Difference between revisions of "Change root"

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(Undo revision 246984 by Freebullets (talk))
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* Be sure that the architecture of the Linux environment you have booted into matches the architecture of the root directory you wish to enter (i.e. i686, x86_64). You can find the architecture of your current environment with:
 
* Be sure that the architecture of the Linux environment you have booted into matches the architecture of the root directory you wish to enter (i.e. i686, x86_64). You can find the architecture of your current environment with:
  
: {{bc|uname -m}}
+
: {{bc|# uname -m}}
  
 
* If you need any kernel modules loaded in the chroot environment, load them before chrooting. It may also be useful to initialize your swap ({{ic|swapon /dev/sdxY}}) and to establish an internet connection before chrooting.
 
* If you need any kernel modules loaded in the chroot environment, load them before chrooting. It may also be useful to initialize your swap ({{ic|swapon /dev/sdxY}}) and to establish an internet connection before chrooting.
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The root partition of the Linux system that you're trying to chroot into needs to be mounted first. To find out the device name assigned by the kernel, run:
 
The root partition of the Linux system that you're trying to chroot into needs to be mounted first. To find out the device name assigned by the kernel, run:
  
  lsblk /dev/sda
+
  # lsblk /dev/sda
  
 
You can also run the following to get an idea of your partition layout.
 
You can also run the following to get an idea of your partition layout.
  
  fdisk -l
+
  # fdisk -l
  
 
Now create a directory where you would like to mount the root partition and mount it:
 
Now create a directory where you would like to mount the root partition and mount it:
  
  mkdir /mnt/arch
+
  # mkdir /mnt/arch
  mount /dev/sda3 /mnt/arch
+
  # mount /dev/sda3 /mnt/arch
  
 
Next, if you have separate partitions for other parts of your system (e.g. {{ic|/boot}}, {{ic|/home}}, {{ic|/var}}, etc), you should mount them, as well:
 
Next, if you have separate partitions for other parts of your system (e.g. {{ic|/boot}}, {{ic|/home}}, {{ic|/var}}, etc), you should mount them, as well:
  
  mount /dev/sda1 /mnt/arch/boot/
+
  # mount /dev/sda1 /mnt/arch/boot/
  mount /dev/sdb5 /mnt/arch/home/
+
  # mount /dev/sdb5 /mnt/arch/home/
  mount ...
+
  # mount ...
  
 
While it's possible to mount filesystems after you've chrooted, it is more convenient to do so beforehand. The reasoning for this is that you'll have to unmount the temporary filesystems after you exit the chroot, so this lets you umount all the filesystems with a single command. This also allows for a safer shutdown. Because the external Linux environment knows all mounted partitions, it can safely unmount them during shutdown.
 
While it's possible to mount filesystems after you've chrooted, it is more convenient to do so beforehand. The reasoning for this is that you'll have to unmount the temporary filesystems after you exit the chroot, so this lets you umount all the filesystems with a single command. This also allows for a safer shutdown. Because the external Linux environment knows all mounted partitions, it can safely unmount them during shutdown.
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To allow the connection to your X server, you have to run the following from a terminal:
 
To allow the connection to your X server, you have to run the following from a terminal:
  
  xhost +
+
  # xhost +
  
 
Then, to direct the applications to your X server, run:
 
Then, to direct the applications to your X server, run:
  
  export DISPLAY=:0.0
+
  # export DISPLAY=:0.0
  
 
== Perform system maintenance ==
 
== Perform system maintenance ==
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When you're finished with system maintenance, exit the chroot:
 
When you're finished with system maintenance, exit the chroot:
  
  exit
+
  # exit
  
 
Then unmount the temporary filesystems and any mounted devices:
 
Then unmount the temporary filesystems and any mounted devices:
  
  umount {proc,sys,dev,boot,[...],}
+
  # umount {proc,sys,dev,boot,[...],}
  
 
Finally, attempt to unmount your root partition:
 
Finally, attempt to unmount your root partition:
  
  cd ..
+
  # cd ..
  umount arch/
+
  # umount arch/
  
 
{{Note|If you get an error saying that {{ic|/mnt}} (or any other partition) is busy, this can mean one of two things:
 
{{Note|If you get an error saying that {{ic|/mnt}} (or any other partition) is busy, this can mean one of two things:
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: If you are still unable to unmount a partition, use the {{ic|--force}} option:
 
: If you are still unable to unmount a partition, use the {{ic|--force}} option:
  
: {{bc|umount -f /mnt}}}}
+
: {{bc|# umount -f /mnt}}}}
  
 
After this, you will be able to safely reboot.
 
After this, you will be able to safely reboot.

Revision as of 21:10, 15 February 2013

Chroot is the process of changing of the apparent disk root directory (and the current running process and its children) to another root directory. When you change root to another directory you cannot access files and commands outside that directory. This directory is called a chroot jail. Changing root is commonly done for system maintenance, such as reinstalling the bootloader or resetting a forgotten password.

