Re-prioritize CrOS boot pair to toggle the active image pair (KERNEL & ROOT) or set the active image pair as specified.

priority

#!/bin/sh -e 
REVISION=14
APPL_PATH="$(readlink -f $0)"
APPLICATION="${0##*/}"
BOOT_KERNEL=""
BOOT_ROOT=""
CURRENT_BOOTPAIR=""
CURRENT_CHROME=""
CURRENT_DRIVE=""
CURRENT_KERNEL=""
CURRENT_KERNVER=""
CURRENT_MILESTONE=""
CURRENT_ROOT=""
CURRENT_TRACK=""
DRIVE_PREFIX=""
ALTERNATE_BOOTPAIR=""
ALTERNATE_CHROME=""
ALTERNATE_KERNEL=""
ALTERNATE_KERNELVER=""
ALTERNATE_MILESTONE=""
ALTERNATE_ROOT=""
ALTERNATE_TRACK=""
ALT_ROOT_NODEV=""
NO_OP=""
ROOTDEV=""
ROOTDEVPFX=""
RPLY=""
TRIES="3"
USAGE="$APPLICATION: REVISION $REVISION

Description:
### A script to toggle the active image pair: A or B $CH_MSG1 (KERNEL & ROOT)
##+ or set the active image (KERNEL & ROOT) as specified on the commandline.

Usage:
$ $APPLICATION [ -s,--showpartition | partition_number | -h,--help | -v, --version ]
"

## Exits the script with exit code $1, spitting out message $@ to stderr
error() {
    local ecode="$1"
    shift
    echo "$*" 1>&2
    exit "$ecode"
}

if   [ "$1" = "-h" -o "$1" = "--help" ]; then
    error 0 "$USAGE"
elif [ "$1" = "-v" -o "$1" = "--version" ]; then
    error 0 "$APPLICATION: REVISION $REVISION"
fi

## If we're not running as root, restart as root.
if [ $(id -u) -ne 0 ]; then
  echo "...elevating $USER to superuser..."
  echo
  exec sudo /bin/sh    "$APPL_PATH" "$@"
fi

## Following routine borrowed from @drinkcat ;)
ROOTDEV="$(rootdev -d -s)"
if [ -z "$ROOTDEV" ]; then
    error 1 "Cannot find root device."
fi
if [ ! -b "$ROOTDEV" ]; then
    error 1 "$ROOTDEV is not a block device."
fi
## If $ROOTDEV ends with a number (e.g. mmcblk0), partitions are named
## ${ROOTDEV}pX (e.g. mmcblk0p1). If not (e.g. sda), they are named
## ${ROOTDEV}X (e.g. sda1).
ROOTDEVPFX="$ROOTDEV"
if [ "${ROOTDEV%[0-9]}" != "$ROOTDEV" ]; then
    ROOTDEVPFX="${ROOTDEV}p"
fi

DRIVE_PREFIX="${ROOTDEVPFX#/dev/}"
CURRENT_ROOT="$(rootdev -s)"
CURRENT_DRIVE="$ROOTDEV"
CURRENT_KERNVER="$(/bin/ls /lib/modules/ | cut -d- -f1,2)"
CURRENT_MILESTONE="$(grep -i chrome_milestone /etc/lsb-release 2>/dev/null | cut -d= -f2)"
CURRENT_CHROME="$(/opt/google/chrome/chrome --version --no-display 2>/dev/null | cut -d ' ' -f3)"
CURRENT_RELEASE="$(grep -i google_release /etc/lsb-release 2>/dev/null | cut -d= -f2)"
CURRENT_TRACK="$(grep -i release_track /etc/lsb-release 2>/dev/null | cut -d= -f2)"

### Neverware CloudReady OS partitions
##/dev/sda
##       start        size    part  contents
##    14929936   235139696      16  Label: "STATE"
##       20496       32768      17  Label: "KERN-A"
##                                  Attr: priority=1 tries=0 successful=1
##     8687632     6242304      18  Label: "ROOT-A"
##       53264       32768      19  Label: "KERN-B"
##                                  Attr: priority=2 tries=0 successful=1
##     2445328     6242304      20  Label: "ROOT-B"
##       16464           1      21  Label: "KERN-C"
##                                  Attr: priority=0 tries=15 successful=0
##       16465           1      22  Label: "ROOT-C"
##       86032     2097152      23  Label: "OEM"
##       16466           1      24  Label: "reserved"
##       16467           1      25  Label: "reserved"
##          80       16384      26  Label: "RWFW"
##     2183184      131072      27  Label: "EFI-SYSTEM"
##                                  Attr: legacy_boot=1

