Continuous-Integration-and-Continuous-Deployment.md

 # Variables
 
+## General Variables
+
+A lot of variables are used by default in the runner environment provided by GitLab:
+
+https://docs.gitlab.com/ee/ci/variables/
+
+They usually come with the prefix `CI_` and are used in the `.gitlab-ci.yml`, e.g.
+
+```yaml
+  rules:
+    - if: $CI_PROJECT_URL != 'https://collaborating.tuhh.de/cfg0846/research-topics-mat-tuhh'
+      when: never
+    - if: '$CI_COMMIT_REF_NAME != "master"'
+```
+
+## Project CI/CD-Variables
+
+The repository stores different CI/CD variables necessary for authentication during ssh-services: https://docs.gitlab.com/ee/ci/variables/#project-cicd-variables
+
+Depending on their domain, they carry different prefixes:
+* `ASTEROID_` for webpage hosted on asteroids
+* `REVIEW_` for the review app hosted on [www3-dev.tuhh.de]()
+* `SSH_` for the review app hosted on [www3.tuhh.de]()
+* `TUHH_` for the deployment on the web server of the institute
+
+Depending on their suffix they are used at certain parts of the corresponding jobs in the pipeline.
+
+## `*_{PRIVATE_KEY,USER,HOST}`
+
+These Variables are used e.g. in `ssh` or `rsync` commands like
+
+```bash
+echo "$REVIEW_SSH_PRIVATE_KEY" | tr -d '\r' | ssh-add -
+```
+or 
+
+```bash
+ssh $ASTEROID_USER@$ASTEROID_HOST ..."
+```
+
 # Pipeline
 
 ## Jobs and Artifacts
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