diff --git a/.scripts/bin-install.sh b/.scripts/bin-install.sh
index 6ee6a98dd37..cd8571e5140 100755
--- a/.scripts/bin-install.sh
+++ b/.scripts/bin-install.sh
@@ -1,17 +1,21 @@
 #!/bin/bash
 
 # This is called by the Makefile *after* an installation into the
-# prefix, so we add the rest of the files that go into a binary pacakge.
+# prefix, so we add the rest of the files that go into a binary package.
 
 set -eu
 
 windows () {
-  [ -v OS ] && [ "$OS" = "Windows_NT" ]
+  # This seems portable enough and does not trigger an
+  # undefined variable error (see set -u above) if $OS
+  # is unset (like in linux/mac). Note: OSX's bash is usually
+  # old and does not support '[ -v OS ]'.
+  [[ "${OS:-}" = "Windows_NT" ]]
 }
 
 if [ $# -ne 1 ]; then
-	echo "Usage: $0 <prefix>" >&2
-	exit 1
+  echo "Usage: $0 <prefix>" >&2
+  exit 1
 fi
 
 PREFIX="$1"
diff --git a/.scripts/make_fstar_version.sh b/.scripts/make_fstar_version.sh
index d4ebe49b567..e47448b6a36 100755
--- a/.scripts/make_fstar_version.sh
+++ b/.scripts/make_fstar_version.sh
@@ -1,11 +1,14 @@
 #!/usr/bin/env bash
 
+windows () {
+  [[ "${OS:-}" = "Windows_NT" ]]
+}
+
 if [[ -z "$FSTAR_VERSION" ]]; then
   FSTAR_VERSION=$(head -n 1 version.txt)~dev
 fi
 
-if [ "$OS" = "Windows_NT" ]
-then
+if windows; then
    if [ "$PROCESSOR_ARCHITECTURE" = "AMD64" ]
    then
      PLATFORM="Windows_x64"
diff --git a/.scripts/mk-package.sh b/.scripts/mk-package.sh
index 1f9b7ec472b..7d37bed1062 100755
--- a/.scripts/mk-package.sh
+++ b/.scripts/mk-package.sh
@@ -25,7 +25,11 @@ PREFIX="$1"
 ARCHIVE="$2"
 
 windows () {
-  [ -v OS ] && [ "$OS" = "Windows_NT" ]
+  # This seems portable enough and does not trigger an
+  # undefined variable error (see set -u above) if $OS
+  # is unset (like in linux/mac). Note: OSX's bash is usually
+  # old and does not support '[ -v OS ]'.
+  [[ "${OS:-}" = "Windows_NT" ]]
 }
 
 # Computes a (hopefully) sensible default for the current system