diff --git a/Makefile.include b/Makefile.include
index 02f5d81f07..12feb826d0 100644
--- a/Makefile.include
+++ b/Makefile.include
@@ -103,8 +103,11 @@ CLEAN = $(filter clean, $(MAKECMDGOALS))
 include $(RIOTMAKE)/utils/variables.mk
 include $(RIOTMAKE)/utils/strings.mk
 
-# get host operating system
-OS := $(shell uname)
+
+# UNAME is always needed so use simple variable expansion so only evaluated once
+UNAME := $(shell uname -m -s)
+OS = $(word 1, $(UNAME))
+OS_ARCH = $(word 2, $(UNAME))
 
 # set python path, e.g. for tests
 PYTHONPATH := $(RIOTBASE)/dist/pythonlibs/:$(PYTHONPATH)
diff --git a/boards/common/remote/Makefile.include b/boards/common/remote/Makefile.include
index 4d5c2a9944..4e3cf15fed 100644
--- a/boards/common/remote/Makefile.include
+++ b/boards/common/remote/Makefile.include
@@ -2,7 +2,6 @@
 export PROGRAMMER ?= cc2538-bsl
 
 ifeq ($(PROGRAMMER),cc2538-bsl)
-  OS := $(shell uname)
   ifeq ($(OS),Linux)
     PORT_BSL ?= $(PORT_LINUX)
   else ifeq ($(OS),Darwin)
diff --git a/boards/native/Makefile.dep b/boards/native/Makefile.dep
index 8eec472da0..fb4182b31a 100644
--- a/boards/native/Makefile.dep
+++ b/boards/native/Makefile.dep
@@ -7,7 +7,7 @@ ifneq (,$(filter mtd,$(USEMODULE)))
 endif
 
 ifneq (,$(filter can,$(USEMODULE)))
-  ifeq ($(shell uname -s),Linux)
+  ifeq ($(OS),Linux)
     USEMODULE += can_linux
     CFLAGS += -DCAN_DLL_NUMOF=2
   endif
diff --git a/boards/native/Makefile.include b/boards/native/Makefile.include
index 8fb4b53300..213c3f6e53 100644
--- a/boards/native/Makefile.include
+++ b/boards/native/Makefile.include
@@ -5,7 +5,7 @@ export NATIVEINCLUDES += -I$(RIOTBASE)/drivers/include/
 
 USEMODULE += native-drivers
 
-ifeq ($(shell uname -s),Darwin)
+ifeq ($(OS),Darwin)
   DEBUGGER ?= lldb
 else
   DEBUGGER ?= gdb
@@ -27,18 +27,18 @@ ifeq (,$(filter -std=%, $(CFLAGS)))
   CFLAGS += -std=gnu99
 endif
 
-ifeq ($(shell uname -m),x86_64)
+ifeq ($(OS_ARCH),x86_64)
   CFLAGS += -m32
 endif
 ifneq (,$(filter -DDEVELHELP,$(CFLAGS)))
   CFLAGS += -fstack-protector-all
 endif
-ifeq ($(shell uname -s),FreeBSD)
-  ifeq ($(shell uname -m),amd64)
+ifeq ($(OS),FreeBSD)
+  ifeq ($(OS_ARCH),amd64)
     CFLAGS += -m32 -DCOMPAT_32BIT -B/usr/lib32
   endif
 endif
-ifeq ($(shell uname -s),Darwin)
+ifeq ($(OS),Darwin)
   CFLAGS += -Wno-deprecated-declarations
 endif
 
@@ -46,11 +46,11 @@ endif
 CXXUWFLAGS +=
 CXXEXFLAGS +=
 
-ifeq ($(shell uname -m),x86_64)
+ifeq ($(OS_ARCH),x86_64)
   export LINKFLAGS += -m32
 endif
-ifeq ($(shell uname -s),FreeBSD)
-  ifeq ($(shell uname -m),amd64)
+ifeq ($(OS),FreeBSD)
+  ifeq ($(OS_ARCH),amd64)
     export LINKFLAGS += -m32 -DCOMPAT_32BIT -L/usr/lib32 -B/usr/lib32
   endif
   export LINKFLAGS += -L $(BINDIR)
@@ -60,7 +60,7 @@ endif
 
