Learn what happens when a container is stopped.
Inside a container, we can run commands just like any other Linux prompt. We can also edit files. Let’s get back to a command prompt inside an Ubuntu container and create a text file.
$ docker run -it --name lab_03_container_01 ubuntu:latest /bin/bash
root@c267f778d237:/# echo "Look, I can write to a file!" > foo.txt
root@c267f778d237:/# ls
bin boot dev etc foo.txt home lib lib64 media mnt opt proc root run sbin srv sys tmp usr var
root@c267f778d237:/# cat foo.txt
Look, I can write to a file!
root@c267f778d237:/# exit
exit
We were able to write to the filesystem inside the container. Cool. Let’s go back and look at that again.
$ docker run -it --name lab_03_container_02 ubuntu:latest /bin/bash
root@cd9f034e05e6:/# cat foo.txt
cat: foo.txt: No such file or directory
root@cd9f034e05e6:/# ls
bin boot dev etc home lib lib64 media mnt opt proc root run sbin srv sys tmp usr var
root@cd9f034e05e6:/# exit
exit
Huh? What happened to our file? It isn’t there anymore. This is happening
because we are in a different container than we were the first time. When we ran
the same docker run command, it started a new container from the same Ubuntu
image. The filesystem inside the container is completely fresh - nothing is
shared with the first container we ran.
Can we get our file back? As a matter of fact, yes. We can get a list of all the
containers on our machine (including the stopped ones) with docker ps -a.
$ docker ps -a
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
cd9f034e05e6 ubuntu:latest "/bin/bash" 5 minutes ago Exited (0) 4 minutes ago lab_03_container_02
ebde4b281dec ubuntu:latest "/bin/bash" 10 minutes ago Exited (0) 5 minutes ago lab_03_container_01
We can see our stopped container there. Let’s get back to bash in the container where we made our file. You can do this with the container’s name or ID. We named our containers earlier in this lab for convenience and easy identification, so we’ll use that now to restart the stopped container.
$ docker start -i lab_03_container_01
root@ebde4b281dec:/# cat foo.txt
Look, I can write to a file!
root@ebde4b281dec:/# ls
bin boot dev etc foo.txt home lib lib64 media mnt opt proc root run sbin srv sys tmp usr var
root@ebde4b281dec:/# exit
exit
Because we restarted our stopped container instead of running a new container,
our changes to the filesystem were present. We can see the contents of the
foo.txt file that we made. With docker start, we can re-start a stopped
container. This can be useful for things like debugging, but is not a normal
part of the Docker workflow, so don’t get used to using it too often.
Feel free to play around with the things we’ve learned so far to get comfortable with them. When you’re ready, move on to the next lab.