diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index 165c2d4e..b1073ae5 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -6,105 +6,102 @@ on:
       - '**'  # Matches all branches
 
 jobs:
-  tests-linux:
+  tests-exe: # Run tests for pnut-exe on all supported platforms and architectures
     strategy:
       matrix:
-        target: ["i386", "x86_64"] # Test on both i386 and x86_64
-    runs-on: ubuntu-latest
+        include:
+          - target: i386
+            host: ubuntu-latest
+          - target: x86_64
+            host: ubuntu-latest
+          - target: mac_os
+            host: macos-latest
+    runs-on: ${{ matrix.host }}
     steps:
       - name: Checkout code
         uses: actions/checkout@v2
 
-      - name: Run ${{ matrix.target }} Linux tests
+      - name: Run ${{ matrix.target }} tests on ${{ matrix.host }}
         run: |
           set -e
           ./run-tests.sh ${{ matrix.target }}
 
-  tests-mac:
-    runs-on: macos-latest
-    steps:
-      - name: Checkout code
-        uses: actions/checkout@v2
-
-      - name: Run MacOS x86_64 tests
-        run: |
-          set -e
-          ./run-tests.sh mac_os
-
-  tests-shell:
+  tests-shell: # Run tests for pnut-sh on all supported shells
     strategy:
       matrix:
-        shell: ["bash", "dash", "ksh", "zsh"]
+        shell: ["bash", "dash", "ksh", "mksh", "yash", "zsh"]
     runs-on: ubuntu-latest
     steps:
       - name: Checkout code
         uses: actions/checkout@v2
 
-      - name: Install shells
+      - name: Install ${{ matrix.shell }} shell
         run: |
           sudo apt-get update
-          sudo apt-get install -y ${{ matrix.shell }}
+          sudo apt-get install -y bash ${{ matrix.shell }}
 
       - name: Run tests with ${{ matrix.shell }}
         run: |
           set -e
-          ./run-tests.sh sh --shell ${{ matrix.shell }}
+          bash ./run-tests.sh sh --shell ${{ matrix.shell }}
 
-  bootstrap-shell:
+  bootstrap-pnut-sh:
+    strategy:
+      matrix:
+        shell: ["bash", "dash", "ksh", "mksh", "yash", "zsh"]
     runs-on: ubuntu-latest
     steps:
       - name: Checkout code
         uses: actions/checkout@v2
 
-      - name: Install shells
+      - name: Install ${{ matrix.shell }} shell
         run: |
           sudo apt-get update
-          sudo apt-get install -y bash ksh dash zsh yash mksh
+          sudo apt-get install -y bash ${{ matrix.shell }}
 
-      - name: Run bootstrap shell with all shells
+      - name: Bootstrap pnut.sh on ${{ matrix.shell }}
         run: |
           set -e
-          chmod +x bootstrap-pnut.sh
-          ./bootstrap-pnut.sh TEST_ALL_SHELLS
-          if [ $? -ne 0 ]; then
-            echo "Bootstrap with all shells failed"
-            exit 1
-          fi
+          bash ./bootstrap-pnut.sh --shell ${{ matrix.shell }}
 
-  bootstrap-x86:
-    runs-on: ubuntu-latest
+  bootstrap-pnut-exe:
+    strategy:
+      matrix:
+        shell: ["dash", "ksh"] # Not testing on other shells to save time
+        include:
+          - target: i386
+            host: ubuntu-latest
+          - target: x86_64
+            host: ubuntu-latest
+          - target: mac_os
+            host: macos-latest
+    runs-on: ${{ matrix.host }}
     steps:
       - name: Checkout code
         uses: actions/checkout@v2
 
-      - name: Install shells
+      - name: Install ${{ matrix.shell }} shell
         run: |
-          sudo apt-get update
-          sudo apt-get install -y bash ksh dash zsh yash mksh
+          if [ ${{ matrix.host }} = "macos-latest" ]; then
+            brew install ${{ matrix.shell }}
+          else
+            sudo apt-get update
+            sudo apt-get install -y ${{ matrix.shell }}
+          fi
 
