diff --git a/README.md b/README.md
index f861359..ba7c6bf 100644
--- a/README.md
+++ b/README.md
@@ -27,14 +27,22 @@ Create an empty hard disk using `qemu-img`, changing the name and size to prefer
 qemu-img create -f qcow2 MyDisk.qcow2 64G
 ```
 
-and add it to the end of `basic.sh`:
-```
-    -drive id=SystemDisk,if=none,file=MyDisk.qcow2 \
-    -device ide-hd,bus=sata.4,drive=SystemDisk \
-```
-> Note: If you're running on a headless system (such as on Cloud providers), you will need `-nographic` and `-vnc :0 -k en-us` for VNC support.
+Then run `basic.sh` to start the machine and install macOS, setting the
+`SYSTEM_DISK` environment variable to the path to your freshly created
+system disk image:
 
-Then run `basic.sh` to start the machine and install macOS. Remember to partition in Disk Utility first!
+```
+SYSTEM_DISK=MyDisk.qcow2 ./basic.sh
+```
+
+If you're running on a headless system (such as on Cloud providers), set
+the `HEADLESS` environment variable to 1:
+
+```
+SYSTEM_DISK=MyDisk.qcow2 HEADLESS=1 ./basic.sh
+```
+
+Remember to partition in Disk Utility first!
 
 ## Step 2a (Virtual Machine Manager)
 If instead of QEMU, you'd like to import the setup into Virt-Manager for further configuration, just run `make.sh --add`.
diff --git a/basic.sh b/basic.sh
index 108e29f..9529d32 100755
--- a/basic.sh
+++ b/basic.sh
@@ -6,6 +6,22 @@ OVMF=$VMDIR/firmware
 #export QEMU_AUDIO_DRV=pa
 #QEMU_AUDIO_DRV=pa
 
+[[ -z "$SYSTEM_DISK" ]] && {
+    echo "Please set the SYSTEM_DISK environment variable"
+    exit 1
+}
+
+[[ -r "$SYSTEM_DISK" ]] || {
+    echo "Can't read system disk image: $SYSTEM_DISK"
+    exit 1
+}
+
+MOREARGS=()
+
+[[ "$HEADLESS" = "1" ]] && {
+    MOREARGS+=(-nographic -vnc :0 -k en-us)
+}
+
 qemu-system-x86_64 \
     -enable-kvm \
     -m 2G \
@@ -26,3 +42,6 @@ qemu-system-x86_64 \
     -device ide-hd,bus=sata.2,drive=ESP \
     -drive id=InstallMedia,format=raw,if=none,file=BaseSystem.img \
     -device ide-hd,bus=sata.3,drive=InstallMedia \
+    -drive id=SystemDisk,if=none,file="${SYSTEM_DISK}" \
+    -device ide-hd,bus=sata.4,drive=SystemDisk \
+    ${MOREARGS[@]}