perl -le 'print$ENV{SHELL}'
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-Blake

You can shave one char off of that; the -l doesn't do anything. Other then that, I think that's as short as it gets for shells that set ${SHELL} (which is POSIX, I think). (Note that the original isn't portable past whatever Unix they were using; ps's format isn't portable at least to my Linux. For that matter, mine doesn't need the -p parameter.)

Thanks,
James Mastros,
Just Another Perl Scribe

-l "sets $\ to the current value of $/" (see perlrun)

In otherwords, it automatically adds a "\n" to all print statements. I like to use it for short one liners, since it helps keep the output separate from my shell prompt.

Oh, and the options usually aren't counted as keystrokes in one-liner golf. i.e both would score a 16:

#         1234567890123456
perl -le 'print$ENV{SHELL}'
[download]

-Blake