Suggest using test -e instead of -a (fixes #3174).

CHANGELOG.md

@@ -3,6 +3,7 @@
 - SC2327/SC2328: Warn about capturing the output of redirected commands.
 - SC2329: Warn when (non-escaping) functions are never invoked.
 - SC2330: Warn about unsupported glob matches with [[ .. ]] in BusyBox.
+- SC2331: Suggest using standard -e instead of unary -a in tests.
 - Precompiled binaries for Linux riscv64 (linux.riscv64)
 ### Changed
 - SC2002 about Useless Use Of Cat is now disabled by default. It can be

src/ShellCheck/Analytics.hs

@@ -204,6 +204,7 @@ nodeChecks = [
     ,checkUnnecessaryParens
     ,checkPlusEqualsNumber
     ,checkExpansionWithRedirection
+    ,checkUnaryTestA
     ]
 
 optionalChecks = map fst optionalTreeChecks
@@ -5098,6 +5099,15 @@ checkExpansionWithRedirection params t =
         err redirectId 2328 $ "This redirection takes output away from the command substitution" ++ if suggestTee then " (use tee to duplicate)." else "."
 
 
+prop_checkUnaryTestA1 = verify checkUnaryTestA "[ -a foo ]"
+prop_checkUnaryTestA2 = verify checkUnaryTestA "[ ! -a foo ]"
+prop_checkUnaryTestA3 = verifyNot checkUnaryTestA "[ foo -a bar ]"
+checkUnaryTestA params t =
+    case t of
+        TC_Unary id _ "-a" _ ->
+            styleWithFix id 2331 "For file existence, prefer standard -e over legacy -a." $
+                fixWith [replaceStart id params 2 "-e"]
+        _ -> return ()
 
 return []
 runTests =  $( [| $(forAllProperties) (quickCheckWithResult (stdArgs { maxSuccess = 1 }) ) |])