fast-reboot

#!/bin/sh

set -e
[ -n "$BASH" ] && set -o pipefail
set -u

# defaults
wait=10
detach_wait=3
nodetach=

usage() {
    cat << EOF
Usage: fast-reboot [option ...] [kernel|boot-index]
Options are:
    --wait, -w N    Wait N seconds before reboot (default $wait)
    --nowait        Don't wait, reboot immediately
    --nodetach, -n  Do not detach. By default it runs --detach $detach_wait
    --detach, -d N  Schedule reboot in N seconds and return
    --help, -h      Show this help message
    --kargs, k      Additional kernel arguments

Example:
    $0 0    Boot kernel with index 0
EOF
    exit $1
}

uerr() {
    echo "ERROR: $*" 2>&1
    echo
    usage 1
}

# Check that all programs we need are available, exit if not
# (relying on 'set -e' and hash returning non-zero for errors)
hash grubby kexec systemctl

# Process options and arguments
target=''
kargs=''
while test $# -gt 0; do
    case $1 in
        -w|--wait)
            if [ "$#" -lt 2 ]; then
                uerr "Option $1 requires an argument"
            fi
            wait=$2
            if ! printf "%f" $wait > /dev/null 2>&1; then
                uerr "Option $1 argument not a number"
            fi
            shift
            ;;
        --nowait)
            wait=0
            ;;
        -n|--nodetach)
            nodetach=yes
            ;;
        -d|--detach)
            if [ "$#" -lt 2 ]; then
                uerr "Option $1 requires an argument"
            fi
            detach_wait=$2
            if ! printf "%f" $detach_wait > /dev/null 2>&1; then
                uerr "Option $1 argument not a number"
            fi
            shift
            ;;
        -h|--help)
            usage 0
            ;;
        -k|--kargs)
            kargs=$2
            shift
            ;;
        -*)
            uerr "Unrecognized option: $1"
            ;;
        *)
            if [ -n "$target" ]; then
                uerr "Extra arguments: $*"
            fi
            target=$1
            ;;
    esac
    shift
done

if [ -z "$target" ]; then
    if ! target=$(grubby --default-kernel); then
        # grubby prints error to stdout
        echo "grubby: $target" 1>&2
        exit 1
    fi
fi

if ! info=$(grubby --info=$target); then
    # grubby prints error to stdout
    echo "grubby: $info" 1>&2
    exit 1
fi
eval $(echo "$info" | sed "s|^\(\S*\)=\([^'\"].*\)$|\1='\2'|")
kernel=${kernel##*@}  # retain the part after the last @ in case of ZFS where the target is indicated
initrd=${initrd##*@}  # retain the part after the last @ in case of ZFS where the target is indicated

printf "\nBooting $title.... "

if [ -z "$nodetach" ]; then
    printf "detached\n"
else
    printf "not detaching\n"
fi

if [ "$wait" -ne 0 ]; then
    printf "\nPress Ctrl-C within $wait seconds to cancel reboot\n\n"
    count=0
    while [ $count -lt $wait ]; do
        printf $count
        sleep 1
        count=$(( $count + 1 ))
    done
    printf "\n"
fi

kexec -l "$kernel" --initrd="$initrd" --command-line="root=$root $args $kargs"

if [ -z "$nodetach" ]; then
    setsid bash -c "sleep $detach_wait; systemctl kexec" &>/dev/null < /dev/null &
else
    systemctl kexec
fi