Building the ISO

Step 1: Setup archiso

The best thing you can do, is follow the official documentation for archiso on Arch Linux Wiki.
But the gist is this:

# mkdir -p ./archiso
# cd ./archiso
# cp -r /usr/share/archiso/configs/releng/* ./

Step 2: Add packages to `packages.x86_64`

git
python
python-setuptools

Step 3: Clone the repo

While standing in the archiso folder, clone using:

# git clone https://github.com/Torxed/archinstall airootfs/root/archinstall-git

Step 4: Create a skel `.zprofile` for autolaunch

While standing in the archiso folder, create and add the following:

# cat <<\EOF >> ./airootfs/root/.zprofile
[[ -z $DISPLAY && $XDG_VTNR -eq 1 ]] && sh -c "cd /root/archinstall-git; git config --global pull.rebase false; git pull; cp examples/guided.py ./; python guided.py"
EOF

This will auto-run when the ISO boots.
It will perform four simple tasks every login on tty1:

Enter the archinstall-git folder
git config and git pull each time
copy guided.py into the root git folder due to how the import works in Python
Finally it will run the latest version of guided.py

This makes it easy to test the latest version, by simply logging out and back in - and the four steps will get repeated.
No need to reboot or re-build.

Step 5: Build

Archiso has great information and the latest info. But in general, all you need to do (standing in the arciso folder), is to run:

# mkarchiso -v -w work/ -o out/ ./

And that should produce your ISO.

Build Script

Requires: archiso, git and qemu

For a simple script that builds, boots and does all this automatically.
The following convenience script is provided. Simply run ./test.sh.

#!/bin/bash

if [ -z "$1" ]
then
	echo "Need to define a output folder for the archiso:"
	echo "Example (build and run):"
	echo "  ./test.sh ./archiso true"
	echo "Example (skip building and run ISO as given path):"
	echo "  ./test.sh ./archiso"
	exit 1
fi

REPO="https://github.com/Torxed/archinstall.git"
ARCHISO_FOLDER=$1
REBUILD=$2
BRANCH="master"

if [ $REBUILD ]
then
	echo "Making a clean build!"
	`rm -rf "${ARCHISO_FOLDER}" 2>/dev/null` || (
		echo "Could not delete protected folder:";
		echo "-> ${ARCHISO_FOLDER}";
		echo "Running as sudo.";
		sudo rm -rf "${ARCHISO_FOLDER}"
	)

	mkdir -p "${ARCHISO_FOLDER}"
	cp -r /usr/share/archiso/configs/releng/* "${ARCHISO_FOLDER}/"

	git clone "${REPO}" "${ARCHISO_FOLDER}/airootfs/root/archinstall-git"
	(cd "${ARCHISO_FOLDER}/airootfs/root/archinstall-git"; git checkout "${BRANCH}" )

	echo "git" >> "${ARCHISO_FOLDER}/packages.x86_64"
	echo "python" >> "${ARCHISO_FOLDER}/packages.x86_64"
	echo "python-setuptools" >> "${ARCHISO_FOLDER}/packages.x86_64"

	cat <<\EOF >> "${ARCHISO_FOLDER}/airootfs/root/.zprofile"
[[ -z $DISPLAY && $XDG_VTNR -eq 1 ]] && sh -c "cd /root/archinstall-git; git config --global pull.rebase false; git pull; cp examples/guided.py ./; python guided.py"
EOF

	( cd "${ARCHISO_FOLDER}/"; sudo mkarchiso -v -w work/ -o out/ ./; )
fi

if [ ! -f "./test.qcow2" ];
then
	qemu-img create -f qcow2 ./test.qcow2 15G
fi

sudo qemu-system-x86_64 \
        -cpu host \
        -enable-kvm \
        -machine q35,accel=kvm \
        -device intel-iommu \
        -m 8192 \
        -drive if=pflash,format=raw,readonly,file=/usr/share/ovmf/x64/OVMF_CODE.fd  \
        -drive if=pflash,format=raw,readonly,file=/usr/share/ovmf/x64/OVMF_VARS.fd \
        -device virtio-scsi-pci,bus=pcie.0,id=scsi0 \
            -device scsi-hd,drive=hdd0,bus=scsi0.0,id=scsi0.0,bootindex=2 \
                -drive file=./test.qcow2,if=none,format=qcow2,discard=unmap,aio=native,cache=none,id=hdd0 \
        -device virtio-scsi-pci,bus=pcie.0,id=scsi1 \
            -device scsi-cd,drive=cdrom0,bus=scsi1.0,bootindex=1 \
                -drive file=$(ls -t $ARCHISO_FOLDER/out/*.iso | head -n 1),media=cdrom,if=none,format=raw,cache=none,id=cdrom0

Modify BRANCH to suit your needs.