if [ "$CURRENT_ROOT" = "${ROOTDEVPFX}3" ]
then CURRENT_KERNEL="2"
     CURRENT_BOOTPAIR="A"
     ALTERNATE_ROOT="${ROOTDEVPFX}5"
     ALTERNATE_KERNEL="4"
     ALTERNATE_BOOTPAIR="B"
elif [ "$CURRENT_ROOT" = "${ROOTDEVPFX}5" ]
then CURRENT_KERNEL="4"
     CURRENT_BOOTPAIR="B"
     ALTERNATE_ROOT="${ROOTDEVPFX}3"
     ALTERNATE_KERNEL="2"
     ALTERNATE_BOOTPAIR="A"
elif [ "$CURRENT_ROOT" = "${ROOTDEVPFX}18" ]
then CURRENT_KERNEL="17"
     CURRENT_BOOTPAIR="A"
     ALTERNATE_KERNEL="19"
     ALTERNATE_ROOT="${ROOTDEVPFX}20"
     ALTERNATE_BOOTPAIR="B"
else CURRENT_KERNEL="19"
     CURRENT_BOOTPAIR="B"
     ALTERNATE_ROOT="${ROOTDEVPFX}18"
     ALTERNATE_KERNEL="17"
     ALTERNATE_BOOTPAIR="A"
fi

ALT_ROOT_NODEV="${ALTERNATE_ROOT#/dev/}"
mkdir -p /var/$ALT_ROOT_NODEV
if mount $ALTERNATE_ROOT /var/$ALT_ROOT_NODEV 2>/dev/null
then
    ALTERNATE_KERNVER="$(/bin/ls /var/$ALT_ROOT_NODEV/lib/modules/ | cut -d- -f1,2)"
    ALTERNATE_MILESTONE="$(grep -i chrome_milestone /var/$ALT_ROOT_NODEV/etc/lsb-release 2>/dev/null | cut -d= -f2)"
    ALTERNATE_CHROME="$(/var/$ALT_ROOT_NODEV/opt/google/chrome/chrome --version --no-display 2>/dev/null | cut -d ' ' -f3)"
    ALTERNATE_RELEASE="$(grep -i google_release /var/$ALT_ROOT_NODEV/etc/lsb-release 2>/dev/null | cut -d= -f2)"
    ALTERNATE_TRACK="$(grep -i release_track /var/$ALT_ROOT_NODEV/etc/lsb-release 2>/dev/null | cut -d= -f2)"
    umount /var/$ALT_ROOT_NODEV && rmdir /var/$ALT_ROOT_NODEV
else
    echo "Cannot mount '/var/$ALT_ROOT_NODEV', perhaps a new update is waiting."
    echo "No alternate releases, milestones, or track info. is available at the moment."
    echo
#   exit 1
fi 

CH_SIZE="$(cgpt show -qnsi 7 $CURRENT_DRIVE)"
if [ "$CH_SIZE" -gt 1 ]; then CH_MSG1="or C"; CH_MSG2=", & 6"; CH_NBR=",6"; fi

if [ "$#" -eq 0 ];
then BOOT_KERNEL="$ALTERNATE_KERNEL"
     BOOT_ROOT="$ALTERNATE_ROOT"
else BOOT_ROOT="${CURRENT_DRIVE}$(($1+1))"
     case $1 in
       [2,4${CH_NBR}]) BOOT_KERNEL="$1"
       echo -n "Re-prioritizing to use KERNEL $BOOT_KERNEL ..."
       sleep 3 && echo; echo
     ;;	
       -s*|--s*) NO_OP="y"
     ;;
       *) echo "KERNEL=$1 is invalid, only 2,4${CH_MSG2} accepted"; exit 2
     ;;
     esac
fi

