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 -it and --rm will run an interactive emphermal container, disappearing when the command is complete.
Let’s walk through an example that runs a Perl script called ps.pl, which prints a simple process table using the CPAN module Proc::ProcessTable and includes -h and -v arguments.
The following Multi-Stage Dockerfile will build a container to run a simple perl script,
ps.pl script as ENTRYPOINT-h as default argumentHere 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" ]
Standard start of a Dockerfile, using the Official Debian image tagged stable-slim as the base layer and a label for the image maintainer,
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
Intermediate 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 Perl CPAN packages and dependencies with cpanminus which works well within a container build environment,
# Install cpan modules
RUN cpanm Proc::ProcessTable Data::Dumper
Create final 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 WORKDIR (aka /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 .
Set 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 ps-perl,
🚀➜ docker build -t ps-perl .
With a container image ready, run it with docker run -it --rm ps-perl, using the -it and --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 ENTRYPOINT,
-h
🚀➜ docker run -it --rm ps-perl -h
A simple perl version of ps
-v
🚀➜ 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
(Updated 9/11/2020)