-      - name: Run bootstrap x86
+      - name: Bootstrap pnut-exe with ${{ matrix.target }} backend
         run: |
           set -e
-          chmod +x bootstrap-pnut-x86.sh
-          ./bootstrap-pnut-x86.sh
-          if [ $? -ne 0 ]; then
-            echo "Bootstrap x86 failed"
-            exit 1
-          fi
+          ./bootstrap-pnut-exe.sh --backend ${{ matrix.target }}
 
-      - name: Run bootstrap x86 with shell
+      - name: Bootstrap pnut-exe with ${{ matrix.target }} backend on ${{ matrix.shell }}
         run: |
           set -e
-          chmod +x bootstrap-pnut-x86.sh
-          ./bootstrap-pnut-x86.sh WITH_SHELL
-          if [ $? -ne 0 ]; then
-            echo "Bootstrap x86 with shell failed"
-            exit 1
-          fi
+          ./bootstrap-pnut-exe.sh --backend ${{ matrix.target }}  --shell ${{ matrix.shell }}
 
   success-message:
     runs-on: ubuntu-latest
-    needs: [tests-linux, tests-mac, tests-shell, bootstrap-shell, bootstrap-x86]
+    needs: [tests-exe, tests-shell, bootstrap-pnut-sh, bootstrap-pnut-exe]
     steps:
       - name: Tests finished
         run: |
diff --git a/bootstrap-pnut-x86.sh b/bootstrap-pnut-exe.sh
similarity index 75%
rename from bootstrap-pnut-x86.sh
rename to bootstrap-pnut-exe.sh
index d8e22555..230ecc73 100755
--- a/bootstrap-pnut-x86.sh
+++ b/bootstrap-pnut-exe.sh
@@ -1,10 +1,12 @@
 #! /bin/sh
 
+set -e -u
+
 TEMP_DIR="bootstrap-results"
 
-# PNUT_x86_OPTIONS="-Dmac_os"
-# PNUT_x86_OPTIONS="-Dx86_64"
-PNUT_x86_OPTIONS="-Di386"
+# PNUT_EXE_OPTIONS="-Dmac_os"
+# PNUT_EXE_OPTIONS="-Dx86_64"
+PNUT_EXE_OPTIONS="-Di386"
 PNUT_SH_OPTIONS="-DRT_NO_INIT_GLOBALS -Dsh"
 
 if [ ! -d "$TEMP_DIR" ]; then mkdir "$TEMP_DIR"; fi
@@ -22,13 +24,13 @@ bootstrap_with_gcc() {
 
   # Compile pnut x86 using GCC, then compile pnut x86 using pnut x86
 
-  gcc -o $TEMP_DIR/pnut-x86-by-gcc.exe $PNUT_x86_OPTIONS pnut.c
-  # gcc -E -P -DPNUT_CC $PNUT_x86_OPTIONS pnut.c > "$TEMP_DIR/pnut-after-cpp.c"
-  ./$TEMP_DIR/pnut-x86-by-gcc.exe $PNUT_x86_OPTIONS pnut.c > $TEMP_DIR/pnut-x86-by-pnut-x86-by-gcc.exe
+  gcc -o $TEMP_DIR/pnut-x86-by-gcc.exe $PNUT_EXE_OPTIONS pnut.c
+  # gcc -E -P -DPNUT_CC $PNUT_EXE_OPTIONS pnut.c > "$TEMP_DIR/pnut-after-cpp.c"
+  ./$TEMP_DIR/pnut-x86-by-gcc.exe $PNUT_EXE_OPTIONS pnut.c > $TEMP_DIR/pnut-x86-by-pnut-x86-by-gcc.exe
 
   chmod +x $TEMP_DIR/pnut-x86-by-pnut-x86-by-gcc.exe
 
-  ./$TEMP_DIR/pnut-x86-by-pnut-x86-by-gcc.exe $PNUT_x86_OPTIONS pnut.c > $TEMP_DIR/pnut-x86-by-pnut-x86-by-pnut-x86-by-gcc.exe
+  ./$TEMP_DIR/pnut-x86-by-pnut-x86-by-gcc.exe $PNUT_EXE_OPTIONS pnut.c > $TEMP_DIR/pnut-x86-by-pnut-x86-by-pnut-x86-by-gcc.exe
 
