An unset variable
Becomes an empty string, and the error you get is about the image name
---
image: myapp:${TAG}Docker Compose Interpolation Tester
See what Compose fills in. Put the .env values above a line of three dashes and the compose text below it.
Paste below, or drop a file anywhere on this panel
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Formatted
Results
Nothing else to flag.
No formatting problems, and nothing the rules object to. Worth remembering what that covers: this reads the file you pasted, not the account or cluster it will be applied to.
No finding matches that filter.
Examples
Real input you can load into the tool above. Each one shows a different thing going wrong, because that is what the tool is for.
An empty value with the dash form
Set but empty counts as set, so the default is skipped
TAG=
---
image: myapp:${TAG-latest}A required variable
Compose stops with your message rather than starting something wrong
---
image: myapp:${TAG:?TAG must be set}Common mistakes
These are the ones that fail silently. The config is accepted, nothing raises an error, and the consequence arrives later.
Relying on a variable being set without a default or a required marker
Compose warns and substitutes an empty string, so the failure appears somewhere else and the warning is off-screen by then.
Instead:Use ${VAR:-default} where a default makes sense and ${VAR:?message} where it does not.
Using ${VAR-default} and finding the default ignored
Without the colon the default applies only when the variable is unset. An .env line reading VAR= sets it to empty, which counts as set.
Instead:Use ${VAR:-default}, which treats empty and unset the same.
Expecting .env to populate the container's environment
It fills in ${ } in the compose file. Nothing from it reaches the container on its own.
Instead:Add the variable under environment: or point env_file: at the file.
Compose warns about an unset variable and carries on
It does not stop. The variable becomes an empty string, so image: app:${TAG} becomes app: and the error you get is about the image name.
What an empty substitution actually breaks
image: app: fails with a confusing reference error. ports: "${PORT}:80" becomes ":80", which binds a random host port rather than failing. An environment value becomes empty and the application starts with a blank credential. The warning about the unset variable scrolled past several lines above whatever went wrong.
The colon changes what counts as missing
${VAR:-default} uses the default when the variable is unset OR empty. ${VAR-default} uses it only when UNSET, so a variable set to nothing, which is what an .env line reading NAME= gives you, keeps its empty value. That pair is a common source of a default that mysteriously does not apply.
Make it required rather than defaulting it
${VAR:?message} stops Compose with your message instead of continuing with an empty value. For anything where there is no sensible default, that is a better failure than a container that starts wrong.
The .env file feeds the compose file, not the container
Compose reads .env from the project directory and uses it to fill in ${ } while parsing the YAML. Nothing in it reaches the container unless the compose file also passes it through environment: or env_file:. Both files look like lists of environment variables, which is exactly why this is confusing.
$$ passes a literal dollar through
Which is how a shell variable, a cron entry or an awk script survives being written in a compose file. A command that works in a terminal and fails under Compose is usually missing these.
The bare $NAME form works too
And is interpolated identically. The braces are only required when the name is followed by characters that could be part of it.
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