how to edit /boot/config.txt from the .img.xz file?

Question

i have installed ubuntu-21.10-preinstalled-desktop-arm64+raspi.img.xz. is it possible to edit /boot/config.txt from the file itself so that we need not boot into the pi and then edit the config.txt and reboot? why not just directly edit the config.txt from the file and boot into the pi with that custom image? note so far i have opened the .img file from the downloaded .img.xz file without extracting and have got 2 mounted volumes "system-boot" and "writable".

Answer 1

You can loop mount the image on a Linux machine then edit using normal tools.

The following is a script I use, but frankly is more work than just editing files.

#!/bin/bash
# 2020-01-19
# Mount partitions on an OS Image
# Assumes only 2 partitions boot & root exist in Image

BOOT_MOUNT='/mnt/SDA1'
ROOT_MOUNT='/mnt/SDA2'
# IMG='/mnt/Image/BusterBackup.img'
IMG="$1"
# echo $IMG

errexit()
{
  echo ""
  echo "$1"
  echo ""
  exit 1
}

if [[ "${IMG}" = "" ]]; then
    errexit "Usage: $0 imagefile"
fi
if [ $(id -u) -ne 0 ]; then
    errexit "$0 must be run as root user"
fi

# Check/create Mount Points
if [ ! -e $BOOT_MOUNT ]; then
    mkdir $BOOT_MOUNT
fi
if [ ! -e $ROOT_MOUNT ]; then
    mkdir $ROOT_MOUNT
fi

# Check Mount Points
if $(mountpoint -q $BOOT_MOUNT); then
    errexit "Mountpoint $BOOT_MOUNT already in use"
fi
if $(mountpoint -q $ROOT_MOUNT); then
    errexit "Mountpoint $ROOT_MOUNT already in use"
fi


# Loop mount all partitions on Image
LOOP="$(losetup --find --partscan --show ${IMG})"
if [ $? -ne 0 ]; then
    errexit "Unable to create loop device"
fi

# echo $LOOP
SD1=$LOOP
SD2=$LOOP
SD1+="p1"   # partition containing boot files
SD2+="p2"   # partition containing OS files

# exit
# Mount Partitions 1 & 2
if ! $(mountpoint -q $BOOT_MOUNT); then
    mount $SD1 $BOOT_MOUNT  # mount partition containing boot files
fi
if ! $(mountpoint -q $ROOT_MOUNT); then
    mount $SD2 $ROOT_MOUNT  # mount root partition containing OS files
fi

echo "When finished, run:"
echo "sudo umount ${BOOT_MOUNT} ${ROOT_MOUNT}"
echo "sudo losetup -d ${LOOP}"
echo ""