website: add a troubleshooting section for the shell provisioner

website/source/docs/provisioners/shell.html.markdown

@@ -127,3 +127,23 @@ reboot so that SSH will eventually be killed automatically:
 reboot
 sleep 60
 ```
+
+## Troubleshooting
+
+*My shell script doesn't work correctly on Ubuntu*
+
+* On Ubuntu the /bin/sh shell is
+[dash](http://en.wikipedia.org/wiki/Debian_Almquist_shell). If your script has
+[bash](http://en.wikipedia.org/wiki/Bash_(Unix_shell\)) specific commands in it
+then put `#!/bin/bash` at the top of your script. Differences
+between dash and bash can be found on the [DashAsBinSh](https://wiki.ubuntu.com/DashAsBinSh) Ubuntu wiki page.
+
+*My shell works when I login but fails with the shell provisioner*
+
+* See the above tip. More than likely your login shell is using /bin/bash
+while the provisioner is using /bin/sh.
+
+*How do I tell what my shell script is doing?*
+
+* Adding a `-x` flag to the shebang at the top of the script (`#!/bin/sh -x`)
+will echo the script statements as it is executing.