docker-scry - examine Docker containers using host tools

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Overview

The docker-scry tool uses the host's pgrep and ps utilities to display information about processes running within a Docker container. It first runs docker inspect, and then uses the Linux-specific namespace parameters to pgrep to find the other processes running in the same PID namespace as the container's main process.

Needs to be run as root

Because of the way pgrep works, docker-scry only runs reliably if invoked from the root account; otherwise pgrep will list all of the processes running on the system as being in the same namespace as the container's main process.

The docker-scry tool will refuse to run pgrep if it is unable to examine the /proc/<main-process-id>/ns/pid filesystem entry; that will happen if it is not run as root. If there is a reason to believe that pgrep will not fall back to listing all the system processes, this check may be disabled by the --force (-f) option to docker-scry pgrep.

Installation

Install the docker-scry tool itself

Install into a virtual environment using uv

One of the fastest ways to get a virtual environment up and running is to use the uv tool:

uv venv /path/to/venv
(set -e; . /path/to/venv/bin/activate; uv pip install docker-scry)
/path/to/venv/bin/docker-scry --help

Let uvx handle the virtual environment

The uv tool's uvx command will automatically handle the installation of Python libraries into virtual environments that it keeps in per-user cache and temporary directories:

uvx docker-scry --help

Install into a virtual environment using venv

Most distributions of Python will already have a venv module installed or ready to be installed by the package manager (e.g. apt install python3-venv):

python3 -m venv /path/to/venv
/path/to/venv/bin/python3 -m pip install docker-scry
/path/to/venv/bin/docker-scry --help

Let docker know about the scry subcommand

Once there is a way to run docker-scry itself, docker must be told about the new scry subcommand. This is done by placing a docker-scry symlink into one of the directories that docker searches for its CLI plugins: either a system-wide one (usually /usr/lib/docker/cli-plugins/ or /usr/libexec/docker/cli-plugins), or a per-user one:

mkdir -p -- "$HOME/.docker/cli-plugins"
ln -s /path/to/venv/bin/docker-scry "$HOME/.docker/cli-plugins/"
docker --help | grep -Fe scry
docker scry --help

If docker-scry is started using uvx, then a small shell program should be placed into the cli-plugins directory under the docker-scry name:

#!/bin/sh

exec uvx -- docker-scry "$@"

Once that file exists (and is made executable), Docker should be able to detect it and run it as the scry subcommand.

Examples

Show the list of processes running within a container:

docker scry pgrep -c sweet_banzai

Same, but display more diagnostic information about the commands run:

docker scry -v pgrep -c sweet_banzai

Do not display even the informational messages; any output on the standard error stream will indicate actual warnings or errors:

docker scry -q pgrep -c sweet_banzai

Only show processes that have python in their command name:

docker scry pgrep -c sweet_banzai -- python

Only show processes that have python anywhere on their command line:

docker scry pgrep -c sweet_banzai -- -f python

Same, but run ps uww on the processes to show more information:

docker scry pgrep -c sweet_banzai --ps uww -- -f python

Pass more command-line arguments to ps:

docker scry -q pgrep -c sweet_banzai --ps '-h -o pid,ppid,cmd' -- -f python

Exit status

The docker scry pgrep subcommand attempts to imitate the exit status of the pgrep utility:

• 0 on success
• 1 if the container was not found or no processes were matched within it
• 3 on another fatal error

Contact

The docker-scry library was written by Peter Pentchev. It is developed in a GitLab repository. This documentation is hosted at Ringlet with a copy at ReadTheDocs.