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NAME | SYNOPSIS | DESCRIPTION | OPTIONS | NOTES | EXAMPLES | AUTHORS | SEE ALSO | REPORTING BUGS | AVAILABILITY |
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UNSHARE(1) User Commands UNSHARE(1)
unshare - run program in new namespaces
unshare [options] [program [arguments]]
The unshare command creates new namespaces (as specified by the
command-line options described below) and then executes the
specified program. If program is not given, then "${SHELL}" is run
(default: /bin/sh).
By default, a new namespace persists only as long as it has member
processes. A new namespace can be made persistent even when it has
no member processes by bind mounting /proc/pid/ns/type files to a
filesystem path. A namespace that has been made persistent in this
way can subsequently be entered with nsenter(1) even after the
program terminates (except PID namespaces where a permanently
running init process is required). Once a persistent namespace is
no longer needed, it can be unpersisted by using umount(8) to
remove the bind mount. See the EXAMPLES section for more details.
unshare since util-linux version 2.36 uses
/proc/[pid]/ns/pid_for_children and
/proc/[pid]/ns/time_for_children files for persistent PID and TIME
namespaces. This change requires Linux kernel 4.17 or newer.
The following types of namespaces can be created with unshare:
mount namespace
Mounting and unmounting filesystems will not affect the rest
of the system, except for filesystems which are explicitly
marked as shared (with mount --make-shared; see
/proc/self/mountinfo or findmnt -o+PROPAGATION for the shared
flags). For further details, see mount_namespaces(7).
unshare since util-linux version 2.27 automatically sets
propagation to private in a new mount namespace to make sure
that the new namespace is really unshared. It’s possible to
disable this feature with option --propagation unchanged. Note
that private is the kernel default.
UTS namespace
Setting hostname or domainname will not affect the rest of the
system. For further details, see uts_namespaces(7).
IPC namespace
The process will have an independent namespace for POSIX
message queues as well as System V message queues, semaphore
sets and shared memory segments. For further details, see
ipc_namespaces(7).
network namespace
The process will have independent IPv4 and IPv6 stacks, IP
routing tables, firewall rules, the /proc/net and
/sys/class/net directory trees, sockets, etc. For further
details, see network_namespaces(7).
PID namespace
Children will have a distinct set of PID-to-process mappings
from their parent. For further details, see pid_namespaces(7).
cgroup namespace
The process will have a virtualized view of /proc/self/cgroup,
and new cgroup mounts will be rooted at the namespace cgroup
root. For further details, see cgroup_namespaces(7).
user namespace
The process will have a distinct set of UIDs, GIDs and
capabilities. For further details, see user_namespaces(7).
time namespace
The process can have a distinct view of CLOCK_MONOTONIC and/or
CLOCK_BOOTTIME which can be changed using
/proc/self/timens_offsets. For further details, see
time_namespaces(7).
-i, --ipc[=file]
Create a new IPC namespace. If file is specified, then the
namespace is made persistent by creating a bind mount at file.
-m, --mount[=file]
Create a new mount namespace. If file is specified, then the
namespace is made persistent by creating a bind mount at file.
Note that file must be located on a mount whose propagation
type is not shared (or an error results). Use the command
findmnt -o+PROPAGATION when not sure about the current
setting. See also the examples below.
-n, --net[=file]
Create a new network namespace. If file is specified, then the
namespace is made persistent by creating a bind mount at file.
-p, --pid[=file]
Create a new PID namespace. If file is specified, then the
namespace is made persistent by creating a bind mount at file.
(Creation of a persistent PID namespace will fail if the
--fork option is not also specified.)
See also the --fork and --mount-proc options.
-u, --uts[=file]
Create a new UTS namespace. If file is specified, then the
namespace is made persistent by creating a bind mount at file.
-U, --user[=file]
Create a new user namespace. If file is specified, then the
namespace is made persistent by creating a bind mount at file.
-C, --cgroup[=file]
Create a new cgroup namespace. If file is specified, then the
namespace is made persistent by creating a bind mount at file.
-T, --time[=file]
Create a new time namespace. If file is specified, then the
namespace is made persistent by creating a bind mount at file.
The --monotonic and --boottime options can be used to specify
the corresponding offset in the time namespace.
-f, --fork
Fork the specified program as a child process of unshare
rather than running it directly. This is useful when creating
a new PID namespace. Note that when unshare is waiting for the
child process, then it ignores SIGINT and SIGTERM and does not
forward any signals to the child. It is necessary to send
signals to the child process.
--keep-caps
When the --user option is given, ensure that capabilities
granted in the user namespace are preserved in the child
process.