 # clean up unused functions
 CFLAGS += -ffunction-sections -fdata-sections
-ifeq ($(shell uname -s),Darwin)
+ifeq ($(OS),Darwin)
   export LINKFLAGS += -Wl,-dead_strip
 else
   export LINKFLAGS += -Wl,--gc-sections
@@ -113,7 +113,7 @@ endif
 
 # backward compatability with glibc <= 2.17 for native
 ifeq ($(CPU),native)
-  ifeq ($(shell uname -s),Linux)
+  ifeq ($(OS),Linux)
     ifeq ($(shell ldd --version |  awk '/^ldd/{if ($$NF < 2.17) {print "yes"} else {print "no"} }'),yes)
 	  LINKFLAGS += -lrt
     endif
@@ -123,7 +123,7 @@ endif
 # clumsy way to enable building native on osx:
 BUILDOSXNATIVE = 0
 ifeq ($(CPU),native)
-  ifeq ($(shell uname -s),Darwin)
+  ifeq ($(OS),Darwin)
     BUILDOSXNATIVE = 1
   endif
 endif
diff --git a/boards/openmote-b/Makefile.include b/boards/openmote-b/Makefile.include
index 5f37a86536..d88914ce57 100644
--- a/boards/openmote-b/Makefile.include
+++ b/boards/openmote-b/Makefile.include
@@ -6,7 +6,6 @@ PORT_DARWIN ?= $(firstword $(sort $(wildcard /dev/tty.usbserial*)))
 export PROGRAMMER ?= cc2538-bsl
 
 ifeq ($(PROGRAMMER),cc2538-bsl)
-  OS := $(shell uname)
   ifeq ($(OS),Linux)
     PORT_BSL ?= $(PORT_LINUX)
   else ifeq ($(OS),Darwin)
diff --git a/cpu/native/Makefile b/cpu/native/Makefile
index 9c2b87b709..cc0605cc5f 100644
--- a/cpu/native/Makefile
+++ b/cpu/native/Makefile
@@ -3,7 +3,7 @@ MODULE = cpu
 DIRS += periph
 DIRS += vfs
 
-ifeq ($(shell uname -s),Darwin)
+ifeq ($(OS),Darwin)
   CFLAGS += -D_XOPEN_SOURCE=600 -D_DARWIN_C_SOURCE
 endif
 
diff --git a/makefiles/tools/bossa.inc.mk b/makefiles/tools/bossa.inc.mk
index 725ba1583d..5297f0ff18 100644
--- a/makefiles/tools/bossa.inc.mk
+++ b/makefiles/tools/bossa.inc.mk
@@ -5,7 +5,6 @@ FFLAGS  ?= -p $(PORT) -e -i -w -v -b -R $(FLASHFILE)
 # some arduino boards need to toggle the serial interface a little bit to get
 # them ready for flashing...
 ifneq (,$(BOSSA_ARDUINO_PREFLASH))
-  OS := $(shell uname)
   ifeq ($(OS),Linux)
     STTY_FLAG = -F
   else ifeq ($(OS),Darwin)
diff --git a/makefiles/tools/serial.inc.mk b/makefiles/tools/serial.inc.mk
index 84144166e0..c6934b0618 100644
--- a/makefiles/tools/serial.inc.mk
+++ b/makefiles/tools/serial.inc.mk
@@ -1,5 +1,4 @@
 # set default port depending on operating system
-OS := $(shell uname)
 ifeq ($(OS),Linux)
   PORT ?= $(call ensure_value,$(PORT_LINUX),No port set)
 else ifeq ($(OS),Darwin)
diff --git a/pkg/fatfs/Makefile.include b/pkg/fatfs/Makefile.include
index 7fe6cb9347..a59773a8d4 100644
--- a/pkg/fatfs/Makefile.include
+++ b/pkg/fatfs/Makefile.include
@@ -16,6 +16,6 @@ else
   CFLAGS += -DFATFS_FFCONF_OPT_FS_NORTC=1
 endif
 
-ifeq ($(shell uname -s),Darwin)
+ifeq ($(OS),Darwin)
   CFLAGS += -Wno-empty-body
 endif