smoser/README.md

## README.md

      
    Raw
  

              README.md
            
          
    Ubuntu Grub Boot Kernel

Boot a specific installed Ubuntu kernel using grub-reboot and grub-set-default.
This allows you to pick what kernel you want to boot on next reboot, or set the default, without
having to know much about how grub works or editing config files.
Usage

  Usage: boot-kernel [options] [kernel]
     call grub-reboot or grub-set-default to boot the provided kernel.
     options:
        -n | --dry-run      do not make changes, only report what would be done.
             --setup-only   only setup 'saved' in /etc/default/grub.
                            do not supply kernel with --setup-only.
             --default      run 'grub-set-default' rather than 'grub-reboot'
     Examples:
       * boot kernel /boot/vmlinuz-4.13.0-17-generic next time.
         $ boot-kernel /boot/vmlinuz-4.13.0-17-generic
       * edit /etc/default/grub to enable 'saved'
         $ boot-kernel --setup-only
       * set the default kernel.
         $ boot-kernel --default /boot/vmlinuz-4.15.0-22-generic
         $ reboot

Example

Here is an example of using boot-kernel on a 16.04 instance to toggle
back and forth between the HWE kernel and the generic -virtual kernel.


Install a second kernel.
 % apt-get install -qy linux-image-virtual-hwe-16.04
 % ls /boot/vmlinuz-*
 /boot/vmlinuz-4.13.0-45-generic  /boot/vmlinuz-4.4.0-128-generic


run with --setup-only to setup.  Note that setup would be done anyway if necessary.
 % ./boot-kernel --setup-only
 changing GRUB_DEFAULT from 0 to "saved" in /etc/default/grub
 apply change to /etc/default/grub
    --- /etc/default/grub	2018-06-14 20:28:15.832442556 +0000
    +++ /tmp/boot-kernel.1FIHGf	2018-06-14 20:28:18.520417934 +0000
    @@ -3,7 +3,7 @@
     # For full documentation of the options in this file, see:
     #   info -f grub -n 'Simple configuration'

    -GRUB_DEFAULT=0
    +GRUB_DEFAULT=saved
     GRUB_HIDDEN_TIMEOUT=0
     GRUB_HIDDEN_TIMEOUT_QUIET=true
     GRUB_TIMEOUT=0
 execute: update-grub
 Generating grub configuration file ...
 Found linux image: /boot/vmlinuz-4.13.0-45-generic
 Found initrd image: /boot/initrd.img-4.13.0-45-generic
 Found linux image: /boot/vmlinuz-4.4.0-128-generic
 Found initrd image: /boot/initrd.img-4.4.0-128-generic
 done


boot into the older kernel.  With a 4.4 and 4.13 kernel installed we would normally reboot into 4.13.
 % sudo ./boot-kernel /boot/vmlinuz-4.4.0-128-generic
 GRUB_DEFAULT already set to 'saved'. no change necessary.
 selected /boot/vmlinuz-4.4.0-128-generic. entry: Advanced options for Ubuntu>Ubuntu, with Linux 4.4.0-128-generic
 execute: grub-reboot "Advanced options for Ubuntu>Ubuntu, with Linux 4.4.0-128-generic"

That used grub-reboot to update /boot/grub/grubenv to set the next_entry.
 % head -n 2 /boot/grub/grubenv
 # GRUB Environment Block
 next_entry=Advanced options for Ubuntu>Ubuntu, with Linux 4.4.0-128-generic

Now reboot and see that it worked.
 % reboot
 ...
 % uname -r
 4.4.0-128-generic

Because it used grub-reboot and not grub-set-default the next time we will reboot back into the default/newest kernel.
 % head -n 2 /boot/grub/grubenv
 # GRUB Environment Block
 next_entry=


Make this the default.
 % sudo ./boot-kernel --default /boot/vmlinuz-4.4.0-128-generic
 GRUB_DEFAULT already set to 'saved'. no change necessary.
 selected /boot/vmlinuz-4.4.0-128-generic. entry: Advanced options for Ubuntu>Ubuntu, with Linux 4.4.0-128-generic
 execute: grub-set-default "Advanced options for Ubuntu>Ubuntu, with Linux 4.4.0-128-generic"
 Searching for GRUB installation directory ... found: /boot/grub

 % head -n 2 /boot/grub/grubenv
 # GRUB Environment Block
 saved_entry=Advanced options for Ubuntu>Ubuntu, with Linux 4.4.0-128-generic


Notes


Power Bare Metal: Note, that on power8 bare metal systems (such as ubuntu ppc64el) the boot loader used is not actually grub2, it is petitboot.  petitboot does not seem to support this function.  I've filed upstream petitboot issue 24.

