document GREP_CMD

Common.mk

@@ -8,8 +8,8 @@ dir_if_exists = $(if $(wildcard $(1)$(2)),$(1))
 # result = $(call READ_BOARD_TXT, 'boardname', 'parameter')
 PARSE_BOARD = $(shell if [ -f $(BOARDS_TXT) ]; \
 then \
-  $(GREP) -Ev '^\#' $(BOARDS_TXT) | \
-  $(GREP) -E "^[ \t]*$(1).$(2)=" | \
+  $(GREP_CMD) -Ev '^\#' $(BOARDS_TXT) | \
+  $(GREP_CMD) -E "^[ \t]*$(1).$(2)=" | \
   cut -d = -f 2- | \
   cut -d : -f 2; \
 fi)
@@ -48,20 +48,20 @@ $(call arduino_output,$(call ardmk_include) Configuration:)
 
 ifeq ($(OS),Windows_NT)
     CURRENT_OS = WINDOWS
-    GREP := grep
+    GREP_CMD := grep
 else
     UNAME_S := $(shell uname -s)
     ifeq ($(UNAME_S),Linux)
         CURRENT_OS = LINUX
-        GREP := grep
+        GREP_CMD := grep
     endif
     ifeq ($(UNAME_S),Darwin)
         CURRENT_OS = MAC
         ifeq (, $(shell which ggrep))
             echo $(info Using macOS BSD grep, please install GNU grep to avoid warnings)
-            GREP := grep
+            GREP_CMD := grep
         else
-            GREP := ggrep
+            GREP_CMD := ggrep
         endif
     endif
 endif

Sam.mk

@@ -414,7 +414,7 @@ CFLAGS_STD += -std=gnu11
 CPPFLAGS += -DMD -D$(USB_TYPE) '-DUSB_PRODUCT=$(USB_PRODUCT)' '-DUSB_MANUFACTURER=$(USB_MANUFACTURER)'
 
 # Get extra define flags from boards.txt
-EXFLAGS := $(shell echo $(call PARSE_BOARD,$(BOARD_TAG),build.extra_flags) | $(GREP) -oE '(-D)\w+')
+EXFLAGS := $(shell echo $(call PARSE_BOARD,$(BOARD_TAG),build.extra_flags) | $(GREP_CMD) -oE '(-D)\w+')
 
 # Strip only defines from extra flags as boards file appends user {build.usb}
 CPPFLAGS += $(EXFLAGS)

arduino-mk-vars.md

@@ -150,6 +150,22 @@ PYTHON_CMD = /usr/bin/python3
 
 ----
 
+### GREP_CMD
+
+**Description:**
+
+Path to GNU grep binary. Only added for macOS, which has BSD grep by default but results in some parsing warnings. macOS users should install GNU grep using Homebrew.
+
+**Example:**
+
+```Makefile
+GREP_CMD = /bin/grep
+```
+
+**Requirement:** *Optional*
+
+----
+
 ## Arduino IDE variables
 
 ### ARDUINO_DIR