[BROKEN] Script for ChromeOS Flex to add cros_debug / dev mode option in grub.cfg

flex.grub

##########################################################################
### THIS SCRIPT NO LONGER WORKS DUE TO RECENT CHANGES IN CHROMEOS Flex ###
##########################################################################

### Script to add cros_debug / dev mode option in grub.cfg 
### ( Each Flex update removes cros_debug / dev mode option
###   Be sure to run this script AFTER each update and BEFORE rebooting )

ANS=
ROOTDEVICE=
ROOTDEVICEPREFIX=
P12MOUNTPOINT=/var/p12
GRUBCFGPATH=$P12MOUNTPOINT/efi/boot
GRUBCFGFILE=grub.cfg

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

# Find the root device
# Sets:
#  - $ROOTDEVICE as the root device (e.g. /dev/sda or /dev/mmcblk0)
#  - $ROOTDEVICEPREFIX as a prefix for partitions (/dev/sda, /dev/mmcblk0p)
findrootdevice() {
    ROOTDEVICE="`rootdev -d -s`"

    if [ -z "$ROOTDEVICE" ]; then
        error 1 "Cannot find root device."
    fi

    if [ ! -b "$ROOTDEVICE" ]; then
        error 1 "$ROOTDEVICE is not a block device."
    fi

    # If $ROOTDEVICE ends with a number (e.g. mmcblk0), partitions are named
    # ${ROOTDEVICE}pX (e.g. mmcblk0p1). If not (e.g. sda), they are named
    # ${ROOTDEVICE}X (e.g. sda1).
    ROOTDEVICEPREFIX="$ROOTDEVICE"
    if [ "${ROOTDEVICE%[0-9]}" != "$ROOTDEVICE" ]; then
        ROOTDEVICEPREFIX="${ROOTDEVICE}p"
    fi
}

findrootdevice

if [ ! -d $P12MOUNTPOINT ]; then
# Make mountpoint
  echo "Making directory to mount partition 12"
  sudo mkdir -p $P12MOUNTPOINT
elif [ ! -d $GRUBCFGPATH ]; then
# Mount EFI partition
  echo "Mounting ${ROOTDEVICEPREFIX}12 on $P12MOUNTPOINT"
  sudo mount ${ROOTDEVICEPREFIX}12 $P12MOUNTPOINT
fi

# Check for cros_debug option, need 5
DEBUGCOUNT="$(grep 'cros_efi cros_debug' $GRUBCFGPATH/$GRUBCFGFILE | wc -l)"
if [ "$DEBUGCOUNT" -eq 5 ]; then
  echo "cros_debug has been added to $GRUBCFGFILE, no changes needed."
  exit
fi

# grub.cfg needs editing
echo "Editing $GRUBCFGFILE to add 'cros_debug' option."
echo "
PLEASE NOTE:
I claim no responsibility for any damage or harm that may occur to your
device as a result of running this script. PROCEED AT YOR OWN RISK.
"
echo -n "[ press ENTER to continue - Ctrl-C to abort ] "; read ANS
cp $GRUBCFGPATH/$GRUBCFGFILE /tmp
# vi /tmp/$GRUBCFGFILE
sed -e 's/cros_efi  /cros_efi cros_debug /' -e 's/cros_efi root/cros_efi cros_debug root/' /tmp/$GRUBCFGFILE
sudo cp $GRUBCFGPATH/$GRUBCFGFILE $GRUBCFGPATH/${GRUBCFGFILE}.bak 
sudo cp /tmp/$GRUBCFGFILE $GRUBCFGPATH/
echo "Done, you can reboot now."

exit

Just tested again for new install of flex on a second laptop, and the "cros_debug" let me get shell at crosh prompt. But then flex updated itself to 236 and after update I no longer got shell at crosh prompt, as apparently the update removed cros_debug. But good news is after I added back cros_debug, I was able to get /bin/bash shell at crosh prompt in newest flex os on my old thinkpad. Also, I think the sed command to use might better be something like this:

# I think sed below does this: for each line: if the line already has "cros_debug" do nothing  (skip  to next line)
# But If line does not have "cros_debug" then change "cros_efi" to "cros_efi cros_debug"
sed "/cros_debug/! s/cros_efi/cros_efi cros_debug/g"
# Should prevent issues if script is run more than once, ensuring grub.cfg has only  one "grub_debug" per line.

I'll update with step-by-step howto and details when I have a chance. But it seems the "BROKEN" warning might be removed for now ;-)

Even after recent 133 update I still can type "shell" at crosh prompt and get bash prompt. Around 2025-02-28, several 10-15 year old laptops updated to 133 version of chromeos-flex. for example: .

- OS variant: Chrome OS
- OS version: reven-release/R133-16151.53.0
- OS channel: stable-channel

BUT, after updates I did have to re-edit grub.cfg (by choosing to boot to Linux in refind). Sometimes, even after the edit, crosh still would not run the shell command. But I did a second edit, and after 2nd reboot, crosh gave me a shell prompt. So, seems like Google and/or the chromium-os folks have (unofficially) restored ability to get dev-mode (shell at crosh prompt) and this ability is no longer broken, at least for most of 2025 for all my 5 or so ChromeOS-flex laptops. And chromebrew can install and run any of the crew packages, including Firefox/brave, etc, on all of them. It is nice not to have to use chrome on flex (or even non-flex) Chromebooks ;-)