build: .
  command: "$$VAR_NOT_INTERPOLATED_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:

services:
  foo:
    labels:
      "$VAR_NOT_INTERPOLATED_BY_COMPOSE": "BAR"

services:
  foo:
    labels:
      - "$VAR_INTERPOLATED_BY_COMPOSE=BAR"