Docker - var/lib/docker/layers folder content

Question

What does exactly var/lib/docker/layers contains? I found, that:

Infomation about docker layer

I have checked out and in it is a file for every container, which contains several characters or empty. I haven't found any logic in content.

Answer 1

If you are referring to /var/lib/docker/aufs/layers . This contains the layers for the images created on your system with docker pull or build .

ex: I have the following image built on my host.

$ docker images 
REPOSITORY          TAG                 IMAGE ID            CREATED             VIRTUAL SIZE
devbox              1.0                 d17a89ba7b49        46 hours ago        717.5 MB

Now find the layers associated with image:

ls /var/lib/docker/aufs/layers -lh | grep d17a89ba7b49
-rw-r--r-- 1 root root 2.6K Oct  7 14:43 d17a89ba7b49d534c0e9f4d24ddf70f490f16b2548e874961a7ef42a93376f2d


cat /var/lib/docker/aufs/layers/d17a89ba7b49d534c0e9f4d24ddf70f490f16b2548e874961a7ef42a93376f2d

    86976168b2da85a2804f08f32529a70ef3c55659943466f7231e10aab57529bb
    664221c455e9e412e95d5505da4139e568043decb87e13cd15198038f554e978
    629e4b1afddb681969bf233b339311a856f3d14f0afb8c13ad70ea0fb7aaccf3
    a1a2a7a152e0002824558fb411b9214715de752cae906cc83732555c65899292
    1e36bd2a92516294f2607dfe49c68a600ca5ca9ca0ab64d65655afd9bac03432
    e52b2f57801a3c8a3de5e6431de32aa965c1ddd2aad56480acdf0929e10cb38f
    412b8dd613c482c70e55267ddb4f98e095be0e263d5aead8950857867b59c97e
    67e97b8f862b6b7fea1aff647161f6885aa3d207451a60c69737152b70742901
    773120c89b700d15afab5a3104ecb26edb4cdb72ecdd6424d5a0680a3f205c02
    ...
    ...

Note: there are other commands like docker history with are better understandable output.

Note: Its best to understand the layers using tools available rather than jumping into the FS. If the built images are pushed on to docker hub , you can get a pictorial view the image layers along with finer details on each layers, including the command that was run to create the layer and the size of the layer added etc.