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

${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 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 the Compose specification.

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 the Compose implementation can't resolve a substituted variable and no default value is defined, it MUST warn the user and substitute 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 MUST be applied before merge on a per-file-basis.