   if [ -s $TEMP_DIR/pnut-x86-by-pnut-x86-by-pnut-x86-by-gcc.exe ] ; then
     if diff $TEMP_DIR/pnut-x86-by-pnut-x86-by-gcc.exe $TEMP_DIR/pnut-x86-by-pnut-x86-by-pnut-x86-by-gcc.exe 2>&1 > /dev/null ; then
@@ -53,7 +55,7 @@ bootstrap_with_shell() {
   ./$TEMP_DIR/pnut-sh-compiled-by-gcc.exe $PNUT_SH_OPTIONS pnut.c > $TEMP_DIR/pnut-sh.sh
 
   # create pnut-i386.sh, the C to i386 machine code compiler as a shell script
-  # gcc -E -P -DPNUT_CC $PNUT_x86_OPTIONS pnut.c > $TEMP_DIR/pnut-i386-after-cpp.c
+  # gcc -E -P -DPNUT_CC $PNUT_EXE_OPTIONS pnut.c > $TEMP_DIR/pnut-i386-after-cpp.c
   printf_timing "pnut-sh.sh compiling pnut.c -> pnut-sh-compiled-by-pnut-sh-sh.sh" \
                 "$1 $TEMP_DIR/pnut-sh.sh $PNUT_SH_OPTIONS pnut.c > $TEMP_DIR/pnut-sh-compiled-by-pnut-sh-sh.sh"
   if diff $TEMP_DIR/pnut-sh.sh $TEMP_DIR/pnut-sh-compiled-by-pnut-sh-sh.sh 2>&1 > /dev/null ; then
@@ -63,14 +65,14 @@ bootstrap_with_shell() {
     exit 1
   fi
   printf_timing "pnut-sh.sh compiling pnut.c -> pnut-i386-compiled-by-pnut-sh-sh.sh" \
-                "$1 $TEMP_DIR/pnut-sh.sh $PNUT_x86_OPTIONS pnut.c > $TEMP_DIR/pnut-i386-compiled-by-pnut-sh-sh.sh"
+                "$1 $TEMP_DIR/pnut-sh.sh $PNUT_EXE_OPTIONS pnut.c > $TEMP_DIR/pnut-i386-compiled-by-pnut-sh-sh.sh"
   printf_timing "pnut-i386-compiled-by-pnut-sh-sh.sh compiling pnut.c -> pnut-i386-compiled-by-pnut-i386-compiled-by-pnut-sh-sh.exe" \
-                "$1 $TEMP_DIR/pnut-i386-compiled-by-pnut-sh-sh.sh $PNUT_x86_OPTIONS pnut.c > $TEMP_DIR/pnut-i386-compiled-by-pnut-i386-compiled-by-pnut-sh-sh.exe"
+                "$1 $TEMP_DIR/pnut-i386-compiled-by-pnut-sh-sh.sh $PNUT_EXE_OPTIONS pnut.c > $TEMP_DIR/pnut-i386-compiled-by-pnut-i386-compiled-by-pnut-sh-sh.exe"
 
   chmod +x $TEMP_DIR/pnut-i386-compiled-by-pnut-i386-compiled-by-pnut-sh-sh.exe
 
   printf_timing "pnut-i386-compiled-by-pnut-i386-compiled-by-pnut-sh-sh.exe compiling pnut.c -> pnut-i386-compiled-by-pnut-i386-compiled-by-pnut-i386-compiled-by-pnut-sh-sh.exe" \
-                "./$TEMP_DIR/pnut-i386-compiled-by-pnut-i386-compiled-by-pnut-sh-sh.exe $PNUT_x86_OPTIONS pnut.c > $TEMP_DIR/pnut-i386-compiled-by-pnut-i386-compiled-by-pnut-i386-compiled-by-pnut-sh-sh.exe"
+                "./$TEMP_DIR/pnut-i386-compiled-by-pnut-i386-compiled-by-pnut-sh-sh.exe $PNUT_EXE_OPTIONS pnut.c > $TEMP_DIR/pnut-i386-compiled-by-pnut-i386-compiled-by-pnut-i386-compiled-by-pnut-sh-sh.exe"
 
   chmod +x $TEMP_DIR/pnut-i386-compiled-by-pnut-i386-compiled-by-pnut-i386-compiled-by-pnut-sh-sh.exe
 
