Use bash from $SHELL when it makes sense

src/Options/Applicative/Builder/Completer.hs

@@ -15,7 +15,9 @@ import Prelude
 import Control.Exception (IOException, try)
 import Data.List (isPrefixOf)
 #ifdef MIN_VERSION_process
+import Data.List (isSuffixOf)
 import System.Process (readProcess)
+import System.Environment (lookupEnv)
 #endif
 
 import Options.Applicative.Types
@@ -40,8 +42,23 @@ bashCompleter :: String -> Completer
 #ifdef MIN_VERSION_process
 bashCompleter action = Completer $ \word -> do
   let cmd = unwords ["compgen", "-A", action, "--", requote word]
-  result <- tryIO $ readProcess "bash" ["-c", cmd] ""
+  bash <- getBash
+  result <- tryIO $ readProcess bash ["-c", cmd] ""
   return . lines . either (const []) id $ result
+
+-- | Determines the bash executable. Ideally we'd invoke the same bash that
+-- is currently active. If $SHELL does not seem to be set to a bash executable
+-- we don't assume $SHELL is bash and we take bash from the $PATH.
+-- This fixes file completion in cases where a virtual environment with a
+-- non-interactive bash is loaded with direnv, nix-shell or similar.
+getBash :: IO String
+getBash = do
+  shellEnv <- lookupEnv "SHELL"
+  pure (case shellEnv of
+      Just exe | "/bash" `isSuffixOf` exe -> exe
+      _ -> "bash"
+    )
+
 #else
 bashCompleter = const $ Completer $ const $ return []
 #endif