Write command to list failed services

Guides/shell.md

@@ -15,3 +15,10 @@ To use a physical link instead of a symlink, use `-S` instead of `-s`
 ```sh
 sudo tar --create --gzip --file bck_var_log.tgz --verbose /var/log
 ```
+
+## Systemctl
+
+```sh
+systemctl list-units --state=failed
+```
+