Manual: unshare
UNSHARE(1) User Commands UNSHARE(1)
NAME
unshare - run program in new namespaces
SYNOPSIS
unshare [options] [program [arguments]]
DESCRIPTION
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).
OPTIONS
-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|subids|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|subids|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).
--owner uid:gid
Set the owner user and group when creating a user namespace. These determine which user in the parent namespace
has CAP_SYS_ADMIN in the new child namespace and can setns(2) into it. This option allows a privileged user to
create a namespace on behalf of an unprivileged one, using its privileges to map ids and/or bind mount the
namespace into the filesystem. It implies --user.
--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
Display version and exit.
NOTES
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.
EXAMPLES
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.
AUTHORS
Mikhail Gusarov , Karel Zak
SEE ALSO
newuidmap(1), newgidmap(1), clone(2), unshare(2), namespaces(7), mount(8)
REPORTING BUGS
For bug reports, use the issue tracker .
AVAILABILITY
The unshare command is part of the util-linux package which can be downloaded from Linux Kernel Archive
.
util-linux 2.41.3 2026-03-13 UNSHARE(1)