a4f5e30249
* Reword lots of instances of 'will' * Reword lots of instances of won't * Reword lots of instances of we'll * Eradicate you'll * Eradicate 'be able to' type of phrases * Eradicate 'unable to' type of phrases * Eradicate 'has / have to' type of phrases * Eradicate 'note that' type of phrases * Eradicate 'in order to' type of phrases * Redirect to official Chef and Puppet docs * Eradicate gratuitous 'please' * Reduce use of e.g. * Reduce use of i.e. * Reduce use of N.B. * Get rid of 'sexagesimal' and correct some errors
2.1 KiB
Your configuration options can contain environment variables. Compose uses the
variable values from the shell environment in which docker-compose is run. For
example, suppose the shell contains POSTGRES_VERSION=9.3 and you supply this
configuration:
db:
image: "postgres:${POSTGRES_VERSION}"
When you run docker-compose up with this configuration, Compose looks for the
POSTGRES_VERSION environment variable in the shell and substitutes its value
in. For this example, Compose resolves the image to postgres:9.3 before
running the configuration.
If an environment variable is not set, Compose substitutes with an empty
string. In the example above, if POSTGRES_VERSION is not set, the value for
the image option is postgres:.
You can set default values for environment variables using a
.env file, which Compose automatically looks for. Values
set in the shell environment override those set in the .env file.
Important
: The
.env filefeature only works when you use thedocker-compose upcommand and does not work withdocker stack deploy. {: .important }
Both $VARIABLE and ${VARIABLE} syntax are supported. Additionally when using
the 2.1 file format, it is possible to
provide inline default values using typical shell syntax:
${VARIABLE:-default}evaluates todefaultifVARIABLEis unset or empty in the environment.${VARIABLE-default}evaluates todefaultonly ifVARIABLEis unset in the environment.
Other extended shell-style features, such as ${VARIABLE/foo/bar}, are not
supported.
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 you forget and use a single dollar sign ($), Compose interprets the value
as an environment variable and warns you:
The VAR_NOT_INTERPOLATED_BY_COMPOSE is not set. Substituting an empty string.