Fix #1369 (Use file extension to detect shell)

The precedence order that is used to determine the shell
is the following:
1. ShellCheck directive
2. Shebang
3. File extension
A new field `asFallbackShell` has been
added to the record type `AnalysisSpec`.

src/ShellCheck/AnalyzerLib.hs

@@ -175,7 +175,7 @@ makeCommentWithFix severity id code str fix =
 makeParameters spec =
     let params = Parameters {
         rootNode = root,
-        shellType = fromMaybe (determineShell root) $ asShellType spec,
+        shellType = fromMaybe (determineShell (asFallbackShell spec) root) $ asShellType spec,
         hasSetE = containsSetE root,
         hasLastpipe =
             case shellType params of
@@ -227,11 +227,13 @@ prop_determineShell4 = determineShellTest "#!/bin/ksh\n#shellcheck shell=sh\nfoo
 prop_determineShell5 = determineShellTest "#shellcheck shell=sh\nfoo" == Sh
 prop_determineShell6 = determineShellTest "#! /bin/sh" == Sh
 prop_determineShell7 = determineShellTest "#! /bin/ash" == Dash
+prop_determineShell8 = determineShellTest' (Just Ksh) "#!/bin/sh" == Sh
 
-determineShellTest = determineShell . fromJust . prRoot . pScript
-determineShell t = fromMaybe Bash $ do
+determineShellTest = determineShellTest' Nothing
+determineShellTest' fallbackShell = determineShell fallbackShell . fromJust . prRoot . pScript
+determineShell fallbackShell t = fromMaybe Bash $ do
     shellString <- foldl mplus Nothing $ getCandidates t
-    shellForExecutable shellString
+    shellForExecutable shellString `mplus` fallbackShell
   where
     forAnnotation t =
         case t of

src/ShellCheck/Checker.hs

@@ -48,6 +48,15 @@ tokenToPosition startMap t = fromMaybe fail $ do
   where
     fail = error "Internal shellcheck error: id doesn't exist. Please report!"
 
+shellFromFilename filename = foldl mplus Nothing candidates
+  where
+    shellExtensions = [(".ksh", Ksh)
+                      ,(".sh", Sh)
+                      ,(".bash", Bash)
+                      ,(".dash", Dash)]
+    candidates =
+        map (\(ext,sh) -> if ext `isSuffixOf` filename then Just sh else Nothing) shellExtensions
+
 checkScript :: Monad m => SystemInterface m -> CheckSpec -> m CheckResult
 checkScript sys spec = do
     results <- checkScript (csScript spec)
@@ -69,6 +78,7 @@ checkScript sys spec = do
                 as {
                     asScript = root,
                     asShellType = csShellTypeOverride spec,
+                    asFallbackShell = shellFromFilename $ csFilename spec,
                     asCheckSourced = csCheckSourced spec,
                     asExecutionMode = Executed,
                     asTokenPositions = tokenPositions

src/ShellCheck/Interface.hs

@@ -25,7 +25,7 @@ module ShellCheck.Interface
     , CheckResult(crFilename, crComments)
     , ParseSpec(psFilename, psScript, psCheckSourced, psShellTypeOverride)
     , ParseResult(prComments, prTokenPositions, prRoot)
-    , AnalysisSpec(asScript, asShellType, asExecutionMode, asCheckSourced, asTokenPositions)
+    , AnalysisSpec(asScript, asShellType, asFallbackShell, asExecutionMode, asCheckSourced, asTokenPositions)
     , AnalysisResult(arComments)
     , FormatterOptions(foColorOption, foWikiLinkCount)
     , Shell(Ksh, Sh, Bash, Dash)
@@ -138,6 +138,7 @@ newParseResult = ParseResult {
 data AnalysisSpec = AnalysisSpec {
     asScript :: Token,
     asShellType :: Maybe Shell,
+    asFallbackShell :: Maybe Shell,
     asExecutionMode :: ExecutionMode,
     asCheckSourced :: Bool,
     asTokenPositions :: Map.Map Id (Position, Position)
@@ -146,6 +147,7 @@ data AnalysisSpec = AnalysisSpec {
 newAnalysisSpec token = AnalysisSpec {
     asScript = token,
     asShellType = Nothing,
+    asFallbackShell = Nothing,
     asExecutionMode = Executed,
     asCheckSourced = False,
     asTokenPositions = Map.empty