basic.sh: Add support for HEADLESS mode via environment

Currently if a user wants to run in headless mode they have to edit the
basic.sh script by hand.  Rather than making them edit the script by hand,
add support for a HEADLESS environment variable which can be set to 1 to
add necessary command line arguments to qemu.

README.md

@@ -27,8 +27,6 @@ Create an empty hard disk using `qemu-img`, changing the name and size to prefer
 qemu-img create -f qcow2 MyDisk.qcow2 64G
 ```
 
-> Note: If you're running on a headless system (such as on Cloud providers), you will need to add `-nographic` and `-vnc :0 -k en-us` (for VNC support) to the end of `basic.sh`.
-
 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:
@@ -37,6 +35,13 @@ system disk image:
 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)

basic.sh

@@ -16,6 +16,12 @@ OVMF=$VMDIR/firmware
     exit 1
 }
 
+MOREARGS=()
+
+[[ "$HEADLESS" = "1" ]] && {
+    MOREARGS+=(-nographic -vnc :0 -k en-us)
+}
+
 qemu-system-x86_64 \
     -enable-kvm \
     -m 2G \
@@ -37,4 +43,5 @@ qemu-system-x86_64 \
     -drive id=InstallMedia,if=none,file=BaseSystem.img \
     -device ide-hd,bus=sata.3,drive=InstallMedia \
     -drive id=SystemDisk,if=none,file="${SYSTEM_DISK}" \
-    -device ide-hd,bus=sata.4,drive=SystemDisk
+    -device ide-hd,bus=sata.4,drive=SystemDisk \
+    ${MOREARGS[@]}