Chrooting with and without upstart in Bash

I’ve posted a chroot script in the past which dealt with upstart. However, if you also have an OS which does not use upstart you need to change the sources of the chroot script. I decided to make a script which does the general things, and have a config file per chroot enviroment. I have need two chrooted enviroments for my dual boot laptop. One for Debian and one for Ubuntu.

Let’s start with creating some directories for our chroot script and the configuration files. Btw, I will use vi in my examples for editing files, use any editor you like, eg nano, gedit, kate, emacs, etc, etc.

# Create /home/youruser/chroot/etc
mkdir -p $HOME/chroot/etc
cd $HOME/chroot

Create the Debian chroot configuration.

vi etc/debian.conf

# This file is read as a shell script
## Debian
root_disk=/dev/sda3
# What is the mount point of the OS?
mnt_point=$HOME/chroot/debian
# Mount various things
# Have seperate mount points for /var/log, /home and /opt,
# include them in the chroot.
mnt_devices_start="proc dev dev/pts sys var/log home opt"
# 1 if your OS uses upstart or 0 if it doesn't.
upstart=0
# Command to start chroot with
# I use my own account to enter the chroot
chroot="su - youruser"

And the Ubuntu config.

vi etc/ubuntu.conf

# This file is read as a shell script
## Ubuntu
root_disk=/dev/sda2
mnt=$HOME/chroot/ubuntu
mnt_devices_start="proc dev dev/pts sys var/log home opt"
upstart=0
chroot="su - youruser"

Then we will create/our chroot script, we define what we want to mount at start/stop (there should be some kind of shell thingy to reverse the list, but I haven’t found it – or just use an array )

vi chroot.sh

#!/bin/bash

mounted=$(mount)
mnt_devices_start="proc dev dev/pts sys var/log home opt"
upstart=0
chroot=bash

SELF=$(basename $0)
SELF_DIR=$(dirname $0)

usage() {
   echo "$SELF [chrootname] "
   exit 0
}

if [ -n "$1" ] ; then
    if [ ! -f "$SELF_DIR/etc/$1.conf" ] ; then
        echo "chroot '$1' does not exist!" >&2
        exit 1
    fi
    source $SELF_DIR/etc/$1.conf
    shift
else
    usage
fi


if [ -z "$mnt_devices_stop" ] ; then
    mnt_devices_stop=$(echo $mnt_devices_start | tac -s ' ')
fi

resolv="$mnt"/etc/resolv.conf
resolv_o="$resolv.orig"

start() {

    [ ! -d "$mnt" ] && sudo mkdir -p "$mnt"

    echo -e "$mounted" | grep -q "$mnt"

    if [ $? -ne 0 ] ; then
        sudo mount $root_disk "$mnt"
        if [ $? -ne 0 ] ; then
            echo "Unable to mount '$root_disk' on '$mnt'" >&2
            exit 1
        fi
    fi

    local i
    for i in $mnt_devices_start ; do

        echo -e "$mounted" | grep -q "$mnt/$i"
        [ $? -eq 0 ] && continue

        sudo mount -o bind /$i "$mnt"/$i

    done

    if [ ! -e "$resolv_o" ] ; then
        sudo cat "/etc/resolv.conf" | sudo tee "$resolv_o" >/dev/null
    fi
    cat /etc/resolv.conf | sudo tee "$resolv" >/dev/null

    if [ $upstart -ne 0 ] && [ ! -e "$mnt/sbin/initctl.distrib" ] ; then
        sudo chroot "$mnt" dpkg-divert --local --rename --add /sbin/initctl
        sudo chroot "$mnt" ln -s /bin/true /sbin/initctl
    fi
    eval sudo chroot "$mnt" "$chroot"
}

stop() {

    [ ! -e "$mnt" ] && return

    if [ "$upstart" -ne 0 ] && [ -e "$mnt/sbin/initctl.distrib" ] ; then
        sudo chroot "$mnt" rm /sbin/initctl
        sudo chroot "$mnt" dpkg-divert --local --rename --remove /sbin/initctl
    fi

    if [ -e "$resolv_o" ] ; then
        cat $resolv_o | tee "$resolv" >/dev/null
        sudo rm "$resolv_o"
    fi

    local i
    for i in $mnt_devices_stop; do
        echo -e "$mounted" | grep -q "$mnt/$i"
        [ $? -ne 0 ] && continue
        sudo umount "$mnt/$i"
    done

    echo -e "$mounted" | grep -q "$mnt"

    if [ $? -eq 0 ] ; then
        sudo umount "$mnt"
        [ $? -ne 0 ] && return
        sudo rmdir "$mnt"
    fi
}


case $1 in
    start|stop) $1;;
    *) usage;;
esac

To be able to call chroot.sh from anywhere, just add it to your path.

vi $HOME/.bashrc # or .zshrc, or whatever your shell requires

export PATH=$PATH:$HOME/chroot

# Load new .bashrc
source $HOME/.bashrc
# or..
bash
# or
export PATH=$PATH:$HOME/chroot

Now you can start/stop chroot by:

chroot.sh ubuntu start
chroot.sh ubuntu stop
# or Debian
chroot.sh debian start
chroot.sh debian stop

I hope you find this useful .