Dockerfile: Output of RUN instruction into a Variable

I couldn't get Andy's (or any other) approach to work in the Dockerfile itself, so I set my Dockerfile entrypoint to a bash file containing:

#!/bin/bash
file="$(conda list --explicit)" && echo $file
echo $(conda list --explicit)

Note the second method doesn't render line breaks, so I found the first method - echo via the $file variable - superior.

You cannot save a variable for later use in other Dockerfile commands (if that is your intention). This is because each RUN happens in a new shell.

However, if you just want to capture the output of ls you should be able to do it in one RUN compound command. For example:

RUN file="$(ls -1 /tmp/dir)" && echo $file

Or just using the subshell inline:

RUN echo $(ls -1 /tmp/dir)

Hope this helps your understanding. If you have an actual error or problem to solve I could expand on this instead of a hypothetical answer.

A full example Dockerfile demonstrating this would be:

FROM alpine:3.7
RUN mkdir -p /tmp/dir && touch /tmp/dir/file1 /tmp//dir/file2
RUN file="$(ls -1 /tmp/dir)" && echo $file
RUN echo $(ls -1 /tmp/dir)

When building you should see steps 3 and 4 output the variable (which contains the list of file1 and file2 creating in step 2):

$ docker build --no-cache -t test .
Sending build context to Docker daemon  2.048kB
Step 1/4 : FROM alpine:3.7
 ---> 3fd9065eaf02
Step 2/4 : RUN mkdir -p /tmp/dir && touch /tmp/dir/file1 /tmp//dir/file2
 ---> Running in abb2fe683e82
Removing intermediate container abb2fe683e82
 ---> 2f6dfca9385c
Step 3/4 : RUN file="$(ls -1 /tmp/dir)" && echo $file
 ---> Running in 060a285e3d8a
file1 file2
Removing intermediate container 060a285e3d8a
 ---> 2e4cc2873b8c
Step 4/4 : RUN echo $(ls -1 /tmp/dir)
 ---> Running in 528fc5d6c721
file1 file2
Removing intermediate container 528fc5d6c721
 ---> 1be7c54e1f29
Successfully built 1be7c54e1f29
Successfully tagged test:latest

Just highlight the answer given in the comments, which is probably the correct one if you are using a modern version of Docker (in my case v20.10.5) and the logs do not show the expected output, when, for example, you run RUN ls.

You should use the option --progress string in the docker build command:

 --progress string         Set type of progress output (auto, plain, tty). Use plain to show container output
                            (default "auto")

For example:

docker build --progress=plain .

In the latest versions of docker, the classic build engine that docker ships with has been upgraded to Builtkit, which displays different information.

You should see output like:

#12 [8/8] RUN ls -alh
#12 sha256:a8cf7b9a7b1f3dc25e3a97700d4cc3d3794862437a5fe2e39683ab229474746c
#12 0.174 total 184K
#12 0.174 drwxr-xr-x    1 root     root        4.0K Mar 28 19:37 .
#12 0.174 drwxr-xr-x    1 root     root        4.0K Mar 28 19:35 ..
#12 0.174 drwxr-xr-x  374 root     root       12.0K Mar 28 19:37 node_modules
#12 0.174 -rw-r--r--    1 root     root        1.1K Mar 28 19:36 package.json
#12 0.174 -rw-r--r--    1 root     root         614 Mar 28 15:48 server.js
#12 0.174 -rw-r--r--    1 root     root      149.5K Mar 28 16:54 yarn.lock
#12 DONE 0.2s

Related question with more details.