Why there is no nsenter in util-linux?

Update:

As of 14.10, the util-linux provides the nsenter command. The solution below has been tested with 14.04.

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The Debian/Ubuntu version is as you said quite old now, even in Trusty.

There's an opened bug and so far no progress unfortunately.

You could try to build it from source:

wget https://www.kernel.org/pub/linux/utils/util-linux/v2.24/util-linux-2.24.1.tar.gz -qO - | tar -xz -C ~/Downloads

Make sure to install the following build dependencies:

sudo apt-get install libncurses5-dev libslang2-dev gettext zlib1g-dev libselinux1-dev debhelper lsb-release pkg-config po-debconf autoconf automake autopoint libtool

And just run in the source directory (~/Downloads/util-linux-2.24.1):

./autogen.sh

./configure && make

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IMPORTANT

Do NOT sudo make install this package on Ubuntu 14.04 LTS until it is officially ready for use, as it definitely demands an unavailable version of libmount, breaking your boot. (If you do this, reinstall the mount package before rebooting your machine, if you can.)

_{Credits: Trevor Alexander for his comment.}

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Finally you'll get:

sylvain@sylvain-ThinkPad-T430s:~/Downloads/util-linux-2.24.1$ ./nsenter -V
nsenter from util-linux 2.24.1

Note: as nsenter is not available in the ubuntu util-linux version, you can install just this file in /usr/bin (or sbin):

sudo cp ./nsenter /usr/bin

If you use docker you can install nsenter in a container and then copy the nsenter command to the host.

From my gist: https://gist.github.com/mbn18/0d6ff5cb217c36419661

# Ubuntu 14.04 don't have nsenter - the straight forward way required me to install build tools and etc.
# I preferred to keep the system clean and install nsenter in a container and then copy the command to the host
# Note - its also possible to run nsenter from a container (didn't tried) https://github.com/jpetazzo/nsenter

# start a container
docker run --name nsenter -it ubuntu:14.04 bash

## in the docker
apt-get update
apt-get install git build-essential libncurses5-dev libslang2-dev gettext zlib1g-dev libselinux1-dev debhelper lsb-release pkg-config po-debconf autoconf automake autopoint libtool

git clone git://git.kernel.org/pub/scm/utils/util-linux/util-linux.git util-linux
cd util-linux/

./autogen.sh
./configure --without-python --disable-all-programs --enable-nsenter
make

## from different shell - on the host
docker cp nsenter:/util-linux/nsenter /usr/local/bin/
docker cp nsenter:/util-linux/bash-completion/nsenter /etc/bash_completion.d/nsenter