Links

This was originally put together from googling.  The following articles were helpful:

blog Configuring GRUB to boot the right kernel after an upgrade
upstream grub doc


## boot-kernel
#!/bin/sh
# https://gist.github.com/smoser/4d6371b9e3823b88a65c84ff40d1fd88
#
# This just selects which kernel to boot on an ubuntu
# system and sets it to boot with 'grub-reboot'
#
# based on http://statusq.org/archives/2012/10/24/4584/
fail() { [ $# -eq 0 ] || msg "$@"; exit 1; }
error() { echo "$@" 1>&2; }
msg() { echo "$@" 1>&2; }
Usage() {
    cat <<EOF
Usage: ${0##*/} [options] [kernel]

   call grub-reboot or grub-set-default to boot the provided kernel.

   options:
      -n | --dry-run      do not make changes, only report what would be done.
           --setup-only   only setup 'saved' in /etc/default/grub.
                          do not supply kernel with --setup-only.
           --default      run 'grub-set-default' rather than 'grub-reboot'

   Examples:
     * boot kernel /boot/vmlinuz-4.13.0-17-generic next time.
       ${0##*/} /boot/vmlinuz-4.13.0-17-generic

     * edit /etc/default/grub to enable 'saved'
       ${0##*/} --setup-only
EOF
}


find_kernel() {
    local kernel_in="$1" kernel=""
    if [ -f "$kernel_in" ]; then
        kernel=${kernel_in}
    elif [ -f "/boot/$kernel_in" ]; then
        kernel="/boot/${kernel_in}"
    else
        kmatch="$kernel_in"
        if [ "${kmatch#*/}" = "$kmatch" ]; then
            kmatch="/boot/vmlinu?-${kmatch#vmlinu?-}*"
        else
            kmatch="/boot/$kmatch"
        fi
        for f in $kmatch; do
            if [ -f "$f" ]; then
                if [ -n "$kernel" ]; then
                    error "multiple kernels match $kmatch"
                    return 1
                fi
                kernel="$f"
            fi
        done
    fi
    [ -f "$kernel" ] || {
        error "did not find kernel matching '$kernel_in'";
        return 1;
    }
    echo "$kernel"
}

read_value() {
    local gdef="" fname="$1"
    gdef=$(sh -ec '
        fail() { ret=$1; shift; echo "$@" 1>&2; exit $ret; }
        fname=$1
        . "$fname" || fail $? "failed source $fname"
        for i in /etc/default/grub.d/*.cfg; do
            [ -f "$i" ] || continue
            . "$i" || fail $? "failed source $i"
        done
        echo $GRUB_DEFAULT' -- "$fname") || {
            error "Failed to read '$var' setting";
            return 1;
        }
    _RET="$gdef"
}

setup_grub() {
    local dry_run="$1" gdef="" var="GRUB_DEFAULT"
    local grubfile="/etc/default/grub" modfile=""
    read_value "$grubfile" ||
        { error "failed reading value of GRUB_DEFAULT"; return 1; }
    gdef="$_RET"

    if [ "$gdef" = "saved" ]; then
        msg "GRUB_DEFAULT already set to 'saved'. no change necessary."
        return 0
    fi

    msg "changing GRUB_DEFAULT from $gdef to \"saved\" in $grubfile"
    modfile=$(mktemp "${TMPDIR:-/tmp/${0##*/}.XXXXXX}") ||
        { error "failed to make temp file"; return 1; }
    sed "s,^$var=.*,$var=saved," "$grubfile" > "$modfile" || {
        error "failed to edit $grubfile to a tempfile"
        rm -f "$modfile"
        return 1
    }

    read_value "$modfile" || {
        error "failed to read GRUB_DEFAULT from edited file";
        rm -f "$modfile"
        return 1
    }
    [ "$_RET" = "saved" ] || {
        error "change of GRUB_DEFAULT in $grubfile did not make a change."
        rm -f "$modfile"
        return 1
    }

    msg "apply change to $grubfile";
    diff -u "$grubfile" "$modfile" | sed 's,^,   ,g' 1>&2
    if [ "$dry_run" != "true" ]; then
        cp "$modfile" "$grubfile" || {
            error "failed to update $grubfile."
            rm -f "$modfile"
            return 1
        }
        rm "$modfile"
    fi

    msg execute: update-grub
    if [ "$dry_run" != "true" ]; then
        update-grub || {
            error "failed update-grub to apply $var=saved";
            return 1;
        }
    fi
}

get_entry() {
    local submenu="Advanced options for Ubuntu"
    local prefix="Ubuntu, with Linux "
    # VER-FLAV like '3.13.0-79-generic'
    local kernel="$1" verflav=""

