Add test to determine shell

The 'sh' filetype in Vim covers sh, bash and ksh. This function looks
for declared variables that results from Vim's own test. The global
variables are checked first since they are typically specifed by the
user in .vimrc

UltiSnips/sh.snippets

@@ -1,3 +1,17 @@
+global !p
+import vim
+def testShell(scope, shell):
+    return vim.eval("exists('" + scope + ":is_" + shell + "')")
+
+def getShell():
+    for scope in ["g", "b"]:
+        for shell in ["bash", "sh", "kornshell"]:
+            if testShell(scope, shell) == "1":
+                if shell == "kornshell":
+                    return "ksh"
+                return shell
+    return "sh"
+endglobal
 
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