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Add note about .env file to variable substitution docs (#801) 

Signed-off-by: Aanand Prasad <aanand.prasad@gmail.com>
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Aanand Prasad 2016-12-07 00:41:21 +00:00 committed by John Mulhausen
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compose/compose-file.md
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@ -1284,6 +1284,16 @@ string. In the example above, if `EXTERNAL_PORT` is not set, the value for the
port mapping is `:5000` (which is of course an invalid port mapping, and will
result in an error when attempting to create the container).
You can set default values for environment variables using a
[`.env` file](env-file.md), which Compose will automatically look for. Values
set in the shell environment will override those set in the `.env` file.
$ unset EXTERNAL_PORT
$ echo "EXTERNAL_PORT=6000" > .env
$ docker-compose up # EXTERNAL_PORT will be 6000
$ export EXTERNAL_PORT=7000
$ docker-compose up # EXTERNAL_PORT will be 7000
Both `$VARIABLE` and `${VARIABLE}` syntax are supported.
Additionally when using the [2.1 file format](compose-file.md#version-21), it
is possible to provide inline default values using typical shell syntax: