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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

   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 (*note
Special built-in utilities::).

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

     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.

     Use this option to specify the supplementary GROUPS to be used by
     the new process.  The items in the list (names or numeric IDs) must
     be separated by commas.

   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.

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