Lintian error: depends-on-essential-package-without-using-version bash

[Drew Parsons <dparsons@emerall.com>]

I'm packaging up a shell script called tzwatch which displays the time from
different timezones.  It's actually a bash script since I thought it had
some bashisms (while getopts case select), although reading through the info
pages for bash, it seems these are standard sh commands after all.  I marked
it as a bash script by comparison with tzselect, on which the script is
based, which is also #!/bin/bash.

Thinking it was truly a bash shell, I added a dependency on the bash package
in debian/control:

Depends: bash

But doing this, lintian gives the warning:

E: tzwatch: depends-on-essential-package-without-using-version bash

I can't find much in Debian policy about this, so I'd like to ask, what
does this error mean, and why is it an error?  Is it correct that I do not
in fact have to declare the dependency on bash, since it is an "Essential"
package?  Even if that's the case, why do I get this error?

Lastly, is there a reliable way of confirming that I do not in fact have any
bashisms, so that #!/bin/sh would be sufficient?  sh is linked to bash, so
simply writing #!/bin/sh in the script is not sufficient.

Drew

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