    # /boot/vmlinuz-VER-FLAV -> vmlinuz-VER-FLAV
    verflav=${kernel##*/}
    # vmlinuz-VER-FLAV - VER-FLAV
    verflav=${verflav#*-}

    entry="${submenu:+${submenu}}>${prefix}$verflav"
    if ! grep -q "$prefix$verflav" "/boot/grub/grub.cfg"; then
        error "no $prefix$verflav entry in /boot/grub/grub.cfg"
        return 1
    fi
    echo "$entry"
}

main() {
	local short_opts="hn"
	local long_opts="help,default,dry-run,setup-only"
	local getopt_out=""
	getopt_out=$(getopt --name "${0##*/}" \
		--options "${short_opts}" --long "${long_opts}" -- "$@") &&
		eval set -- "${getopt_out}" ||
		{ Usage 1>&2; return; }

	## <<insert default variables here>>
    local dry_run=false setup_only=false mode="reboot"
	local cur="" next="" kernel="" kernel_in=""
	while [ $# -ne 0 ]; do
		cur="$1"; next="$2";
		case "$cur" in
			-h|--help) Usage ; exit 0;;
			   --default) mode="set-default";;
			-n|--dry-run) dry_run=true;;
			   --setup-only) setup_only=true;;
			--) shift; break;;
		esac
		shift;
	done

    if [ "$setup_only" = "true" ]; then
        [ $# -eq 0 ] || {
            Usage 1>&2; echo "got $# args, expected 0 for setup-only"
            return 1;
        }
    else
        [ $# -eq 1 ] || {
            Usage 1>&2; echo "got $# args, expected 1. must provide kernel."
            return 1;
        }
        if [ "$setup_only" = "false" ]; then
            kernel_in="$1"
            kernel=$(find_kernel "${kernel_in}") || return
        fi
    fi

    setup_grub "$dry_run" || return
    if [ "$setup_only" = "true" ]; then
        return
    fi

    entry=$(get_entry "$kernel") || fail
    msg "selected $kernel. entry: ${entry}"

    cmd="grub-$mode"

    msg "execute: $cmd \"$entry\""
    if [ "$dry_run" != "true" ]; then
        "$cmd" "$entry" || {
            error "failed: $cmd \"$entry\""
            return 1
        }
    fi
    return 0
}

main "$@"

# vi: ts=4 expandtab
	#!/bin/sh
	# https://gist.github.com/smoser/4d6371b9e3823b88a65c84ff40d1fd88
	#
	# This just selects which kernel to boot on an ubuntu
	# system and sets it to boot with 'grub-reboot'
	#
	# based on http://statusq.org/archives/2012/10/24/4584/
	fail() { [ $# -eq 0 ] \|\| msg "$@"; exit 1; }
	error() { echo "$@" 1>&2; }
	msg() { echo "$@" 1>&2; }
	Usage() {
	cat <<EOF
	Usage: ${0##*/} [options] [kernel]

	call grub-reboot or grub-set-default to boot the provided kernel.

	options:
	-n \| --dry-run do not make changes, only report what would be done.
	--setup-only only setup 'saved' in /etc/default/grub.
	do not supply kernel with --setup-only.
	--default run 'grub-set-default' rather than 'grub-reboot'

	Examples:
	* boot kernel /boot/vmlinuz-4.13.0-17-generic next time.
	${0##*/} /boot/vmlinuz-4.13.0-17-generic

	* edit /etc/default/grub to enable 'saved'
	${0##*/} --setup-only
	EOF
	}


	find_kernel() {
	local kernel_in="$1" kernel=""
	if [ -f "$kernel_in" ]; then
	kernel=${kernel_in}
	elif [ -f "/boot/$kernel_in" ]; then
	kernel="/boot/${kernel_in}"
	else
	kmatch="$kernel_in"
	if [ "${kmatch#*/}" = "$kmatch" ]; then
	kmatch="/boot/vmlinu?-${kmatch#vmlinu?-}*"
	else
	kmatch="/boot/$kmatch"
	fi
	for f in $kmatch; do
	if [ -f "$f" ]; then
	if [ -n "$kernel" ]; then
	error "multiple kernels match $kmatch"
	return 1
	fi
	kernel="$f"
	fi
	done
	fi
	[ -f "$kernel" ] \|\| {
	error "did not find kernel matching '$kernel_in'";
	return 1;
	}
	echo "$kernel"
	}

	read_value() {
	local gdef="" fname="$1"
	gdef=$(sh -ec '
	fail() { ret=$1; shift; echo "$@" 1>&2; exit $ret; }
	fname=$1
	. "$fname" \|\| fail $? "failed source $fname"
	for i in /etc/default/grub.d/*.cfg; do
	[ -f "$i" ] \|\| continue
	. "$i" \|\| fail $? "failed source $i"
	done
	echo $GRUB_DEFAULT' -- "$fname") \|\| {
	error "Failed to read '$var' setting";
	return 1;
	}
	_RET="$gdef"
	}

