Execute authentication preparation script lines only if executables exist

mixins.yml

 .prepareaccess:
   before_script:
-    - if [[ -n $SSH_PRIVATE_KEY ]]; then eval $(ssh-agent -s); fi
-    - if [[ -n $SSH_PRIVATE_KEY ]]; then echo "$SSH_PRIVATE_KEY" | tr -d '\r' | ssh-add - > /dev/null; fi
-    - if [[ -n $SSH_PRIVATE_KEY ]]; then mkdir -p ~/.ssh; fi
-    - if [[ -n $SSH_PRIVATE_KEY ]]; then chmod 700 ~/.ssh; fi
-    - if [[ -n $SSH_PRIVATE_KEY ]]; then ssh-keyscan ${CI_SERVER_HOST} >> ~/.ssh/known_hosts; fi
-    - if [[ -n $SSH_PRIVATE_KEY ]]; then chmod 644 ~/.ssh/known_hosts; fi
-    - if [[ -n $SSH_PRIVATE_KEY ]]; then ssh git@${CI_SERVER_HOST}; fi
-    - if [[ -n $GITLAB_ACCESS_TOKEN ]]; then glab auth login --hostname ${CI_SERVER_HOST} --token $GITLAB_ACCESS_TOKEN; fi
-    - if [[ -n $GITLAB_PRIVATE_TOKEN ]]; then glab auth login --hostname ${CI_SERVER_HOST} --token $GITLAB_PRIVATE_TOKEN; fi
+    - if [[ command -v ssh-agent ]] && [[ -n $SSH_PRIVATE_KEY ]]; then eval $(ssh-agent -s); fi
+    - if [[ command -v ssh-add ]] && [[ -n $SSH_PRIVATE_KEY ]]; then echo "$SSH_PRIVATE_KEY" | tr -d '\r' | ssh-add - > /dev/null; fi
+    - if [[ command -v ssh-add ]] && [[ -n $SSH_PRIVATE_KEY ]]; then mkdir -p ~/.ssh; fi
+    - if [[ command -v ssh-add ]] && [[ -n $SSH_PRIVATE_KEY ]]; then chmod 700 ~/.ssh; fi
+    - if [[ command -v ssh-keyscan ]] && [[ -n $SSH_PRIVATE_KEY ]]; then ssh-keyscan ${CI_SERVER_HOST} >> ~/.ssh/known_hosts; fi
+    - if [[ command -v ssh-keyscan ]] && [[ -n $SSH_PRIVATE_KEY ]]; then chmod 644 ~/.ssh/known_hosts; fi
+    - if [[ command -v ssh ]] && [[ -n $SSH_PRIVATE_KEY ]]; then ssh git@${CI_SERVER_HOST}; fi
+    - if [[ command -v glab ]] && [[ -n $GITLAB_ACCESS_TOKEN ]]; then glab auth login --hostname ${CI_SERVER_HOST} --token $GITLAB_ACCESS_TOKEN; fi
+    - if [[ command -v glab ]] && [[ -n $GITLAB_PRIVATE_TOKEN ]]; then glab auth login --hostname ${CI_SERVER_HOST} --token $GITLAB_PRIVATE_TOKEN; fi
     - if [[ -n $GITLAB_ACCESS_TOKEN ]]; then composer config -g --no-plugins --no-interaction gitlab-token.${CI_SERVER_HOST} $GITLAB_ACCESS_TOKEN; fi
     - if [[ -n $GITLAB_PRIVATE_TOKEN ]]; then composer config -g --no-plugins --no-interaction gitlab-token.${CI_SERVER_HOST} $GITLAB_PRIVATE_TOKEN; fi
     - if [[ -n $DOCKER_AUTH_CONFIG ]]; then mkdir -p ~/.docker; echo $DOCKER_AUTH_CONFIG > ~/.docker/config.json; fi