diff --git a/require-ess/src/require.c b/require-ess/src/require.c
index 8ed30e50427bb3555e35ee6a0776e334295e5ec7..879edb56a61bae1de8b8d4a109fc5ec3abfef8b7 100644
--- a/require-ess/src/require.c
+++ b/require-ess/src/require.c
@@ -36,10 +36,10 @@ DBCORE_API int epicsStdCall iocshCmd(const char *cmd);
 
 int requireDebug;
 
-#if defined(__unix)
+#if defined(__unix) || defined(UNIX)
 
 #ifndef OS_CLASS
-#ifdef __linux
+#if defined(__linux) || defined(linux)
 #define OS_CLASS "Linux"
 #endif  // __linux