--kill-child[=signame]
When unshare terminates, have signame be sent to the forked
child process. Combined with --pid this allows for an easy and
reliable killing of the entire process tree below unshare. If
not given, signame defaults to SIGKILL. This option implies
--fork.
--mount-proc[=mountpoint]
Just before running the program, mount the proc filesystem at
mountpoint (default is /proc). This is useful when creating a
new PID namespace. It also implies creating a new mount
namespace since the /proc mount would otherwise mess up
existing programs on the system. The new proc filesystem is
explicitly mounted as private (with MS_PRIVATE|MS_REC).
--mount-binfmt[=mountpoint]
Just before running the program, mount the binfmt_misc
filesystem at mountpoint (default is
/proc/sys/fs/binfmt_misc). It also implies creating a new
mount namespace since the binfmt_misc mount would otherwise
mess up existing programs on the system. The new binfmt_misc
filesystem is explicitly mounted as private (with
MS_PRIVATE|MS_REC).
--map-user=uid|name
Run the program only after the current effective user ID has
been mapped to uid. If this option is specified multiple
times, the last occurrence takes precedence. This option
implies --user.
--map-users=inneruid:outeruid:count|auto|all
Run the program only after the block of user IDs of size count
beginning at outeruid has been mapped to the block of user IDs
beginning at inneruid. This mapping is created with
newuidmap(1) if unshare was run unprivileged. If the range of
user IDs overlaps with the mapping specified by --map-user,
then a "hole" will be removed from the mapping. This may
result in the highest user ID of the mapping not being mapped.
Use --map-users multiple times to map more than one block of
user IDs. The special value auto will map the first block of
user IDs owned by the effective user from /etc/subuid to a
block starting at user ID 0. The special value subids will
identity map the same block. The special value all will create
a pass-through map for every user ID available in the parent
namespace. This option implies --user.
Before util-linux version 2.39, this option expected a
comma-separated argument of the form outeruid,inneruid,count
but that format is now deprecated for consistency with the
ordering used in /proc/[pid]/uid_map and the X-mount.idmap
mount option.
--map-group=gid|name
Run the program only after the current effective group ID has
been mapped to gid. If this option is specified multiple
times, the last occurrence takes precedence. This option
implies --setgroups=deny and --user.
--map-groups=innergid:outergid:count|auto|all
Run the program only after the block of group IDs of size
count beginning at outergid has been mapped to the block of
group IDs beginning at innergid. This mapping is created with
newgidmap(1) if unshare was run unprivileged. If the range of
group IDs overlaps with the mapping specified by --map-group,
then a "hole" will be removed from the mapping. This may
result in the highest group ID of the mapping not being
mapped. Use --map-groups multiple times to map more than one
block of group IDs. The special value auto will map the first
block of user IDs owned by the effective user from /etc/subgid
to a block starting at group ID 0. The special value subids
will identity map the same block. The special value all will
create a pass-through map for every group ID available in the
parent namespace. This option implies --user.
Before util-linux version 2.39, this option expected a
comma-separated argument of the form outergid,innergid,count
but that format is now deprecated for consistency with the
ordering used in /proc/[pid]/gid_map and the X-mount.idmap
mount option.
--map-auto
Map the first block of user IDs owned by the effective user
from /etc/subuid to a block starting at user ID 0. In the same
manner, also map the first block of group IDs owned by the
effective group from /etc/subgid to a block starting at group
ID 0. This option is intended to handle the common case where
the first block of subordinate user and group IDs can map the
whole user and group ID space. This option is equivalent to
specifying --map-users=auto and --map-groups=auto.
--map-subids
Identity map the first block of user IDs owned by the
effective user from /etc/subuid. In the same manner, also
identity map the first block of group IDs owned by the
effective group from /etc/subgid. This option is equivalent to
specifying --map-users=subids and --map-groups=subids.
-r, --map-root-user
Run the program only after the current effective user and
group IDs have been mapped to the superuser UID and GID in the
newly created user namespace. This makes it possible to
conveniently gain capabilities needed to manage various
aspects of the newly created namespaces (such as configuring
interfaces in the network namespace or mounting filesystems in
the mount namespace) even when run unprivileged. As a mere
convenience feature, it does not support more sophisticated
use cases, such as mapping multiple ranges of UIDs and GIDs.
This option implies --setgroups=deny and --user. This option
is equivalent to --map-user=0 --map-group=0.
-c, --map-current-user
Run the program only after the current effective user and
group IDs have been mapped to the same UID and GID in the
newly created user namespace. This option implies
--setgroups=deny and --user. This option is equivalent to
--map-user=$(id -ru) --map-group=$(id -rg).
