This blog shows you how you can copy stuff from your host machine to the running container without the docker cp command that we usually use.

Steps in text

Here I have a script on the host, which looks following:

#!/bin/bash

tput bold
echo "OS Information:"
tput sgr0
echo

cat /etc/os-release

After running which looks like following:

$ ls
script.sh

$ ./script.sh
OS Information:

NAME="Flatcar Linux by Kinvolk"
ID=flatcar
ID_LIKE=coreos
VERSION=2079.6.0
VERSION_ID=2079.6.0
BUILD_ID=2019-06-18-0855
PRETTY_NAME="Flatcar Linux by Kinvolk 2079.6.0 (Rhyolite)"
ANSI_COLOR="38;5;75"
HOME_URL="https://flatcar-linux.org/"
BUG_REPORT_URL="https://issues.flatcar-linux.org"
FLATCAR_BOARD="amd64-usr"

And here is the running container in another tab to which I want to copy the file.

$ docker run -it fedora bash
[root@0d6d865626ff /]#

Note: All the console with shell prompt [root@0d6d865626ff /]# means that the command is run inside the container.

I can always run the docker cp command like following to copy file:

$ docker ps
CONTAINER ID        IMAGE                                     COMMAND                  CREATED              STATUS              PORTS               NAMES
0d6d865626ff        fedora                                    "bash"                   About a minute ago   Up About a minute                       hardcore_roentgen
$ docker cp script.sh hardcore_roentgen:/

Now if I check it from the container:

[root@0d6d865626ff /]# ls / | grep script
script.sh

Let’s clean up the file and use another easier and container runtime agnostic method to do the same.

[root@0d6d865626ff /]# rm script.sh
rm: remove regular file 'script.sh'? y
[root@0d6d865626ff /]# ls /
bin  boot  dev  etc  home  lib  lib64  lost+found  media  mnt  opt  proc  root  run  sbin  srv  sys  tmp  usr  var

To figure out the file system root of the container process let’s start a sleep process.

[root@0d6d865626ff /]# sleep 3000

Now the sleep process will help us identify the PID of this process on the host.

$ ps aufx | grep sleep
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root      1668  0.0  0.0   5252   692 pts/0    S+   16:47   0:00          \_ sleep 3000

Since we figured out the PID of the process to be 1668, now let’s copy the file to the root of the container by running following command. Down here I have used the PID that I got output of above, replace it with the PID you get for your process.

$ sudo cp -v ./script.sh /proc/1668/root/
'./script.sh' -> '/proc/1668/root/script.sh'

Now we can stop the sleep process we started earlier in the container see in the root that the file has been copied successfully.

[root@0d6d865626ff /]# sleep 3000
^C
[root@0d6d865626ff /]# ls / | grep script
script.sh

Video of above steps

I learnt this trick while pair coding/learning things about container security with my colleague Alban, thanks Alban!.