Dockerfile: Output of RUN instruction into a Variable

Question

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\

Answer 1

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.

Answer 2

I've used this specifically for printing colors to console. If your container has python, you can do:

python -c 'print(" \033[31m RED TEXT \033[0m ")'
python -c 'print(" \033[32m GREEN TEXT \033[0m ")'
python -c 'print(" \033[34m BLUE TEXT \033[0m ")'

Answer 3

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