--propagation private|shared|slave|unchanged
Recursively set the mount propagation flag in the new mount
namespace. The default is to set the propagation to private.
It is possible to disable this feature with the argument
unchanged. The option is silently ignored when the mount
namespace (--mount) is not requested.
--setgroups allow|deny
Allow or deny the setgroups(2) system call in a user
namespace.
To be able to call setgroups(2), the calling process must at
least have CAP_SETGID. But since Linux 3.19 a further
restriction applies: the kernel gives permission to call
setgroups(2) only after the GID map (/proc/pid*/gid_map*) has
been set. The GID map is writable by root when setgroups(2) is
enabled (i.e., allow, the default), and the GID map becomes
writable by unprivileged processes when setgroups(2) is
permanently disabled (with deny).
-R, --root=dir
run the command with root directory set to dir.
-w, --wd=dir
change working directory to dir.
-S, --setuid uid
Set the user ID which will be used in the entered namespace.
-G, --setgid gid
Set the group ID which will be used in the entered namespace
and drop supplementary groups.
-l, --load-interp=string
Load binfmt_misc definition in the namespace (implies
--mount-binfmt). The string argument is
:name:type:offset:magic:mask:interpreter:flags. For more
details about new binary type registration see
https://www.kernel.org/doc/Documentation/admin-guide/binfmt-misc.rst.
To manage the F flag in flags with --root parameter,
binfmt_misc is mounted twice, once before the chroot to load
the interpreter from the caller filesystem and once after to
make it available from the chroot userspace.
--monotonic offset
Set the offset of CLOCK_MONOTONIC which will be used in the
entered time namespace. This option requires unsharing a time
namespace with --time.
--boottime offset
Set the offset of CLOCK_BOOTTIME which will be used in the
entered time namespace. This option requires unsharing a time
namespace with --time.
-h, --help
Display help text and exit.
-V, --version
Print version and exit.
The proc and sysfs filesystems mounting as root in a user
namespace have to be restricted so that a less privileged user
cannot get more access to sensitive files that a more privileged
user made unavailable. In short the rule for proc and sysfs is as
close to a bind mount as possible.
The following command creates a PID namespace, using --fork to
ensure that the executed command is performed in a child process
that (being the first process in the namespace) has PID 1. The
--mount-proc option ensures that a new mount namespace is also
simultaneously created and that a new proc(5) filesystem is
mounted that contains information corresponding to the new PID
namespace. When the readlink(1) command terminates, the new
namespaces are automatically torn down.
# unshare --fork --pid --mount-proc readlink /proc/self
1
As an unprivileged user, create a new user namespace where the
user’s credentials are mapped to the root IDs inside the
namespace:
$ id -u; id -g
1000
1000
$ unshare --user --map-root-user \
sh -c 'whoami; cat /proc/self/uid_map /proc/self/gid_map'
root
0 1000 1
0 1000 1
As an unprivileged user, create a user namespace where the first
65536 IDs are all mapped, and the user’s credentials are mapped to
the root IDs inside the namespace. The map is determined by the
subordinate IDs assigned in subuid(5) and subgid(5). Demonstrate
this mapping by creating a file with user ID 1 and group ID 1. For
brevity, only the user ID mappings are shown:
$ id -u
1000
$ cat /etc/subuid
1000:100000:65536
$ unshare --user --map-auto --map-root-user
# id -u
0
# cat /proc/self/uid_map
0 1000 1
1 100000 65535
# touch file; chown 1:1 file
# ls -ln --time-style=+ file
-rw-r--r-- 1 1 1 0 file
# exit
$ ls -ln --time-style=+ file
-rw-r--r-- 1 100000 100000 0 file
The first of the following commands creates a new persistent UTS
namespace and modifies the hostname as seen in that namespace. The
namespace is then entered with nsenter(1) in order to display the
modified hostname; this step demonstrates that the UTS namespace
continues to exist even though the namespace had no member
processes after the unshare command terminated. The namespace is
then destroyed by removing the bind mount.
# touch /root/uts-ns
# unshare --uts=/root/uts-ns hostname FOO
# nsenter --uts=/root/uts-ns hostname
FOO
# umount /root/uts-ns
The following commands establish a persistent mount namespace
referenced by the bind mount /root/namespaces/mnt. In order to
ensure that the creation of that bind mount succeeds, the parent
directory (/root/namespaces) is made a bind mount whose
propagation type is not shared.
# mount --bind /root/namespaces /root/namespaces
# mount --make-private /root/namespaces
# touch /root/namespaces/mnt
# unshare --mount=/root/namespaces/mnt
The following commands demonstrate the use of the --kill-child
option when creating a PID namespace, in order to ensure that when
unshare is killed, all of the processes within the PID namespace
are killed.
