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londoncalling merged 2 commits into from
Oct 31, 2016
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Document Compose's syntax for variable defaults #84

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Document Compose's syntax for variable defaults
nVitius Oct 12, 2016
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incorporated misty's comments
Oct 31, 2016
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16 changes: 14 additions & 2 deletions compose/compose-file.md
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Expand Up @@ -1143,8 +1143,20 @@ 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).

Both `$VARIABLE` and `${VARIABLE}` syntax are supported. Extended shell-style
features, such as `${VARIABLE-default}` and `${VARIABLE/foo/bar}`, are not
In the case of environment variables that are not set, it is also possible to
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@mdlinville mdlinville Oct 12, 2016

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Consider "you can" instead of "it is possible to".

define default values using one of the following syntax variants:


* To provide a default value that will be used if `VARIABLE` is either *unset* or *empty*:

`${VARIABLE:-default}`

* To provide a default value only if `VARIABLE` is *empty*:

`${VARIABLE-default}`

Both `$VARIABLE` and `${VARIABLE}` syntax are supported. 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
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