How can I inspect the file system of a failed docker build

Question

I m trying to build a new Docker image for our development process  using cpanm to install a bunch of Perl modules as a base image for various projects   While developing the Dockerfile  cpanm returns a failure code because some of the modules did not install cleanly   I m fairly sure I need to get apt to install some more things   My question is  where can I find the   cpanm work directory quoted in the output  in order to inspect the logs  In the general case  how can I inspect the file system of a failed docker build command   Morning edit After biting the bullet and running a find I discovered   var lib docker aufs diff 3afa404e       cpanm   Is this reliable  or am I better off building a  bare  container and running stuff manually until I have all the things I need

User · Answer

What I would do is comment out the Dockerfile below and including the offending line. Then you can run the container and run the docker commands by hand, and look at the logs in the usual way. E.g. if the Dockerfile is

RUN foo
RUN bar
RUN baz

and it's dying at bar I would do

RUN foo
# RUN bar
# RUN baz

Then

$ docker build -t foo .
$ docker run -it foo bash
container# bar
...grep logs...

User · Answer

The top answer works in the case that you want to examine the state immediately prior to the failed command   However  the question asks how to examine the state of the failed container itself   In my situation  the failed command is a build that takes several hours  so rewinding prior to the failed command and running it again takes a long time and is not very helpful   The solution here is to find the container that failed     docker ps -a CONTAINER ID        IMAGE               COMMAND                  CREATED             STATUS                          PORTS               NAMES 6934ada98de6        42e0228751b3          bin sh -c    utils     24 minutes ago      Exited  1  About a minute ago                       sleepy bell   Commit it to an image     docker commit 6934ada98de6 sha256 7015687976a478e0e94b60fa496d319cdf4ec847bcd612aecf869a72336e6b83   And then run the image  if necessary  running bash      docker run -it 7015687976a4  bash -il    Now you are actually looking at the state of the build at the time that it failed  instead of at the time before running the command that caused the failure

User · Answer

Docker caches the entire filesystem state after each successful RUN line   Knowing that    to examine the latest state before your failing RUN command  comment it out in the Dockerfile  as well as any and all subsequent RUN commands   then run docker build and docker run again  to examine the state after the failing RUN command  simply add     true to it to force it to succeed  then proceed like above  keep any and all subsequent RUN commands commented out  run docker build and docker run    Tada  no need to mess with Docker internals or layer IDs  and as a bonus Docker automatically minimizes the amount of work that needs to be re-done

User · Answer

Everytime docker successfully executes a RUN command from a Dockerfile  a new layer in the image filesystem is committed  Conveniently you can use those layers ids as images to start a new container   Take the following Dockerfile   FROM busybox RUN echo  foo   gt   tmp foo txt RUN echo  bar   gt  gt   tmp foo txt   and build it     docker build -t so-2622957   Sending build context to Docker daemon 47 62 kB Step 1 3   FROM busybox  --- gt  00f017a8c2a6 Step 2 3   RUN echo  foo   gt   tmp foo txt  --- gt  Running in 4dbd01ebf27f  --- gt  044e1532c690 Removing intermediate container 4dbd01ebf27f Step 3 3   RUN echo  bar   gt  gt   tmp foo txt  --- gt  Running in 74d81cb9d2b1  --- gt  5bd8172529c1 Removing intermediate container 74d81cb9d2b1 Successfully built 5bd8172529c1   You can now start a new container from 00f017a8c2a6  044e1532c690 and 5bd8172529c1     docker run --rm 00f017a8c2a6 cat  tmp foo txt cat   tmp foo txt  No such file or directory    docker run --rm 044e1532c690 cat  tmp foo txt foo    docker run --rm 5bd8172529c1 cat  tmp foo txt foo bar   of course you might want to start a shell to explore the filesystem and try out commands     docker run --rm -it 044e1532c690 sh           ls -l  tmp total 4 -rw-r--r--    1 root     root             4 Mar  9 19 09 foo txt     cat  tmp foo txt  foo     When one of the Dockerfile command fails  what you need to do is to look for the id of the preceding layer and run a shell in a container created from that id   docker run --rm -it  lt id last working layer gt  bash -il   Once in the container    try the command that failed  and reproduce the issue then fix the command and test it finally update your Dockerfile with the fixed command     If you really need to experiment in the actual layer that failed instead of working from the last working layer  see Drew s answer

User · Answer

Debugging build step failures is indeed very annoying   The best solution I have found is to make sure that each step that does real work succeeds  and adding a check after those that fails  That way you get a committed layer that contains the outputs of the failed step that you can inspect   A Dockerfile  with an example after the   Run  DB2 silent installer line       DB2 10 5 Client Dockerfile  Part 1      Requires     - DB2 10 5 Client for 64bit Linux ibm data server runtime client linuxx64 v10 5 tar gz     - Response file for DB2 10 5 Client for 64bit Linux db2rtcl nr rsp        Using Ubuntu 14 04 base image as the starting point  FROM ubuntu 14 04  MAINTAINER David Carew  lt carew us ibm com gt     DB2 prereqs  also installing sharutils package as we use the utility uuencode to generate password - all others are required for the DB2 Client   RUN dpkg --add-architecture i386  amp  amp  apt-get update  amp  amp  apt-get install -y sharutils binutils libstdc  6 i386 libpam0g i386  amp  amp  ln -s  lib i386-linux-gnu libpam so 0  lib libpam so 0 RUN apt-get install -y libxml2     Create user db2clnt   Generate strong random password and allow sudo to root w o password   RUN       adduser --quiet --disabled-password -shell  bin bash -home  home db2clnt --gecos  DB2 Client  db2clnt  amp  amp       echo db2clnt  dd if  dev urandom bs 16 count 1 2 gt  dev null   uuencode -  head -n 2   grep -v begin   cut -b 2-10    chgpasswd  amp  amp       adduser db2clnt sudo  amp  amp       echo   sudo ALL  ALL  NOPASSWD ALL   gt  gt   etc sudoers    Install DB2 RUN mkdir  install   Copy DB2 tarball - ADD command will expand it automatically ADD v10 5fp9 linuxx64 rtcl tar gz  install    Copy response file COPY  db2rtcl nr rsp  install    Run  DB2 silent installer RUN mkdir  logs RUN   install rtcl db2setup -t  logs trace -l  logs log -u  install db2rtcl nr rsp  amp  amp  touch  install done      bin true RUN test -f  install done     echo ERROR-------  echo install failed  see files in container  logs directory of the last container layer  echo run docker run   lt last image id gt    bin cat  logs trace  echo ----------  RUN test -f  install done    Clean up unwanted files RUN rm -fr  install rtcl    Login as db2clnt user CMD su - db2clnt

[debugging] How can I inspect the file system of a failed `docker build`?

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