# set +m # Don't print job status messages
# unshare --pid --fork --mount-proc --kill-child -- \
bash --norc -c '(sleep 555 &) && (ps a &) && sleep 999' &
[1] 53456
# PID TTY STAT TIME COMMAND
1 pts/3 S+ 0:00 sleep 999
3 pts/3 S+ 0:00 sleep 555
5 pts/3 R+ 0:00 ps a
# ps h -o 'comm' $! # Show that background job is unshare(1)
unshare
# kill $! # Kill unshare(1)
# pidof sleep
The pidof(1) command prints no output, because the sleep processes
have been killed. More precisely, when the sleep process that has
PID 1 in the namespace (i.e., the namespace’s init process) was
killed, this caused all other processes in the namespace to be
killed. By contrast, a similar series of commands where the
--kill-child option is not used shows that when unshare
terminates, the processes in the PID namespace are not killed:
# unshare --pid --fork --mount-proc -- \
bash --norc -c '(sleep 555 &) && (ps a &) && sleep 999' &
[1] 53479
# PID TTY STAT TIME COMMAND
1 pts/3 S+ 0:00 sleep 999
3 pts/3 S+ 0:00 sleep 555
5 pts/3 R+ 0:00 ps a
# kill $!
# pidof sleep
53482 53480
The following example demonstrates the creation of a time
namespace where the boottime clock is set to a point several years
in the past:
# uptime -p # Show uptime in initial time namespace
up 21 hours, 30 minutes
# unshare --time --fork --boottime 300000000 uptime -p
up 9 years, 28 weeks, 1 day, 2 hours, 50 minutes
The following example execute a chroot into the directory
/chroot/powerpc/jessie and install the interpreter
/bin/qemu-ppc-static to execute the powerpc binaries.
$ unshare --map-root-user --fork --pid --load-interp=":qemu-ppc:M::\\x7fELF\x01\\x02\\x01\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x02\\x00\\x14:\\xff\\xff\\xff\\xff\\xff\\xff\\xff\\x00\\xff\\xff\\xff\\xff\\xff\\xff\\xff\\xff\\xff\\xfe\\xff\\xff:/bin/qemu-ppc-static:OCF" --root=/chroot/powerpc/jessie /bin/bash -l
The load-interp parameter can be read as following
qemu-ppc
is the name of the new file created below
/proc/sys/fs/binfmt_misc to register the interpreter
M
defines the interpreter for a given type of magic number
\\x7fELF\x01\\x02\\x01\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x02\\x00\\x1
is the magic number to recognize the file to interpret (in
this case, the ELF header for PPC32)
\\xff\\xff\\xff\\xff\\xff\\xff\\xff\\x00\\xff\\xff\\xff\\xff\\xff\\xff\\xff\\xff\\xff\\xfe\\xff\\xff
the mask to apply to the magic number
/bin/qemu-ppc-static
the interpreter to use with the file
OCF
the file is open by the kernel with credential and
security tokens of the file itself and loaded as soon as
we register it.
Mikhail Gusarov <dottedmag@dottedmag.net>, Karel Zak
<kzak@redhat.com>
newuidmap(1), newgidmap(1), clone(2), unshare(2), namespaces(7),
mount(8)
For bug reports, use the issue tracker
<https://github.com/util-linux/util-linux/issues>.
The unshare command is part of the util-linux package which can be
downloaded from Linux Kernel Archive
<https://www.kernel.org/pub/linux/utils/util-linux/>. This page is
part of the util-linux (a random collection of Linux utilities)
project. Information about the project can be found at
⟨https://www.kernel.org/pub/linux/utils/util-linux/⟩. If you have a
bug report for this manual page, send it to
util-linux@vger.kernel.org. This page was obtained from the
project's upstream Git repository
⟨git://git.kernel.org/pub/scm/utils/util-linux/util-linux.git⟩ on
2025-02-02. (At that time, the date of the most recent commit that
was found in the repository was 2025-01-30.) If you discover any
rendering problems in this HTML version of the page, or you
believe there is a better or more up-to-date source for the page,
or you have corrections or improvements to the information in this
COLOPHON (which is not part of the original manual page), send a
mail to man-pages@man7.org
util-linux 2.41.devel-938-0a... 2025-01-15 UNSHARE(1)
Pages that refer to this page: unshare(2), cgroup_namespaces(7), ipc_namespaces(7), mount_namespaces(7), namespaces(7), network_namespaces(7), time_namespaces(7), uts_namespaces(7), findmnt(8), lsns(8)
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HTML rendering created 2025-02-02 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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