Portably capture curl exit status, http status code and response body when creating gitlab merge request

create-merge-request.sh

#!/bin/sh
# see https://superuser.com/a/862395/1001553 if you can use bash and want to avoid temp file

# let curl fail on error and print response code while redirecting body to file
# read curl stdout into variable
http_status=$(curl --request POST \
  --fail \
  --silent --write-out '%{http_code}' \
  --output response.json \
  --url http://gitlab.dzbw.de/api/v4/projects/4560/merge_requests \
  --header 'content-type: application/json' \
  --header 'private-token: the-application-token' \
  --data '{
	"id": "4560",
	"source_branch": "develop",
	"target_branch": "master",
	"title": "Deploye auf Stage Kons",
	"assignee_id" : 129,
	"allow_collaboration": true,
	"squash": true
}')

exit_status="$?"
echo http_status: $http_status

if [ $http_status -eq 409 ]; then
  echo "Merge request already exists, nothing to do"
  exit_status=0
else
  if [ $exit_status -eq 0 ]; then
      echo "Merge request created"
    else
      echo "HTTP ${http_status}: Failed to create merge request"
  fi
  cat response.json
  echo
fi

exit $exit_status