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Support metaarg expansion #151

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72cc105
Support meta arg expansion
jedinym Aug 26, 2025
6e0f523
Edit example to showcase nested ARGs
jedinym Aug 27, 2025
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3 changes: 2 additions & 1 deletion README.md
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Original file line number Diff line number Diff line change
Expand Up @@ -274,7 +274,8 @@ $ dockerfile-json --jsonpath=..As Dockerfile

`Dockerfile`
```Dockerfile
ARG ALPINE_TAG=3.10
ARG ALPINE_MINOR=10
ARG ALPINE_TAG=3.${ALPINE_MINOR}
ARG APP_BASE=scratch

FROM alpine:$ALPINE_TAG AS build
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3 changes: 2 additions & 1 deletion examples/Dockerfile.3
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@@ -1,4 +1,5 @@
ARG ALPINE_TAG=3.10
ARG ALPINE_MINOR=10
ARG ALPINE_TAG=3.${ALPINE_MINOR}
ARG APP_BASE=scratch

FROM alpine:${ALPINE_TAG} AS build
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19 changes: 11 additions & 8 deletions pkg/dockerfile/expand.go
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Expand Up @@ -39,21 +39,24 @@ func (d *Dockerfile) metaArgsEnvExpander(env instructions.SingleWordExpander) in
if defaultValue := arg.DefaultValue; defaultValue != nil {
metaArgsEnv[arg.Key] = *defaultValue
}

if value, err := env(arg.Key); err == nil {
arg.ProvidedValue = &value
metaArgsEnv[arg.Key] = value
arg.Value = &value
}
err := arg.Expand(env)
if err != nil {
continue
}
for _, kv := range arg.ArgCommand.Args {
if kv.Value != nil {
metaArgsEnv[arg.Key] = *kv.Value

exp := os.Expand(*arg.Value, func(argval string) string {
if val, ok := metaArgsEnv[argval]; ok {
return val;
}
}

return argval;
})
arg.Value = &exp;
metaArgsEnv[arg.Key] = exp;
}

return func(key string) (string, error) {
if value, ok := metaArgsEnv[key]; ok {
return value, nil
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