Requirements

  • You'll need to boot from another working Linux environment (e.g. from a LiveCD or USB flash media, or from another installed Linux distribution).
  • Root privileges are required in order to chroot.
  • Be sure that the architecture of the Linux environment you have booted into matches the architecture of the root directory you wish to enter (i.e. i686, x86_64). You can find the architecture of your current environment with:
# uname -m
  • If you need any kernel modules loaded in the chroot environment, load them before chrooting. It may also be useful to initialize your swap (swapon /dev/sdxY) and to establish an internet connection before chrooting.

Mount the partitions

The root partition of the Linux system that you're trying to chroot into needs to be mounted first. To find out the device name assigned by the kernel, run:

# lsblk /dev/sda

You can also run the following to get an idea of your partition layout.

# fdisk -l

Now create a directory where you would like to mount the root partition and mount it:

# mkdir /mnt/arch
# mount /dev/sda3 /mnt/arch

Next, if you have separate partitions for other parts of your system (e.g. /boot, /home, /var, etc), you should mount them, as well:

# mount /dev/sda1 /mnt/arch/boot/
# mount /dev/sdb5 /mnt/arch/home/
# mount ...

While it's possible to mount filesystems after you've chrooted, it is more convenient to do so beforehand. The reasoning for this is that you'll have to unmount the temporary filesystems after you exit the chroot, so this lets you umount all the filesystems with a single command. This also allows for a safer shutdown. Because the external Linux environment knows all mounted partitions, it can safely unmount them during shutdown.

Change root

Mount the temporary filesystems as root:

Note: Using a newer (2012) Arch release, the following commands can be replaced with arch-chroot /mnt/arch. You must have arch-install-scripts installed to run arch-chroot. The following commands may still be used if you're using a different Linux distribution.
cd /mnt/arch
mount -t proc proc proc/
mount -t sysfs sys sys/
mount -o bind /dev dev/
mount -t devpts pts dev/pts/

If you have established an internet connection and want to use it in the chroot environment, you may have to copy over your DNS configuration to be able to resolve hostnames.

cp -L /etc/resolv.conf etc/resolv.conf

Now chroot into your installed system and define your shell:

chroot /mnt/arch /bin/bash
Note: If you see the error chroot: cannot run command '/bin/bash': Exec format error, it is likely that the two architectures do not match.

Optionally, to source your Bash configuration (~/.bashrc and /etc/bash.bashrc), run:

source ~/.bashrc
source /etc/profile

Optionally, create a unique prompt to be able to differentiate your chroot environment:

export PS1="(chroot) $PS1"

Run graphical chrooted applications

If you have X running on your system, you can start graphical applications from the chroot environment.

To allow the connection to your X server, you have to run the following from a terminal:

# xhost +

Then, to direct the applications to your X server, run:

# export DISPLAY=:0.0

Perform system maintenance

At this point you can perform whatever system maintenance you require inside the chroot environment. A few common examples are:

Exit the chroot environment

When you're finished with system maintenance, exit the chroot:

# exit

Then unmount the temporary filesystems and any mounted devices:

# umount {proc,sys,dev,boot,[...],}

Finally, attempt to unmount your root partition:

# cd ..
# umount arch/
Note: If you get an error saying that /mnt (or any other partition) is busy, this can mean one of two things:
  • A program was left running inside of the chroot.
  • Or, more frequently, a sub-mount still exists (e.g. /mnt/arch/boot within /mnt/arch). Check with lsblk to see if there are any mountpoints left:
lsblk /dev/sda
If you are still unable to unmount a partition, use the --force option:
# umount -f /mnt

After this, you will be able to safely reboot.

Example

This may protect your system from Internet attacks during browsing:

## as root: 
# cd
# mkdir myroot
# pacman -S arch-install-scripts
# mount --bind myroot myroot # pacstrap must see myroot as mounted 
# pacstrap -i myroot base base-devel
# mount -t proc proc myroot/proc/
# mount -t sysfs sys myroot/sys/
# mount -o bind /dev myroot/dev/
# mount -t devpts pts myroot/dev/pts/
# cp -i /etc/resolv.conf myroot/etc/
# chroot myroot
## inside chroot: 
# passwd # set a password 
# useradd -m -s /bin/bash user
# passwd user # set a password
## in a shell outside the chroot: 
# pacman -S xorg-server-xnest
## in a shell outside the chroot you can run this as user: 
$ Xnest -ac -geometry 1024x716+0+0 :1
## continue inside the chroot: 
# pacman -S xterm
# xterm
## xterm is now running in Xnest 
# pacman -S xorg-server xorg-xinit xorg-server-utils
# pacman -S openbox
# pacman -S ttf-dejavu
# pacman -S firefox
## firefox is now running in Xnest 
# exit
## now do this as user: 
## outside chroot: 
# chroot --userspec=user myroot
## inside chroot as user: 
$ DISPLAY=:1
$ openbox &
$ HOME="/home/user"
$ firefox

See also: Basic Chroot