How to check if a process is running inside docker container

[Updated1] I have a shell which will change TCP kernel parameters in some functions, but now I need to make this shell run in Docker container, that means, the shell need to know it is running inside a container and stop configuring the kernel.

Now I'm not sure how to achieve that, here is the contents of /proc/self/cgroup inside the container:

9:hugetlb:/
8:perf_event:/
7:blkio:/
6:freezer:/
5:devices:/
4:memory:/
3:cpuacct:/
2:cpu:/docker/25ef774c390558ad8c4e9a8590b6a1956231aae404d6a7aba4dde320ff569b8b
1:cpuset:/

Any flags above can I use to figure out if this process is running inside a container?

[Updated2]: I have also noticed Determining if a process runs inside lxc/Docker, but it seems not working in this case, the content in /proc/1/cgroup of my container is:

8:perf_event:/
7:blkio:/
6:freezer:/
5:devices:/
4:memory:/
3:cpuacct:/
2:cpu:/docker/25ef774c390558ad8c4e9a8590b6a1956231aae404d6a7aba4dde320ff569b8b
1:cpuset:/

No /lxc/containerid

Answers


To check inside a Docker container if you are inside a Docker container or not can be done via /proc/1/cgroup. As this post suggests you can to the following:

Outside a docker container all entries in /proc/1/cgroup end on / as you can see here:

vagrant@ubuntu-13:~$ cat /proc/1/cgroup
11:name=systemd:/
10:hugetlb:/
9:perf_event:/
8:blkio:/
7:freezer:/
6:devices:/
5:memory:/
4:cpuacct:/
3:cpu:/
2:cpuset:/

Inside a Docker container some of the control groups will belong to Docker (or LXC):

vagrant@ubuntu-13:~$ docker run busybox cat /proc/1/cgroup
11:name=systemd:/
10:hugetlb:/
9:perf_event:/
8:blkio:/
7:freezer:/
6:devices:/docker/3601745b3bd54d9780436faa5f0e4f72bb46231663bb99a6bb892764917832c2
5:memory:/
4:cpuacct:/
3:cpu:/docker/3601745b3bd54d9780436faa5f0e4f72bb46231663bb99a6bb892764917832c2
2:cpuset:/

Docker creates .dockerenv and .dockerinit (removed in v1.11) files at the top of the container's directory tree so you might want to check if those exist.

Something like this should work.

#!/bin/bash
if [ -f /.dockerenv ]; then
    echo "I'm inside matrix ;(";
else
    echo "I'm living in real world!";
fi

We use the proc's sched (/proc/$PID/sched) to extract the PID of the process. The process's PID inside the container will differ then it's PID on the host (a non-container system).

For example, the output of /proc/1/sched on a container will return:

root@33044d65037c:~# cat /proc/1/sched | head -n 1
bash (5276, #threads: 1)

While on a non-container host:

$ cat /proc/1/sched  | head -n 1
init (1, #threads: 1)

This helps to differentiate if you are in a container or not. eg you can do:

if [[ ! $(cat /proc/1/sched | head -n 1 | grep init) ]]; then {
    echo in docker
} else {
    echo not in docker
} fi

Thomas' solution as code:

running_in_docker() {
  (awk -F/ '$2 == "docker"' /proc/self/cgroup | read non_empty_input)
}

Note

The read with a dummy variable is a simple idiom for Does this produce any output?. It's a compact method for turning a possibly verbose grep or awk into a test of a pattern.

Additional note on read


We needed to exclude processes running in containers, but instead of checking for just docker cgroups we decided to compare /proc/<pid>/ns/pid to the init system at /proc/1/ns/pid. Example:

pid=$(ps ax | grep "[r]edis-server \*:6379" | awk '{print $1}')
if [ $(readlink "/proc/$pid/ns/pid") == $(readlink /proc/1/ns/pid) ]; then
   echo "pid $pid is the same namespace as init system"
else
   echo "pid $pid is in a different namespace as init system"
fi

Or in our case we wanted a one liner that generates an error if the process is NOT in a container

bash -c "test -h /proc/4129/ns/pid && test $(readlink /proc/4129/ns/pid) != $(readlink /proc/1/ns/pid)"

which we can execute from another process and if the exit code is zero then the specified PID is running in a different namespace.


What works for me is to check for the inode number of the '/.' Inside the docker, its a very high number. Outside the docker, its a very low number like '2'. I reckon this approach would also depend on the FileSystem being used.

Example

Inside the docker:

# ls -ali / | sed '2!d' |awk {'print $1'}
1565265

Outside the docker

$ ls -ali / | sed '2!d' |awk {'print $1'}
2

In a script:

#!/bin/bash
INODE_NUM=`ls -ali / | sed '2!d' |awk {'print $1'}`
if [ $INODE_NUM == '2' ];
then
        echo "Outside the docker"
else
        echo "Inside the docker"
fi

I've created a small python script. Hope someone finds it useful. :-)

#!/usr/bin/env python3
#@author Jorge III Altamirano Astorga 2018
import re
import math

total = None
meminfo = open('/proc/meminfo', 'r')
for line in meminfo:
    line = line.strip()
    if "MemTotal:" in line:
        line = re.sub("[^0-9]*", "", line)
        total = int(line)
meminfo.close()
print("Total memory: %d kB"%total)

procinfo = open('/proc/self/cgroup', 'r')
for line in procinfo: 
    line = line.strip()
    if re.match('.{1,5}:name=systemd:', line):
        dockerd = "/sys/fs/cgroup/memory" + \
            re.sub("^.{1,5}:name=systemd:", "", line) + \
            "/memory.stat"
        #print(dockerd)
        memstat = open(dockerd, 'r')
        for memline in memstat:
            memline = memline.strip()
            if re.match("hierarchical_memory_limit", memline):
                memline = re.sub("[^0-9]*", \
                    "", memline)  
                total = math.floor(int(memline) / 2**10)
        memstat.close()
procinfo.close()
print("Total available memory to the container: %d kB"%total)

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