diff --git a/cmake/RunTests.cmake b/cmake/RunTests.cmake
index d470793a27..8d5b0d2402 100644
--- a/cmake/RunTests.cmake
+++ b/cmake/RunTests.cmake
@@ -64,6 +64,11 @@ endif()
 
 set(ENV{SYSTEM_NAME} ${CMAKE_HOST_SYSTEM_NAME})  # used by test/helpers.lua.
 
+if(NOT WIN32)
+  # Tests assume POSIX "sh" and may fail if SHELL=fish. #24941 #6172
+  set(ENV{SHELL} sh)
+endif()
+
 execute_process(
   # Note: because of "-ll" (low-level interpreter mode), some modules like
   # _editor.lua are not loaded.
diff --git a/test/functional/vimscript/system_spec.lua b/test/functional/vimscript/system_spec.lua
index 762e8877ce..90aab48d61 100644
--- a/test/functional/vimscript/system_spec.lua
+++ b/test/functional/vimscript/system_spec.lua
@@ -335,12 +335,12 @@ describe('system()', function()
       if is_os('win') then
         eq("echoed\n", eval('system("echo echoed")'))
       else
-        eq("echoed", eval('system("echo -n echoed")'))
+        eq("echoed", eval('system("printf echoed")'))
       end
     end)
     it('to backgrounded command does not crash', function()
       -- This is indeterminate, just exercise the codepath. May get E5677.
-      feed_command('call system(has("win32") ? "start /b /wait cmd /c echo echoed" : "echo -n echoed &")')
+      feed_command('call system(has("win32") ? "start /b /wait cmd /c echo echoed" : "printf echoed &")')
       local v_errnum = string.match(eval("v:errmsg"), "^E%d*:")
       if v_errnum then
         eq("E5677:", v_errnum)