chroot - Changes the root directory of a command
/usr/sbin/chroot directory command
Only root can use the chroot command. The chroot command
changes the root directory from / to the specified directory
when the command executes. (The command specified
includes both the command name as well as any arguments.)
Consequently, the root of any path (as indicated by the
first / (slash) in the pathname) changes to directory and
is always relative to the current root. Even if the
chroot command is in effect, directory is relative to the
current root of the running process.
Several programs may not operate properly after chroot
executes. You must ensure that all vital files are present
in the new root file system and the relevant pathnames
for the files map correctly in the new root file
system.
For example, the ls -l command fails to give user and
group names if the new root file system does not have a
copy of the /etc/passwd and /etc/group files. If the
/etc/passwd and /etc/group files in the new root file system
represent different user and group names, then the
output from the ls -l command will be based on those
names, not the ones for the system's own name database.
Utilities that depend on description files produced by the
ctab command may also fail if the required description
files are not present in the new root file system.
The chroot program uses the execv() function to invoke the
specified command. As a consequence, the command specified
must be an executable binary, not a shell script. Further,
if the program requires indirect loading (for example,
due to unresolved symbols requiring use of a shared
library), then /sbin/loader as well as any files it
requires (for example, shared libraries) must be present
in the new root file system in the appropriate locations.
If special files in the new root have different major and
minor device numbers than the initial root directory, it
is possible to overwrite the file system.
To run a subshell with another file system as the root,
enter a command similar to the following. Note in this
example, the file system is on the /dev/disk/dsk13a device
and is mounted to /mnt/dsk13a:
chroot /mnt/dsk13a /sbin/sh
The command shown in the previous example specifies
a change from the current root file system to the
one mounted on /mnt/dsk13a while /sbin/sh (which
itself is relative to the new root file system)
executes. When /bin/sh executes, the original root
file system is inaccessible. The file system
mounted on /mnt/dsk13a must contain the standard
directories of a root file system. In particular,
the shell looks for commands in /sbin, /bin, and
/usr/bin (among others) on the new root file system.
Running the /sbin/sh command creates a subshell
that runs as a separate process from the original
shell. Press Ctrl-d to exit the subshell and
return to the original shell. This restores the
environment of the original shell, including the
meanings of the current directory (.) and the root
directory (/). To run a command in another root
file system and save the output on the initial root
file system, enter a command similar to the following.
Note in this example, the file system is on
the /dev/disk/dsk13a device and is mounted to
/mnt/dsk13a:
chroot /mnt/dsk13a /bin/cc -E /u/bob/prog.c >
prep.out
The previous command runs the /bin/cc command with
/mnt/dsk13a as the specified root file system. It
compiles the /mnt/dsk13a/u/bob/prog.c file, reads
the #include files from the /mnt/dsk13a/usr/include
directory, and puts the compiled text in the
prep.out file on the initial root file system. To
create a file relative to the original root rather
than the new one, use this syntax and enter:
chroot directory command > file
Specifies the command path.
Commands: cc(1), cpp(1), ls(1), sh(1)
Functions: chdir(2), chroot(2) exec(2)
chroot(8)
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