Almost every IT department has one; a Perl script written decades ago by a long-gone employee that’s critical to production. It does it’s job well, but no one wants to touch or take responsibility for it. You just want to upgrade your infrastructure and bring it along, but there are so many CPAN modules and inconsitencies you want never want to look at it again. What to do?
Most articles on containers focus on running a service, exposing ports and data volumes, but one feature frequently overlooked is the ability to run a container as an executable.
In Docker, the ENTRYPOINT instruction makes this possible by running a command and accpeting command line arguments from
docker run. Combining this with
--rm will run an interactive emphermal container, disappearing when the command is complete.
The following Multi-Stage Dockerfile will build a container to run a simple perl script,
ps.plscript as ENTRYPOINT
-has default argument
Here is the Dockerfile in full,
#This Dockerfile uses a Multi-Stage Build: https://docs.docker.com/develop/develop-images/multistage-build/ FROM debian:stable-slim AS base LABEL maintainer="Micheal Waltz <[email protected]>" # Environment ENV DEBIAN_FRONTEND=noninteractive \ LANG=en_US.UTF-8 \ LC_ALL=C.UTF-8 \ LANGUAGE=en_US.UTF-8 # Install runtime packages RUN apt-get update \ && apt-get install -y \ perl # Set app dir WORKDIR /app # Intermediate build layer FROM base AS build #Update system and install packages RUN apt-get update \ && apt-get install -yq \ build-essential \ cpanminus # Install cpan modules RUN cpanm Proc::ProcessTable Data::Dumper # Runtime layer FROM base AS run # Copy build artifacts from build layer COPY --from=build /usr/local /usr/local # Copy perl script COPY ./ps.pl . # Set Entrypoint ENTRYPOINT [ "/app/ps.pl" ]
FROM debian:stable-slim AS base LABEL maintainer="Micheal Waltz <[email protected]>"
Basic environment variables for Debian packages through
apt non-interactive and set locale to UTF-8,
ENV DEBIAN_FRONTEND=noninteractive \ LANG=en_US.UTF-8 \ LC_ALL=C.UTF-8 \ LANGUAGE=en_US.UTF-8
Install “runtime” packages for the
base layer that other layers will inheriet. Sets
/app as WORKDIR,
# Install runtime packages RUN apt-get update \ && apt-get install -y \ perl # Set app dir WORKDIR /app
build layer which is used temporarily to install packages and modules that won’t be used directly at runtime. Using an intermeiate layer will save space in the resulting image and no include packages that aren’t used explicitly for runtime operation, improving security posture.
# Intermediate build layer FROM base AS build #Update system and install packages RUN apt-get update \ && apt-get install -yq \ build-essential \ cpanminus
# Install cpan modules RUN cpanm Proc::ProcessTable Data::Dumper
run layer for the resulting image and copy
/usr/local from the intermediate
build layer to include required CPAN modules,
# Runtime layer FROM base AS run # Copy build artifacts from build layer COPY --from=build /usr/local /usr/local
Copy ps.pl script into
/app) of the container. This is put near the end of the
Dockerfile since it is the file most likely to change and will allow re-using of cache for previous layers, speeding up subsequent builds.
# Copy perl script COPY ./ps.pl .
ENTRYPOINT to the ps.pl script,
# Set entrypoint ENTRYPOINT [ "/app/ps.pl" ]
Now that the Dockerfile is complete, build a container image with the name
🚀➜ docker build -t ps-perl .
With a container image ready, run it with
docker run -it --rm ps-perl, using the
--rm options so the container is removed when it exits,,
🚀➜ docker run -it --rm ps-perl PID TTY STAT START COMMAND 1 /dev/pts/0 run Fri Sep 11 17:45:16 2020 /usr/bin/perl -w /app/ps.pl -------------------------------- uid: 0 gid: 0 pid: 1 fname: ps.pl ppid: 0 pgrp: 1 sess: 1 ttynum: 34816 flags: 4210944 minflt: 1684 cminflt: 0 majflt: 0 cmajflt: 0 utime: 40000 stime: 10000 cutime: 0 cstime: 0 priority: 20 start: 1599846316 size: 12779520 rss: 8192000 wchan: 0 time: 50000 ctime: 0 state: run euid: 0 suid: 0 fuid: 0 egid: 0 sgid: 0 fgid: 0 pctcpu: 5.00 pctmem: 0.07 cmndline: /usr/bin/perl -w /app/ps.pl exec: /usr/bin/perl cwd: /app cmdline: ARRAY(0x55fb475bb9f0) environ: ARRAY(0x55fb472dd500) tracer: 0
Command line arguments will also pass into the container when run using the CMD to pass to
🚀➜ docker run -it --rm ps-perl -h A simple perl version of ps
🚀➜ docker run -it --rm ps-perl -v Version: 1.0
By using the
-slim variant of Debian for the base image and multi-stage builds, the resulting container size is just over 130MB. Using a smaller image like Alpine Linux could further reduce the image size, but could introduce known caveats because of musl libc and glibc differences.
🚀➜ docker images ps-perl latest 938c3dde86d7 29 minutes ago 134MB