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12-interpolation.md

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Interpolation

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. Default values can be defined inline using typical shell syntax:

  • ${VARIABLE:-default} evaluates to default if VARIABLE is unset or empty in the environment.
  • ${VARIABLE-default} evaluates to default only if VARIABLE is unset in the environment.

Similarly, the following syntax allows you to specify mandatory variables:

  • ${VARIABLE:?err} exits with an error message containing err if VARIABLE is unset or empty in the environment.
  • ${VARIABLE?err} exits with an error message containing err only if VARIABLE is unset in the environment.

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 will process any string following a $ sign as long as it makes it a valid variable definition: either an alphanumeric name ([_a-z][_a-z0-9]*) or a braced string starting with ${. In other circumstances, it will be preserved without attempt 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.

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"