GNU screen not seeing $SHELL env var?

[Tom Rodman <cygzr at trodman dot com>]

Not sure when this problem started.  The issue:
GNU screen is starting w/an empty or undefined value for $SHELL, so
the login shell for all screen windows is incorrect:

  $ uname -a; cygcheck -c cygwin
  CYGWIN_NT-5.1 aqua 1.7.16s(0.261/5/3) 20120604 01:26:35 i686 Cygwin
  Cygwin Package Information
  Package              Version        Status
  cygwin               1.7.15-1       OK
  $ cygcheck -f /bin/screen
  screen-4.0.3-7
  $ grep SHELL ~/.screenrc
  shell -$SHELL
  $ echo $SHELL
  /bin/bash
  $ bash -c 'echo SHELL: $SHELL'  # does this prove SHELL is exported?
  SHELL: /bin/bash
  $ screen 
  --snip/switch to a separate shell to run [procps -wwH -o pid,ppid,user,tty,args -e]:
   5988  2200 adm_tsr  pty3       /bin/bash --login
   1492  5988 adm_tsr  pty3         screen
   4476  1492 adm_tsr  ?              SCREEN
   5420  4476 adm_tsr  pty4             -
  --snip/now to shell w/pid 5420 inside screen:
  $ echo SHELL: \[$SHELL]
  SHELL: []
  $ 

Notice above that pid 5420 is named '-', which corresponds
to /bin/sh, instead of /bin/bash.

  $ command ps -elW |grep 5420
  I    5420    4476    5420       2476  pty4    1008 07:54:08 /usr/bin/sh

The workaround is to export SHELL prior to starting screen.
I do not understand why that works, it seems that SHELL was already
exported, since (in a non screen login shell): 

   sh -c 'echo SHELL: $SHELL'

echos /bin/bash

Is anyone else seeing this issue?

--
thanks,
Tom

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