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I am trying to install Ubuntu base(Ubuntu from scratch) but I have a problem...after the installation I get a system with read-only file system and also, /boot is not automounted.

here is my /etc/fstab:

/dev/sda1   /boot vfat defaults,noatime 0 2
/dev/sda2   /     ext4 defaults,noatime 0 1

here is how I chroot into the base system for installation:

mount /dev/sda2 /mnt
cp /etc/resolv.conf /mnt/etc/resolv.conf
mount --types proc /proc /mnt/proc
mount --rbind /sys /mnt/sys
mount --make-rslave /mnt/sys
mount --rbind /dev /mnt/dev
mount --make-rslave /mnt/dev
test -L /dev/shm && rm /dev/shm && mkdir /dev/shm
mount --types tmpfs --options nosuid,nodev,noexec shm /dev/shm
chmod 1777 /dev/shm
chroot /mnt /bin/bash
source /etc/profile
export PS1="(chroot) ${PS1}"
<installation steps>
exit
cd
umount -l /mnt/dev{/shm,/pts,}
umount -R /mnt
reboot

when I run the following command the system becomes writable:

mount -o remount,rw /

what did I do wrong ?

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  • Can you provide more details about what happens after you reboot? What gets printed on the console before you run the mount command? Commented Oct 23, 2020 at 2:41
  • I get kernel messages and then get a # prompt.. Commented Oct 23, 2020 at 4:20
  • Most likely your installation steps are wrong. A correct Ubuntu install shouldn't dump you directly into a root shell with a plain # prompt like that, even with a broken filesystem. Sounds like you're getting /bin/sh for init instead of systemd. You'll need to provide more details about your installation steps, or link to instructions that you followed. Also, why are you installing into a chroot rather than using the Ubuntu installer? Sounds like a possible XY Problem. Commented Oct 23, 2020 at 5:32
  • Here is my installation steps: pastebin.com/GbwtEDsV Commented Oct 23, 2020 at 13:52

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Short Answer

In your installation steps, you need to install the systemd-sysv package.

Long Answer

Looks like you're installing Ubuntu Focal. The Ubuntu Base image for Focal does not include a /sbin/init. Assuming you're using systemd, which is the default, this binary is provided by the systemd-sysv package. Note that the Ubuntu Base image for some earlier version of Ubuntu do include /sbin/init, so older instructions may not mention this. (Why doesn't the image have /sbin/init anymore? Probably because it's generally not needed in containers, which is an increasingly large use case of images like this.)

So how did you end up at a shell prompt without an init? The init script in the initrd, which is responsible for execing a "real" init, has a fallback. If /sbin/init isn't found, it tries a list of alternate executables, one of which is /bin/sh. You probably also got a message like Target filesystem doesn't have requested /sbin/init printed to the console.

So how come your root filesystem was mounted read only? Because that's what your GRUB config tells it to do. Run cat /proc/cmdline and you should see that the ro option is included, which instructs the initrd to mount / read-only initially. This allows init to, if necessary, run fsck to fix errors on your root filesystem. Init will then remount / read-write.

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  • Should I install systemd after installing grub+initramfs+kernel or before ? Commented Oct 23, 2020 at 17:36
  • I suspect the regular Ubuntu installer installs systemd-sysv first, so that probably what I'd choose. But order probably doesn't matter. Commented Oct 23, 2020 at 17:41
  • Also, How do I change GRUB's config so that it wouldn't make the filesystem read-only ? Commented Oct 23, 2020 at 17:42
  • You don't need to make it mount read-write, systemd remounts it read-write for you. If you really want to mount it read-write at boot, I suggest asking a new question. Commented Oct 23, 2020 at 17:49

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