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23.1 chroot: Run a command with a different root directory

chroot runs a command with a specified root directory. On many systems, only the super-user can do this.1 Synopses:

     chroot option newroot [command [args]...]
     chroot option

Ordinarily, file names are looked up starting at the root of the directory structure, i.e., /. chroot changes the root to the directory newroot (which must exist) and then runs command with optional args. If command is not specified, the default is the value of the SHELL environment variable or /bin/sh if not set, invoked with the -i option. command must not be a special built-in utility (see Special built-in utilities).

The program accepts the following options. Also see Common options. Options must precede operands.

--userspec=user[:group]
By default, command is run with the same credentials as the invoking process. Use this option to run it as a different user and/or with a different primary group. If a user is specified then the supplementary groups are set according to the system defined list for that user, unless overridden with the --groups option.
--groups=groups
Use this option to override the supplementary groups to be used by the new process. The items in the list (names or numeric IDs) must be separated by commas. Use ‘--groups=''’ to disable the supplementary group look-up implicit in the --userspec option.

The user and group name look-up performed by the --userspec and --groups options, is done both outside and inside the chroot, with successful look-ups inside the chroot taking precedence. If the specified user or group items are intended to represent a numeric ID, then a name to ID resolving step is avoided by specifying a leading ‘+’. See Disambiguating names and IDs.

Here are a few tips to help avoid common problems in using chroot. To start with a simple example, make command refer to a statically linked binary. If you were to use a dynamically linked executable, then you'd have to arrange to have the shared libraries in the right place under your new root directory.

For example, if you create a statically linked ls executable, and put it in /tmp/empty, you can run this command as root:

     $ chroot /tmp/empty /ls -Rl /

Then you'll see output like this:

     /:
     total 1023
     -rwxr-xr-x 1 0 0 1041745 Aug 16 11:17 ls

If you want to use a dynamically linked executable, say bash, then first run ‘ldd bash’ to see what shared objects it needs. Then, in addition to copying the actual binary, also copy the listed files to the required positions under your intended new root directory. Finally, if the executable requires any other files (e.g., data, state, device files), copy them into place, too.

Exit status:

     125 if chroot itself fails
     126 if command is found but cannot be invoked
     127 if command cannot be found
     the exit status of command otherwise

Footnotes

[1] However, some systems (e.g., FreeBSD) can be configured to allow certain regular users to use the chroot system call, and hence to run this program. Also, on Cygwin, anyone can run the chroot command, because the underlying function is non-privileged due to lack of support in MS-Windows.