Don't print colors when $TERM is 'dumb' or unset (fixes #2260)

CHANGELOG.md

@@ -6,6 +6,7 @@
 ### Fixed
 - SC2102 about repetitions in ranges no longer triggers on [[ -v arr[xx] ]]
 - SC2290: Warn about misused = in declare & co, which were not caught by SC2270+
+- The flag --color=auto no longer outputs color when TERM is "dumb" or unset
 
 ### Changed
 - SC2048: Warning about $\* now also applies to ${array[\*]}

src/ShellCheck/Formatter/Format.hs

@@ -28,6 +28,7 @@ import Data.Array
 import Data.List
 import System.IO
 import System.Info
+import System.Environment
 
 -- A formatter that carries along an arbitrary piece of data
 data Formatter = Formatter {
@@ -68,12 +69,14 @@ makeNonVirtual comments contents =
 
 
 shouldOutputColor :: ColorOption -> IO Bool
-shouldOutputColor colorOption = do
-    term <- hIsTerminalDevice stdout
+shouldOutputColor colorOption =
+    case colorOption of
+        ColorAlways -> return True
+        ColorNever -> return False
+        ColorAuto -> do
+            isTerminal <- hIsTerminalDevice stdout
+            term <- lookupEnv "TERM"
             let windows = "mingw" `isPrefixOf` os
-    let isUsableTty = term && not windows
-    let useColor = case colorOption of
-                       ColorAlways -> True
-                       ColorNever -> False
-                       ColorAuto -> isUsableTty
-    return useColor
+            let dumbTerm = term `elem` [Just "dumb", Just "", Nothing]
+            let isUsableTty = isTerminal && not windows && not dumbTerm
+            return isUsableTty