tinyramfs/init

#!/bin/sh
#
# Tiny init
#
# https://www.shellcheck.net/wiki/SC2154
# shellcheck disable=2154

print()
{
    printf "%b %s\n" "${2:-"\033[1;37m>>\033[m"}" "$1"
}

panic()
{
    print "${1:-unexpected error occurred}" \
          "\033[1;31m!!\033[m" >&2

    sh
}

resolve_device()
{
    count=0; device="$1"

    case "${device%%=*}" in
        UUID)     device="/dev/disk/by-uuid/${device#*=}"     ;;
        LABEL)    device="/dev/disk/by-label/${device#*=}"    ;;
        PARTUUID) device="/dev/disk/by-partuuid/${device#*=}" ;;
        /dev/*)            ;;
        *)        return 0 ;;
    esac

    # Race condition may occur if device manager is not yet initialized device.
    # To fix this, we simply waiting until device is available. If device
    # didn't appear in specified time, we panic.
    while :; do
        if [ -b "$device" ]; then
            return 0
        elif [ "$((count += 1))" = "${rootdelay:=30}" ]; then
            break
        else
            sleep 1
        fi
    done

    panic "failed to lookup partition"
}

run_hook()
{
    type="$1"

    # Run hooks if any exist.
    #
    # https://www.shellcheck.net/wiki/SC1090
    # shellcheck disable=1090
    for hook in $hooks; do
        [ -f "/usr/share/tinyramfs/hooks/${hook}/${hook}.${type}" ] || continue
           . "/usr/share/tinyramfs/hooks/${hook}/${hook}.${type}"
    done
}

prepare_environment()
{
    # https://www.shellcheck.net/wiki/SC1091
    # shellcheck disable=1091
    . /etc/tinyramfs/config

    export \
        PATH=/bin TERM=linux SHELL=/bin/sh \
        LANG=C LC_ALL=C PS1="# " HOME=/root

    mount -t proc     -o nosuid,noexec,nodev     proc /proc
    mount -t sysfs    -o nosuid,noexec,nodev     sys  /sys
    mount -t tmpfs    -o nosuid,nodev,mode=0755  run  /run
    mount -t devtmpfs -o nosuid,noexec,mode=0755 dev  /dev

    ln -s /proc/self/fd /dev/fd
    ln -s fd/0          /dev/stdin
    ln -s fd/1          /dev/stdout
    ln -s fd/2          /dev/stderr
}

parse_cmdline()
{
    read -r cmdline < /proc/cmdline

    for line in $cmdline; do case "$line" in
        rootfstype=*) root_type="${line#*=}" ;;
        rootflags=*)  root_opts="${line#*=}" ;;
        debug=1)      set -x ;;
        ro | rw)      rorw="$line" ;;
        --*)          init_args="${cmdline#*-- }"; break ;;
        *=*)          command export "$line"     ;;
        *)            command export "${line}=1" ;;
    esac 2> /dev/null || :; done
}

mount_root()
{
    [ "$break" = root ] && { print "break before mount_root()"; sh; }

    resolve_device "$root"

    # https://www.shellcheck.net/wiki/SC2086
    # shellcheck disable=2086
    mount \
        -o "${rorw:-ro}${root_opts:+,$root_opts}" ${root_type:+-t $root_type} \
        -- "$device" /mnt/root || panic "failed to mount root"
}

boot_system()
{
    [ "$break" = boot ] && { print "break before boot_system()"; sh; }

    for dir in run dev sys proc; do
        mount -o move "$dir" "/mnt/root/${dir}"
    done

    # POSIX 'exec' has no '-c' flag to execute command with empty environment.
    # Using 'env -i' instead to prevent leaking exported variables.
    #
    # Some implementations of 'switch_root' doesn't conform to POSIX utility
    # guidelines and doesn't support '--'. This means that we can't guarantee
    # safety of init_args.
    # shellcheck disable=2086
    exec \
        env -i TERM=linux PATH=/bin:/sbin:/usr/bin:/usr/sbin \
        switch_root /mnt/root "${init-/sbin/init}" $init_args ||
        panic "failed to boot system"
}

# Exit if command fails and disable globbing.
set -ef

# Run emergency shell if init unexpectedly exiting due to error.
trap panic EXIT

# TODO display fancy colored status info

prepare_environment
parse_cmdline
run_hook init
mount_root
run_hook init.late
boot_system