diff --git a/pkgs/build-support/setup-hooks/patch-shebangs.sh b/pkgs/build-support/setup-hooks/patch-shebangs.sh
index 5bd1f5078ad00..6ee5987f905d0 100644
--- a/pkgs/build-support/setup-hooks/patch-shebangs.sh
+++ b/pkgs/build-support/setup-hooks/patch-shebangs.sh
@@ -133,9 +133,9 @@ patchShebangs() {
                 sed -e "1 s|.*|#\!$escapedInterpreterLine|" "$f" > "$tmpFile"
 
                 # Make original file writable if it is read-only
-                local restoreReadOnly
+                local restoreReadOnly=
                 if [[ ! -w "$f" ]]; then
-                    chmod +w "$f"
+                    chmod u+w "$f"
                     restoreReadOnly=true
                 fi
 
@@ -144,7 +144,7 @@ patchShebangs() {
                 cat "$tmpFile" > "$f"
                 rm "$tmpFile"
                 if [[ -n "${restoreReadOnly:-}" ]]; then
-                    chmod -w "$f"
+                    chmod u-w "$f"
                 fi
 
                 touch --date "$timestamp" "$f"
diff --git a/pkgs/test/stdenv/patch-shebangs.nix b/pkgs/test/stdenv/patch-shebangs.nix
index 487d144e77337..12d26ccb5a974 100644
--- a/pkgs/test/stdenv/patch-shebangs.nix
+++ b/pkgs/test/stdenv/patch-shebangs.nix
@@ -164,19 +164,37 @@ let
             . ${../../stdenv/generic/setup.sh}
             . ${../../build-support/setup-hooks/patch-shebangs.sh}
             mkdir -p $out/bin
-            echo "#!/bin/bash" > $out/bin/test
-            echo "echo -n hello" >> $out/bin/test
-            chmod 555 $out/bin/test
-            original_perms=$(stat -c %a $out/bin/test)
-            patchShebangs $out/bin/test
-            new_perms=$(stat -c %a $out/bin/test)
-            if ! [ "$original_perms" = "$new_perms" ]; then
-              echo "Permissions changed from $original_perms to $new_perms"
-              exit 1
-            fi
+            modes=(555 575 755 775 777)
+            for mode in "''${modes[@]}"
+            do
+              target=$out/bin/test-$mode
+              echo "#!/bin/bash" > "$target"
+              echo "echo -n hello" >> "$target"
+              chmod $mode "$target"
+              if ! [ "$mode" = "$(stat -c %a "$target")" ]; then
+                echo "chmod didn't set up test permissions as expected"
+                exit 1
+              fi
+            done
+            # This is structured as two loops + one patchShebangs so that at
+            # least one test exercises patchShebangs on multiple scripts in one
+            # invocation.
+            patchShebangs $out/bin
+            for mode in "''${modes[@]}"
+            do
+              target=$out/bin/test-$mode
+              new_perms=$(stat -c %a "$target")
+              if ! [ "$mode" = "$new_perms" ]; then
+                echo "Permissions changed from $mode to $new_perms"
+                exit 1
+              fi
+            done
           ''
         ];
-        assertion = "grep '^#!${lib.getExe earlyBash}' $out/bin/test > /dev/null";
+        # This tests that any of the test files have this line, when we should
+        # really be testing all of them. But it's a little redundant with other
+        # tests anyway.
+        assertion = "grep '^#!${lib.getExe earlyBash}' $out/bin/test-* > /dev/null";
       })
       // {
         meta = { };