Docker How To Tell If You Are In A Container

Better way to check if you are running inside a docker container:

ps -p 1

See the explaination for this further down.

OUTDATED Method: The generally recommended way to check if you are running inside a docker container is to use this commmand:

cat /proc/self/cgroup

I the past this used to give pretty obvious output showing that you are in a docker container. It doesn’t seem to be as clear anymore so this might be a bit trickier.

This is what the output looks like on my Ubuntu 22.04.2 system:

-bash-5.1# cat /proc/self/cgroup
0::/user.slice/user-1000.slice/user@1000.service/app.slice/app-org.gnome.Terminal.slice/vte-spawn-c4d90668-9745-4d14-99b0-0b8f17bbee84.scope
-bash-5.1# 

This is what it looks like inside an Docker container with the latest ubuntu image on that same host:

root@50c79e17fabd:/# cat /proc/self/cgroup
0::/
root@50c79e17fabd:/# 

Another way to tell would be to just check the number of processes running. Technically, a docker container can contain any number of processes but in practice they usually only have a few ( ex 2 or 3 processes ).

On a normal physical or virtual host you would generally see more than 100 processes running at a time. For example on my Ubuntu desktop I saw 354 processes when I checked:

-bash-5.1# ps -ef | wc -l
354
-bash-5.1# 

In a docker container I only saw two processes running.

root@50c79e17fabd:/# ps -ef
UID          PID    PPID  C STIME TTY          TIME CMD
root           1       0  0 00:20 pts/0    00:00:00 /bin/bash
root          29       1  0 01:34 pts/0    00:00:00 ps -ef
root@50c79e17fabd:/# 

Also note that the process with PID 1 is “/bin/bash”. For a docker container PID 1 will generally be whatever the main command for the image/container was. On a physical/virtual host PID 1 would usually be an init system like systemd.

You can quickly check the processe running with PID 1 using the following two examples:

Physical / Virtual:

-bash-5.1# ps -p 1
    PID TTY          TIME CMD
      1 ?        00:00:07 systemd
-bash-5.1# 

Inside Docker container:

root@50c79e17fabd:/# ps -p 1
    PID TTY          TIME CMD
      1 pts/0    00:00:00 bash
root@50c79e17fabd:/#