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 $file = ls /tmp/dir
You cannot save a variable for later use in other Dockerfile commands (if that is your intention). This is because each RUN happens in a new shell.
However, if you just want to capture the output of ls you should be able to do it in one RUN compound command. For example:
RUN file="$(ls -1 /tmp/dir)" && echo $file
Or just using the subshell inline:
RUN echo $(ls -1 /tmp/dir)
If you have an actual error or problem to solve I could expand on this instead of a hypothetical answer.
A full example Dockerfile demonstrating this would be:
FROM alpine:3.7
RUN mkdir -p /tmp/dir && touch /tmp/dir/file1 /tmp//dir/file2
RUN file="$(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). The option for --progress plain forces the output to show the steps in later version of Docker:
$ docker build --no-cache --progress plain -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 $(ls -1 /tmp/dir)
---> Running in 528fc5d6c721
file1 file2
Removing intermediate container 528fc5d6c721
---> 1be7c54e1f29
Successfully built 1be7c54e1f29
Successfully tagged test:latest
Just highlight the answer given in the comments, which is probably the correct one if you are using a modern version of Docker (in my case v20.10.5) and the logs do not show the expected output, when, for example, you run RUN ls.
You should use the option --progress <string> in the docker build command:
--progress string Set type of progress output (auto, plain, tty). Use plain to show container output
(default "auto")
For example:
docker build --progress=plain .
In the latest versions of docker, the classic build engine that docker ships with has been upgraded to Buildkit, which displays different information.
You should see output like:
#12 [8/8] RUN ls -alh
#12 sha256:a8cf7b9a7b1f3dc25e3a97700d4cc3d3794862437a5fe2e39683ab229474746c
#12 0.174 total 184K
#12 0.174 drwxr-xr-x 1 root root 4.0K Mar 28 19:37 .
#12 0.174 drwxr-xr-x 1 root root 4.0K Mar 28 19:35 ..
#12 0.174 drwxr-xr-x 374 root root 12.0K Mar 28 19:37 node_modules
#12 0.174 -rw-r--r-- 1 root root 1.1K Mar 28 19:36 package.json
#12 0.174 -rw-r--r-- 1 root root 614 Mar 28 15:48 server.js
#12 0.174 -rw-r--r-- 1 root root 149.5K Mar 28 16:54 yarn.lock
#12 DONE 0.2s
Related question with more details.
docker compose alternative to:
docker build --progress=plain .
is
BUILDKIT_PROGRESS=plain docker compose build
or in compose.yml file
services:
build:
progress: plain
I couldn't get Andy's (or any other) approach to work in the Dockerfile itself, so I set my Dockerfile entrypoint to a bash file containing:
#!/bin/bash
file="$(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.
Another option is to save result in a temporary file and use it in a consequent RUN statement or even copy it to another step, with COPY --from command:
Here is an example:
FROM alpine:3.17.0
WORKDIR /output
# here we can store result of any command
# or even store multiple variables in a same file
RUN echo "2 + 3" >> /output/temp_file
# will print 5, almost anything can be retrieved from a
# text file in bash and be used in following commands
RUN echo $(( $(cat /output/temp_file) ))
CMD tail -f /dev/null
