Dockerfile: Output of RUN instruction into a Variable

I am writing a dockerfile and want to put the output of the "ls" command into a variable as shown below:

$file = ls /tmp/dir

Here, "dir" only has one file inside it.

The following RUN instruction within a dockerfile is not working

RUN $file = ls /tmp/dir

Solution 1:

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

Solution 2:

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.

Solution 3:

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")

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

Related question with more details.