	setup_grub() {
	local dry_run="$1" gdef="" var="GRUB_DEFAULT"
	local grubfile="/etc/default/grub" modfile=""
	read_value "$grubfile" \|\|
	{ error "failed reading value of GRUB_DEFAULT"; return 1; }
	gdef="$_RET"

	if [ "$gdef" = "saved" ]; then
	msg "GRUB_DEFAULT already set to 'saved'. no change necessary."
	return 0
	fi

	msg "changing GRUB_DEFAULT from $gdef to \"saved\" in $grubfile"
	modfile=$(mktemp "${TMPDIR:-/tmp/${0##*/}.XXXXXX}") \|\|
	{ error "failed to make temp file"; return 1; }
	sed "s,^$var=.*,$var=saved," "$grubfile" > "$modfile" \|\| {
	error "failed to edit $grubfile to a tempfile"
	rm -f "$modfile"
	return 1
	}

	read_value "$modfile" \|\| {
	error "failed to read GRUB_DEFAULT from edited file";
	rm -f "$modfile"
	return 1
	}
	[ "$_RET" = "saved" ] \|\| {
	error "change of GRUB_DEFAULT in $grubfile did not make a change."
	rm -f "$modfile"
	return 1
	}

	msg "apply change to $grubfile";
	diff -u "$grubfile" "$modfile" \| sed 's,^, ,g' 1>&2
	if [ "$dry_run" != "true" ]; then
	cp "$modfile" "$grubfile" \|\| {
	error "failed to update $grubfile."
	rm -f "$modfile"
	return 1
	}
	rm "$modfile"
	fi

	msg execute: update-grub
	if [ "$dry_run" != "true" ]; then
	update-grub \|\| {
	error "failed update-grub to apply $var=saved";
	return 1;
	}
	fi
	}

	get_entry() {
	local submenu="Advanced options for Ubuntu"
	local prefix="Ubuntu, with Linux "
	# VER-FLAV like '3.13.0-79-generic'
	local kernel="$1" verflav=""

	# /boot/vmlinuz-VER-FLAV -> vmlinuz-VER-FLAV
	verflav=${kernel##*/}
	# vmlinuz-VER-FLAV - VER-FLAV
	verflav=${verflav#*-}

	entry="${submenu:+${submenu}}>${prefix}$verflav"
	if ! grep -q "$prefix$verflav" "/boot/grub/grub.cfg"; then
	error "no $prefix$verflav entry in /boot/grub/grub.cfg"
	return 1
	fi
	echo "$entry"
	}

	main() {
	local short_opts="hn"
	local long_opts="help,default,dry-run,setup-only"
	local getopt_out=""
	getopt_out=$(getopt --name "${0##*/}" \
	--options "${short_opts}" --long "${long_opts}" -- "$@") &&
	eval set -- "${getopt_out}" \|\|
	{ Usage 1>&2; return; }

	## <<insert default variables here>>
	local dry_run=false setup_only=false mode="reboot"
	local cur="" next="" kernel="" kernel_in=""
	while [ $# -ne 0 ]; do
	cur="$1"; next="$2";
	case "$cur" in
	-h\|--help) Usage ; exit 0;;
	--default) mode="set-default";;
	-n\|--dry-run) dry_run=true;;
	--setup-only) setup_only=true;;
	--) shift; break;;
	esac
	shift;
	done

	if [ "$setup_only" = "true" ]; then
	[ $# -eq 0 ] \|\| {
	Usage 1>&2; echo "got $# args, expected 0 for setup-only"
	return 1;
	}
	else
	[ $# -eq 1 ] \|\| {
	Usage 1>&2; echo "got $# args, expected 1. must provide kernel."
	return 1;
	}
	if [ "$setup_only" = "false" ]; then
	kernel_in="$1"
	kernel=$(find_kernel "${kernel_in}") \|\| return
	fi
	fi

	setup_grub "$dry_run" \|\| return
	if [ "$setup_only" = "true" ]; then
	return
	fi

	entry=$(get_entry "$kernel") \|\| fail
	msg "selected $kernel. entry: ${entry}"

	cmd="grub-$mode"

	msg "execute: $cmd \"$entry\""
	if [ "$dry_run" != "true" ]; then
	"$cmd" "$entry" \|\| {
	error "failed: $cmd \"$entry\""
	return 1
	}
	fi
	return 0
	}

	main "$@"

	# vi: ts=4 expandtab