> You should check for $ENV{SHELL} and emit "setenv VAR VALUE"
> lines if appropriate.

$ENV{SHELL} is not a way to determine your current shell. $ENV{SHELL} is your »preferred shell« (aka login shell in /etc/passwd), see The Single UNIX Specification, Version 3.

Example:

# we are in bash, all is fine
$ eval `perl -le 'print $ENV{SHELL} =~ /csh/ ? "setenv FOO bar" : "exp
+ort FOO=bar"'`

# now we switch to tcsh
$ tcsh
$ eval `perl -le 'print $ENV{SHELL} =~ /csh/ ? "setenv FOO bar" : "exp
+ort FOO=bar"'`
$ export: Command not found.
$ echo $SHELL
/bin/bash
[download]

It seems that there is no portable way to find out the name of your current shell. We discussed that some time ago at de.comp.os.unix.shell.

Looking at $ENV{SHELL} is probably the best we can do --- other programs seem to do it the same way (and therefore do it wrong):

$ env SHELL=/bin/sh ssh-agent
SSH_AUTH_SOCK=/tmp/ssh-XXN54U4g/agent.3950; export SSH_AUTH_SOCK;
SSH_AGENT_PID=3951; export SSH_AGENT_PID;
echo Agent pid 3951;
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$ env SHELL=/bin/tcsh ssh-agent
setenv SSH_AUTH_SOCK /tmp/ssh-XXPC7hgb/agent.3948;
setenv SSH_AGENT_PID 3949;
echo Agent pid 3949;
[download]

Similar output for /usr/X11/bin/resize