Merge pull request #2285 from ucb-bar/mac_bash

Find a modern bash on MacOS

Author: jerryz123

common.mk

@@ -1,9 +1,9 @@
-SHELL=/bin/bash
 SED ?= sed
 
 # Note: Individual rules that use RISCV or external tools perform local checks to avoid
 # blocking unrelated targets. Use $(require_riscv) and $(call require_cmd,<tool>) inside recipes.
 
+
 define require_riscv
 	@if [ -z "$(RISCV)" ]; then \
 	  echo "RISCV is unset. Source env.sh (which activates the default conda env) before building sims." 1>&2; \

variables.mk

@@ -1,3 +1,12 @@
+# Ensure GNU Make uses a modern bash for recipes
+# - Set early (before other includes) so old make versions honor it.
+# - Prefer Homebrew or /usr/local bash, else fall back to /bin/bash.
+SHELL := $(shell \
+  if [ -x /opt/homebrew/bin/bash ]; then printf /opt/homebrew/bin/bash; \
+  elif [ -x /usr/local/bin/bash ]; then printf /usr/local/bin/bash; \
+  elif command -v bash >/dev/null 2>&1; then command -v bash; \
+  else printf /bin/bash; fi)
+
 #########################################################################################
 # makefile variables shared across multiple makefiles
 # - to use the help text, your Makefile should have a 'help' target that just