echo -n "CURRENT_BOOTPAIR=$CURRENT_BOOTPAIR"
echo "			ALTERNATE_BOOTPAIR=$ALTERNATE_BOOTPAIR"
echo -n "-------------------------------"
echo "		-------------------------------"
echo -n "CURRENT_KERNEL=$CURRENT_KERNEL"
echo "			ALTERNATE_KERNEL=$ALTERNATE_KERNEL"
echo -n "CURRENT_ROOT=$CURRENT_ROOT"
echo "		ALTERNATE_ROOT=$ALTERNATE_ROOT"
echo -n "CURRENT_MILESTONE=$CURRENT_MILESTONE"
[ -n "$ALTERNATE_MILESTONE" ] && echo "			ALTERNATE_MILESTONE=$ALTERNATE_MILESTONE" ||echo
echo -n "CURRENT_CHROME=$CURRENT_CHROME"
[ -n "$ALTERNATE_CHROME" ] && echo "		ALTERNATE_CHROME=$ALTERNATE_CHROME" || echo
echo -n "CURRENT_RELEASE=$CURRENT_RELEASE"
[ -n "$ALTERNATE_RELEASE" ] && echo "		ALTERNATE_RELEASE=$ALTERNATE_RELEASE" || echo
echo -n "CURRENT_KERNVER=$CURRENT_KERNVER"
[ -n "$ALTERNATE_KERNVER" ] && echo "		ALTERNATE_KERNVER=$ALTERNATE_KERNVER"
echo -n "CURRENT_TRACK=$CURRENT_TRACK"
[ -n "$ALTERNATE_TRACK" ] && echo "		ALTERNATE_TRACK=$ALTERNATE_TRACK" || echo
echo
[ -z "$NO_OP" ] && echo "BOOT_KERNEL=$BOOT_KERNEL (prioritized)"
[ -z "$NO_OP" ] && echo "BOOT_ROOT=$BOOT_ROOT" && echo

cgpt show -i 2 "$CURRENT_DRIVE" | grep --color=always -e start -e size -e part -e KERN -e priority -e ROOT -e contents
for i in 3 4 5 17 18 19 20
do
    cgpt show -i "$i" "$CURRENT_DRIVE" | grep --color=always -e KERN -e priority -e ROOT
done

if [ "$CH_SIZE" -gt 1 ]; then
    cgpt show -i 6 "$CURRENT_DRIVE" | grep --color=always -e KERN -e priority -e ROOT -e CHRUBUNTU
    cgpt show -i 7 "$CURRENT_DRIVE" | grep --color=always -e KERN -e priority -e ROOT -e CHRUBUNTU
fi

if [ -n "$NO_OP" ]; then echo; exit 0; fi

echo
echo "cgpt add $CURRENT_DRIVE -i $BOOT_KERNEL -S0 -T"$TRIES""
echo "cgpt prioritize $CURRENT_DRIVE -i $BOOT_KERNEL"
echo
echo -n "Press ENTER to perform the above ROLLBACK - Ctrl+C to abort "; read RPLY
if [ -n "${RPLY}" ]; then error 0 "Key entered, ABORTING ..."; fi

## Mark the kernel as successfully booted (success=0, tries="$TRIES").
cgpt add "$CURRENT_DRIVE" -i "$BOOT_KERNEL" -S0 -T"$TRIES"
## Mark the kernel as highest priority
cgpt prioritize "$CURRENT_DRIVE" -i "$BOOT_KERNEL"

echo
cgpt show -i 2 "$CURRENT_DRIVE" | grep --color=always -e start -e size -e part -e KERN -e priority -e ROOT -e contents
for i in 3 4 5 17 18 19 20
do
    cgpt show -i "$i" "$CURRENT_DRIVE" | grep --color=always -e KERN -e priority -e ROOT
done

if [ "$CH_SIZE" -gt 1 ]; then
    cgpt show -i 6 "$CURRENT_DRIVE" | grep --color=always -e KERN -e priority -e ROOT -e CHRUBUNTU
    cgpt show -i 7 "$CURRENT_DRIVE" | grep --color=always -e KERN -e priority -e ROOT -e CHRUBUNTU
fi

echo
exit 0

Fixed a few root drive glitches and updated current & alternate info. displayed.

Revision #8 - fairly major update.

Revision #9 - updated for CloudReady OS partition scheme.

Revision #10 - Changed field for chrome version from 3 to 2, added another abort option.

Revision #11 - Added Boot pair and kernel version.

Revision #13 - Rearranged some logic, added revision/version output, fixed revision number.

Revision #14 - Reinstated '-s' option blunder - my bad & added the word ROLLBACK to the prompt.