You can run a command with a different root folder with the chroot command. Let's try it out.
Listing dependencies
First, let's get the library dependencies of sh, echo, cat, and pwd:
$ ldd /bin/sh /bin/echo /bin/cat /bin/pwd
/bin/sh:
linux-vdso.so.1 (0x00007ffd8b764000)
libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f415d09f000)
/lib64/ld-linux-x86-64.so.2 (0x00007f415d2d4000)
/bin/echo:
linux-vdso.so.1 (0x00007ffd04383000)
libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f3120c73000)
/lib64/ld-linux-x86-64.so.2 (0x00007f3120e90000)
/bin/cat:
linux-vdso.so.1 (0x00007fffe1d7a000)
libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007fcf49a42000)
/lib64/ld-linux-x86-64.so.2 (0x00007fcf49c60000)
/bin/pwd:
linux-vdso.so.1 (0x00007fffdbfad000)
libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f60ec35c000)
/lib64/ld-linux-x86-64.so.2 (0x00007f60ec57a000)
Creating a new custom root
Now let's create a barebones root filesystem for those commands to work, with their respective libraries:
$ mkdir customroot/
$ mkdir customroot/bin/ customroot/lib/ customroot/lib64/
$ cp /bin/sh /bin/echo /bin/cat /bin/pwd customroot/bin/
$ cp /lib/x86_64-linux-gnu/libc.so.6 customroot/lib/
$ cp /lib64/ld-linux-x86-64.so.2 customroot/lib64/
Running a new shell on with the custom root
Now running our shell with customroot folder as the root:
$ sudo chroot customroot/ sh
# echo "Hello World!" > hello.txt
# cat hello.txt
Hello World!
# pwd
/
# ls
sh: 4: ls: not found
# exit
Notice pwd outputting / as expected, and ls not running since with didn't copy its binaries (and dependencies).
Creating a custom Debian root
Copying each binary file and it's dependencies as done above is cumbersome. What if we could generate a complete base root filesystem in an easier way? debootstrap to the rescue!
$ sudo apt install debootstrap
$ sudo debootstrap bionic customdebroot
Now let's run a new shell with our new root:
$ sudo chroot customdebroot/ bash
# pwd
/
# ls
bin boot dev etc home lib lib64 media mnt opt proc root run sbin srv sys tmp usr var
Let's install some package:
# apt install dict
# which dict
/usr/bin/dict
# dict nintendo
1 definition found
From The Free On-line Dictionary of Computing (30 December 2018) [foldoc]:
Nintendo
<company, games> A Japanese {video game} hardware manufacturer
and software publisher. Nintendo started by making playing
cards, but was later dominant in video games throughout the
1980s and early 1990s worldwide. They make lots of games
consoles including the Gameboy, Gameboy Advance SP, DS, DS
Lite and the Wii.
{Nintendo home (http://nintendo.com/)}.
(2008-03-08)
Back to our original shell with the 'real' root:
# exit
$ file customdebroot/usr/bin/dict
customdebroot/usr/bin/dict: ELF 64-bit LSB shared object, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux 2.6.32, BuildID[sha1]=1ae2d6542c13f51dcaaf6510a18e1b4bf49cf7c8, stripped
dict was only installed on our custom Debian root as expected.
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