docs: Fix missing quotes in su zulip -c documentation.

This fixes an actual user-facing issue in our mobile push
notifications documentation (where we were incorrectly failing to
quote the argument to `./manage.py register_server` making it not
work), as well as preventing future similar issues from occurring
again via a linter rule.

docs/production/mobile-push-notifications.md

@@ -29,7 +29,7 @@ follows:
     registration command:
     ```
     # As root:
-    su zulip -c /home/zulip/deployments/current/manage.py register_server
+    su zulip -c '/home/zulip/deployments/current/manage.py register_server'
     # Or as the zulip user, you can skip the `su zulip -c`:
     /home/zulip/deployments/current/manage.py register_server
     ```

docs/production/settings.md

@@ -16,7 +16,7 @@ administrator can do.  To change any of the following settings, edit
 the `/etc/zulip/settings.py` file on your Zulip server, and then
 restart the server with the following command:
 ```
-su zulip -c /home/zulip/deployments/current/scripts/restart-server
+su zulip -c '/home/zulip/deployments/current/scripts/restart-server'
 ```
 
 ## Specific settings

docs/subsystems/full-text-search.md

@@ -77,7 +77,7 @@ messages until the migration finishes.
 
 Once the migrations are complete, restart Zulip:
 
-    su zulip -c /home/zulip/deployments/current/scripts/restart-server
+    su zulip -c '/home/zulip/deployments/current/scripts/restart-server'
 
 Now, you can use full-text search across all languages.
 
@@ -100,7 +100,7 @@ Then, set `USING_PGROONGA = False` in `/etc/zulip/settings.py`:
 
 And, restart Zulip:
 
-    su zulip -c /home/zulip/deployments/current/scripts/restart-server
+    su zulip -c '/home/zulip/deployments/current/scripts/restart-server'
 
 Now, full-text search feature based on PGroonga is disabled.  If you'd
 like, you can also remove the `pgroonga = enabled` line in

scripts/lib/install

@@ -315,7 +315,7 @@ if [ "$has_appserver" = 0 ]; then
         # If we're installing from a git checkout, we need to run
         # `tools/update-prod-static` in order to build the static
         # assets.
-        su zulip -c "/home/zulip/deployments/current/tools/update-prod-static --authors-not-required"
+        su zulip -c '/home/zulip/deployments/current/tools/update-prod-static --authors-not-required'
     fi
 fi
 
@@ -331,7 +331,7 @@ if [ -n "$NO_INIT_DB" ]; then
 
  Stopping because --no-init-db was passed.  To complete the installation, run:
 
-   su zulip -c /home/zulip/deployments/current/scripts/setup/initialize-database
+   su zulip -c '/home/zulip/deployments/current/scripts/setup/initialize-database'
 EOF
     exit 0
 fi

tools/linter_lib/custom_check.py

@@ -770,6 +770,10 @@ def build_custom_checkers(by_lang):
          'include_only': set(['docs/']),
          'description': "Use relative links (../foo/bar.html) to other documents in docs/",
          },
+        {'pattern': "su zulip -c [^']",
+         'include_only': set(['docs/']),
+         'description': "Always quote arguments using `su zulip -c '` to avoid confusion about how su works.",
+         },
         {'pattern': r'\][(][^#h]',
          'include_only': set(['README.md', 'CONTRIBUTING.md']),
          'description': "Use absolute links from docs served by GitHub",