Parse let arguments as arithmetic expressions.

ShellCheck/Analytics.hs

@@ -1195,12 +1195,20 @@ prop_checkArithmeticDeref4 = verifyNot checkArithmeticDeref "(( ! $? ))"
 prop_checkArithmeticDeref5 = verifyNot checkArithmeticDeref "(($1))"
 prop_checkArithmeticDeref6 = verify checkArithmeticDeref "(( a[$i] ))"
 prop_checkArithmeticDeref7 = verifyNot checkArithmeticDeref "(( 10#$n ))"
+prop_checkArithmeticDeref8 = verifyNot checkArithmeticDeref "let i=$i+1"
 checkArithmeticDeref params t@(TA_Expansion _ [T_DollarBraced id l]) =
-    unless (excepting $ bracedString l) $
+    unless ((isException $ bracedString l) || (not isNormal)) $
         style id 2004 "$ on variables in (( )) is unnecessary."
   where
-    excepting [] = True
-    excepting s = any (`elem` "/.:#%?*@") s || isDigit (head s)
+    isException [] = True
+    isException s = any (`elem` "/.:#%?*@") s || isDigit (head s)
+    isNormal = fromMaybe True $ msum $ map isNormalContext $ (parents params t)
+    isNormalContext t =
+        case t of
+            T_Arithmetic {} -> return True
+            T_DollarArithmetic {} -> return True
+            T_SimpleCommand {} -> return False
+            _ -> fail "Irrelevant"
 checkArithmeticDeref _ _ = return ()
 
 prop_checkArithmeticBadOctal1 = verify checkArithmeticBadOctal "(( 0192 ))"
@@ -2265,6 +2273,7 @@ prop_checkUnused15= verifyNotTree checkUnusedAssignments "x=(1); n=0; (( x[n] ))
 prop_checkUnused16= verifyNotTree checkUnusedAssignments "foo=5; declare -x foo"
 prop_checkUnused17= verifyNotTree checkUnusedAssignments "read -i 'foo' -e -p 'Input: ' bar; $bar;"
 prop_checkUnused18= verifyNotTree checkUnusedAssignments "a=1; arr=( [$a]=42 ); echo \"${arr[@]}\""
+prop_checkUnused19= verifyNotTree checkUnusedAssignments "a=1; let b=a+1; echo $b"
 checkUnusedAssignments params t = snd $ runWriter (mapM_ checkAssignment flow)
   where
     flow = variableFlow params

ShellCheck/Parser.hs

@@ -1345,20 +1345,20 @@ readSimpleCommand = called "simple command" $ do
     case cmd of
       Nothing -> return $ makeSimpleCommand id1 id2 prefix [] []
       Just cmd -> do
-            suffix <- option [] $
-                        if isModifierCommand cmd
-                        then readModifierSuffix
-                        else if isTimeCommand cmd
-                             then readTimeSuffix
-                             else readCmdSuffix
+            suffix <- option [] $ getParser readCmdSuffix cmd [
+                        (["declare", "export", "local", "readonly", "typeset"], readModifierSuffix),
+                        (["time"], readTimeSuffix),
+                        (["let"], readLetSuffix)
+                    ]
             return $ makeSimpleCommand id1 id2 prefix [cmd] suffix
   where
-    isModifierCommand (T_NormalWord _ [T_Literal _ s]) =
-        s `elem` ["declare", "export", "local", "readonly", "typeset"]
-    isModifierCommand _ = False
-    -- Might not belong in T_SimpleCommand. Fixme?
-    isTimeCommand (T_NormalWord _ [T_Literal _ "time"]) = True
-    isTimeCommand _ = False
+    isCommand strings (T_NormalWord _ [T_Literal _ s]) = s `elem` strings
+    isCommand _ _ = False
+    getParser def cmd [] = def
+    getParser def cmd ((list, action):rest) =
+        if isCommand list cmd
+        then action
+        else getParser def cmd rest
 
 prop_readPipeline = isOk readPipeline "! cat /etc/issue | grep -i ubuntu"
 prop_readPipeline2 = isWarning readPipeline "!cat /etc/issue | grep -i ubuntu"
@@ -1782,6 +1782,22 @@ readTimeSuffix = do
         lookAhead $ char '-'
         readCmdWord
 
+-- Fixme: this is a hack that doesn't handle let '++c' or let a\>b
+readLetSuffix = many1 (readIoRedirect <|> try readLetExpression <|> readCmdWord)
+  where
+    readLetExpression = do
+        startPos <- getPosition
+        expression <- readStringForParser readCmdWord
+        subParse startPos readArithmeticContents expression
+
+-- Get whatever a parser would parse as a string
+readStringForParser parser = do
+    pos <- lookAhead (parser >> getPosition)
+    s <- readUntil pos
+    return s
+  where
+    readUntil endPos = anyChar `reluctantlyTill` (getPosition >>= guard . (== endPos))
+
 prop_readAssignmentWord = isOk readAssignmentWord "a=42"
 prop_readAssignmentWord2 = isOk readAssignmentWord "b=(1 2 3)"
 prop_readAssignmentWord3 = isWarning readAssignmentWord "$b = 13"