How to use yaml template variable as a part of value in Concourse CI - concourse

Question

When I try to use template variable, e.g. {{hostname}} as a part of value, it gets wrapped with double quotes.
How to add a variable w/o quotes?
Example:
---
resource_types:
- name: maven
type: docker-image
source:
repository: patrickcrocker/maven-resource
tag: latest
resources:
- name: maven-snapshot
type: maven
source:
url: http://{{hostname}}:8081/repository/maven-snapshots/
- name: repo
type: git
source:
uri: "git#bitbucket.org:foo/bar.git"
branch: master{{hostname}}
And the result for the command fly -t ci set-pipeline --pipeline test --config test.yml --var="hostname=localhost" is as follows (look at "localhost"):
resources:
resource maven-snapshot has been added:
name: maven-snapshot
type: maven
source:
url: http://"localhost":8081/repository/maven-snapshots/
resource repo has been added:
name: repo
type: git
source:
branch: master"localhost"
uri: git#bitbucket.org:foo/bar.git
resource types:
resource type maven has been added:
name: maven
type: docker-image
source:
repository: patrickcrocker/maven-resource
tag: latest
The reason I've included a 3rd-party maven resource is that git resource does not allow {{}} in the uri, leading to the error:
failed to unmarshal configStructure: yaml: line 17: did not find expected key

Answer 1

UPDATE
As of concourse v3.2.0 {{someValue}} syntax is deprecated in favor of ((someValue)). New syntax will understand you are trying to interpolate the string and place the value accordingly.
Replacing {{hostname}} with ((hostname)) will solve your issue:
resources:
- name: maven-snapshot
type: maven
source:
url: http://((hostname)):8081/repository/maven-snapshots/
Concourse does not support this.
The Concourse yaml templating is very primitive and you can not insert variables in the middle of strings.
You will need to set your url parameter as http://localhost:8081/repository/maven-snapshots/ and your branch parameter as localmaster or whatever it should be.
We know this is a problem and we are working on it, but for now you can not set variables in the way you want.

Answer 2

While waiting for this feature from concourse team, I've written this small executable to work around the issue in this GitHub repo:
https://github.com/sercant/inline-yaml
I prepare my config.yml like this:
ftp-username: username
ftp-password: password
ftp-uri: 192.168.1.2
ftp-dir: home/ftp/
ftp-uri-combined: ftp://{{ftp-username}}:{{ftp-password}}#{{ftp-uri}}/{{ftp-dir}}
ftp-uri-combined-html5: {{ftp-uri-combined}}html5
ftp-uri-combined-android: {{ftp-uri-combined}}android
And prepared a create-pipeline.sh:
#!/usr/bin/env sh
TEMP=$(mktemp)
java -jar inline-yaml.jar $3 ${TEMP};
fly -t lite set-pipeline -p $2 -c $1 --load-vars-from ${TEMP};
rm ${TEMP};
Whenever I need to create a pipeline, I ran:
./create-pipeline.sh build-plan.yml build-plan-name config.yml