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Add support for more advanced ${xxx:...} syntax 

Just ${xxx:+...} and ${xxx:-...} for now

Signed-off-by: Doug Davis <dug@us.ibm.com>
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Doug Davis 2015-01-28 18:28:48 -08:00 committed by Tibor Vass
parent 07995c4ef3
commit efc478e33c
1 changed files with 16 additions and 4 deletions
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18
docs/sources/reference/builder.md
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@ -113,15 +113,27 @@ images.
> replacement at the time. After 1.3 this behavior will be preserved and
> canonical.
Environment variables (declared with [the `ENV` statement](#env)) can also be used in
certain instructions as variables to be interpreted by the `Dockerfile`. Escapes
are also handled for including variable-like syntax into a statement literally.
Environment variables (declared with [the `ENV` statement](#env)) can also be
used in certain instructions as variables to be interpreted by the
`Dockerfile`. Escapes are also handled for including variable-like syntax
into a statement literally.
Environment variables are notated in the `Dockerfile` either with
`$variable_name` or `${variable_name}`. They are treated equivalently and the
brace syntax is typically used to address issues with variable names with no
whitespace, like `${foo}_bar`.
The `${variable_name}` syntax also supports a few of the standard `bash`
modifiers as specified below:
* `${variable:-word}` indicates that if `variable` is set then the result
will be that value. If `variable` is not set then `word` will be the result.
* `${variable:+word}` indiates that if `variable` is set then `word` will be
the result, otherwise the result is the empty string.
In all cases, `word` can be any string, including additional environment
variables.
Escaping is possible by adding a `\` before the variable: `\$foo` or `\${foo}`,
for example, will translate to `$foo` and `${foo}` literals respectively.