Interpolation
${VARIABLE}
. Both $VARIABLE
and ${VARIABLE}
syntax is supported.
For braced expressions, the following formats are supported:
- Direct substitution
${VAR}
-> value ofVAR
- Default value
${VAR:-default}
-> value ofVAR
if set and non-empty, otherwisedefault
${VAR-default}
-> value ofVAR
if set, otherwisedefault
- Required value
${VAR:?error}
-> value ofVAR
if set and non-empty, otherwise exit with error${VAR?error}
-> value ofVAR
if set, otherwise exit with error
- Alternative value
${VAR:+replacement}
->replacement
ifVAR
is set and non-empty, otherwise empty${VAR+replacement}
->replacement
ifVAR
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"