init-systemd

#!/bin/bash
# Copyright 2020 (c) Ayane Satomi
# Licensed under MIT
# Parts of this code is based from https://forum.snapcraft.io/t/running-snaps-on-wsl2-insiders-only-for-now/13033
set -eo pipefail


if [ -z "$1" ]; then 
    echo "Usage: $0 -u (USER) -c (command)"
    echo "This script spawns a namespace to be able to use systemd in a specified user."
    echo "This requires unshare (from util-linux)."
    echo "If -c is provided, the command will be executed in the namespace's context non-interactively."
    exit 1;
fi 

# I ran out of ideas here so feel free to do some weird lettering shit that matches this more
while getopts ":u:c:" o; do
    case "${o}" in
        u)
            user=${OPTARG}
            ;;
        c)
            exec_command=${OPTARG}
            ;;
        *)
            echo "Usage: $0 -u (USER) -c (command)"
            echo "This script spawns a namespace to be able to use systemd in a specified user."
            echo "This requires unshare (from util-linux)."
            echo "If -c is provided, the command will be executed in the namespace's context non-interactively."
            exit 1;
            ;;
    esac
done

shift $((OPTIND-1))

# Required checks: Obviously we're looking for
#   * root
#   * unshare
#   * a existing user based on what was provided.
user_exists=$(id -u $user > /dev/null 2>&1; echo $?)
if [ ! $(whoami) == "root" ]; then echo "Must run as root!"; exit 1; fi 
if [ -z $(command -v unshare) ]; then echo "util-linux is not installed!"; exit 1; fi 
if [ -z $(command -v nsenter) ]; then echo "nsenter is not installed!"; exit 1; fi
if [ "$user_exists" -eq  1  ]; then echo "Cannot proceed, user does not exist!"; exit 127; fi


# We want to spawn a namespace but not spawn anymore if it exists already.
SYSTEMD_PID=$(ps -eo pid=,args= | awk '$2=="/lib/systemd/systemd" && $3=="--system-unit=basic.target" {print $1}')
if [ -z "$SYSTEMD_PID" ]; then
        unshare --kill-child --fork --pid --mount-proc /lib/systemd/systemd --system-unit=basic.target <&- &
        SYSTEMD_PID=$(ps -eo pid=,args= | awk '$2=="/lib/systemd/systemd" && $3=="--system-unit=basic.target" {print $1}')
fi

if [ -n "$exec_command" ]; then
    # Run this inside the namespace!
    nsenter -t "$SYSTEMD_PID" -m -p su $(id -un $user) - sh -c "cd $PWD; exec $exec_command"
    
    # FIXME: Since we keep forking systemd per namespace session its wise to dispose them after.
    kill -9 "$SYSTEMD_PID"
else
    # The idea here is we want to preserve WSL's PWD so we don't need to walk manually back to the directory.
    # So when "bash" is typed on PowerShell, the experience is seamless - it behaves similarly and the directory is already walked for your convenience.
    nsenter -t "$SYSTEMD_PID" -m -p su $(id -un $user) - sh -c 'cd $PWD; exec "${SHELL:-sh}"'

    # FIXME: Since we keep forking systemd per namespace session its wise to dispose them after.
    kill -9 "$SYSTEMD_PID"
fi