SC2236/SC2237-equivalent to binary operators: =, !=, -eq, -ne, -gt, -ge, -lt, and -le

[e-kwsm]

For new checks and feature suggestions

• shellcheck.net (i.e. the latest commit) currently gives no useful warnings about this
• I searched through https://github.com/koalaman/shellcheck/issues and didn't find anything related

Here's a snippet or screenshot that shows the problem:

#!/bin/sh
[ ! "$USER" = me ]
! [ "$USER" != me ]
[ ! $$ -eq 0 ]
! [ $$ -gt 0 ]

Here's what shellcheck currently says:

nothing

Here's what I wanted or expected to see:

[ ! "$USER" = me ]
  ^-- SCXXXX (style): Use a != b instead of ! a = b
! [ "$USER" != me ]
^-- SCXXXX (style): Use [ a = b ] instead of ! [ a != b ]
[ ! $$ -eq 0 ]
  ^-- SCXXXX (style): Use a -ne b instead of ! a -eq b
! [ $$ -gt 0 ]
^-- SCXXXX (style): Use [ a -le b ] instead of ! [ a -gt b ]

similarly to SC2236 and SC2237 (double negation).

[e-kwsm]

I am not familiar with Haskell, but something like below will address the issue

--- a/src/ShellCheck/Analytics.hs
+++ b/src/ShellCheck/Analytics.hs
@@ -3906,12 +3906,35 @@ checkInvertedStringTest _ t =
                 "-n" -> style (getId t) 2236 "Use -z instead of ! -n."
                 "-z" -> style (getId t) 2236 "Use -n instead of ! -z."
                 _ -> return ()
+        TC_Unary _ _ "!" (TC_Binary _ _ op _ _) ->
+            case op of
+                "=" -> style (getId t) 9999 "Use a != b instead of ! a = b."
+                "!=" -> style (getId t) 9999 "Use a = b instead of ! a != b."
+                "-eq" -> style (getId t) 9999 "Use a -ne b instead of ! a -eq b."
+                "-ne" -> style (getId t) 9999 "Use a -eq b instead of ! a -ne b."
+                "-gt" -> style (getId t) 9999 "Use a -le b instead of ! a -gt b."
+                "-ge" -> style (getId t) 9999 "Use a -lt b instead of ! a -ge b."
+                "-lt" -> style (getId t) 9999 "Use a -ge b instead of ! a -lt b."
+                "-le" -> style (getId t) 9999 "Use a -gt b instead of ! a -le b."
+                _ -> return ()
         T_Banged _ (T_Pipeline _ _
           [T_Redirecting _ _ (T_Condition _ _ (TC_Unary _ _ op _))]) ->
             case op of
                 "-n" -> style (getId t) 2237 "Use [ -z .. ] instead of ! [ -n .. ]."
                 "-z" -> style (getId t) 2237 "Use [ -n .. ] instead of ! [ -z .. ]."
                 _ -> return ()
+        T_Banged _ (T_Pipeline _ _
+          [T_Redirecting _ _ (T_Condition _ _ (TC_Binary _ _ op _ _))]) ->
+            case op of
+                "=" -> style (getId t) 9999 "Use [ a != b ] instead of ! [ a = b ]."
+                "!=" -> style (getId t) 9999 "Use [ a = b ] instead of ! [ a != b ]."
+                "-eq" -> style (getId t) 9999 "Use [ a -ne b ] instead of ! [ a -eq b ]."
+                "-ne" -> style (getId t) 9999 "Use [ a -eq b ] instead of ! [ a -ne b ]."
+                "-gt" -> style (getId t) 9999 "Use [ a -le b ] instead of ! [ a -gt b ]."
+                "-ge" -> style (getId t) 9999 "Use [ a -lt b ] instead of ! [ a -ge b ]."
+                "-lt" -> style (getId t) 9999 "Use [ a -ge b ] instead of ! [ a -lt b ]."
+                "-le" -> style (getId t) 9999 "Use [ a -gt b ] instead of ! [ a -le b ]."
+                _ -> return ()
         _ -> return ()

 prop_checkRedirectionToCommand1 = verify checkRedirectionToCommand "ls > rm"