@@ -116,23 +118,39 @@ bootstrap_with_shell() {
   fi
 }
 
-# Handle runtime options
-WITH_SHELL=0
-TEST_ALL_SHELLS=0
-
-if [ $# -gt 0 ] && [ $1 = "WITH_SHELL" ] ; then WITH_SHELL=1; shift; fi
-if [ $# -gt 0 ] && [ $1 = "TEST_ALL_SHELLS" ] ; then TEST_ALL_SHELLS=1; shift; fi
+# Parse the arguments
+backend="x86_64"  # Default to x86_64 Linux
+shell=            # Defined if doing the full bootstrap using pnut.sh on Posix shell. "all" to test with all shells (slow).
+
+while [ $# -gt 0 ]; do
+  case $1 in
+    --backend) backend="$2"; shift 2 ;;
+    --shell) shell="$2"; shift 2 ;;
+    *) echo "Unknown option: $1"; exit 1;;
+  esac
+done
+
+case $backend in
+  i386 | x86_64 | mac_os)
+    PNUT_EXE_OPTIONS="-D$backend" ;;
+  *)
+    echo "Unknown backend: $backend"
+    exit 1
+    ;;
+esac
 
-if [ $WITH_SHELL -ne 0 ]; then
-  bootstrap_with_shell "ksh"
+if [ -z "$shell" ]; then
+  bootstrap_with_gcc
 else
-  bootstrap_with_gcc # Much faster than bootstrapping on the Shell
-fi
-
-if [ $TEST_ALL_SHELLS -ne 0 ]; then
-  bootstrap_with_shell "dash"
-  bootstrap_with_shell "bash"
-  bootstrap_with_shell "zsh"
-  bootstrap_with_shell "yash"
-  bootstrap_with_shell "mksh"
+  if [ "$shell" = "all" ]; then
+    set +e # Don't exit on error because we want to test all shells.
+    bootstrap_with_shell "dash"
+    bootstrap_with_shell "ksh"
+    bootstrap_with_shell "bash"
+    bootstrap_with_shell "yash"
+    bootstrap_with_shell "mksh"
+    bootstrap_with_shell "zsh"
+  else
+    bootstrap_with_shell "$shell"
+  fi
 fi
diff --git a/bootstrap-pnut.sh b/bootstrap-pnut.sh
index e1afe72b..dbd78da7 100755
--- a/bootstrap-pnut.sh
+++ b/bootstrap-pnut.sh
@@ -1,5 +1,7 @@
 #! /bin/sh
 
+set -e
+
 TEMP_DIR="bootstrap-results"
 PNUT_SH_OPTIONS="-DRT_NO_INIT_GLOBALS -Dsh"
 
@@ -22,17 +24,24 @@ bootstrap_with_shell() {
   wc pnut.c "$TEMP_DIR/pnut.sh" "$TEMP_DIR/pnut-twice-bootstrapped.sh"
 }
 
-# Handle runtime options
-TEST_ALL_SHELLS=0
-
-if [ $# -gt 0 ] && [ $1 = "TEST_ALL_SHELLS" ] ; then TEST_ALL_SHELLS=1; shift; fi
+# Parse the arguments
+shell="$SHELL" # Use current shell as the default. "all" to test all shells.
 
-bootstrap_with_shell "ksh"
+while [ $# -gt 0 ]; do
+  case $1 in
+    --shell) shell="$2"; shift 2 ;;
+    *) echo "Unknown option: $1"; exit 1;;
+  esac
+done
 
-if [ $TEST_ALL_SHELLS -ne 0 ]; then
+if [ "$shell" = "all" ]; then
+  set +e # Don't exit on error because we want to test all shells.
   bootstrap_with_shell "dash"
+  bootstrap_with_shell "ksh"
   bootstrap_with_shell "bash"
-  bootstrap_with_shell "zsh"
   bootstrap_with_shell "yash"
   bootstrap_with_shell "mksh"
+  bootstrap_with_shell "zsh"
+else
+  bootstrap_with_shell "$shell"
 fi