Created SC2117 (markdown)

SC2117.md

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+## To run commands as another user, use su -c or sudo.
+
+### Problematic code:
+
+    whoami
+    su
+    whoami
+
+### Correct code:
+
+    whoami
+    sudo whoami
+
+### Rationale:
+
+It's commonly believed that `su` makes a session run as another user. In reality, it starts an entirely new shell, independent of the one currently running your script.
+
+`su; whoami` will start a root shell and wait for it to exit before running `whoami`. It will not start a root shell and then proceed to run `whoami` in it. 
+
+To run commands as another user, use `sudo some command` or `su -c 'some command'`. `sudo` is preferred when available, as it doesn't require additional quoting and can be configured to run passwordless if desired.
+
+
+### Contraindications
+
+If you're aware of the above and want to e.g. start an interactive shell for a user, feel free to ignore this message.