dumb-init

dumb-init is a simple process designed to run as PID 1 inside Docker containers and proxy signals to child processes.

In Docker containers, a process typically runs as PID 1, which means that signals like TERM will just bounce off your process unless it goes out of its way to handle them (see the "Why" section below). This is a big problem with scripts in languages like Python, Ruby, or Bash, and can lead to leaking Docker containers if you're not careful.

Why you need a signal proxy

When processes are sent a signal on a normal Linux system, the kernel will first check for any custom handlers the process has registered for that signal, and otherwise fall back to default behavior (for example, killing the process on TERM).

However, if the process receiving the signal is PID 1, it gets special treatment by the kernel; if it hasn't registered a handler for the signal, the kernel won't fall back to default behavior, and nothing happens. In other words, if your process doesn't explicitly handle these signals, a TERM will have no effect at all.

A common example is CI jobs that do docker run my-container script: sending TERM to the docker run process will typically kill the docker run command, but leave the container running in the background.

What dumb-init does

dumb-init runs as PID 1, acting like a simple init system. It launches a single process, and then proxies all received signals to that child process.

Since your actual process is no longer PID 1, when it receives signals from dumb-init, the default signal handlers will be applied, and your process will behave as you would expect. If your process dies, dumb-init will also die.

Process group behavior

In its default mode, dumb-init establishes a process group (and "session", via setsid(2)) rooted at the child, and sends signals to the entire process group. This is useful if you have a poorly-behaving child (such as a shell script) which won't normally signal its children before dying.

This can actually be useful outside of Docker containers in regular process supervisors like daemontools or supervisord for supervising shell scripts. Normally, a signal like SIGTERM received by a shell isn't forwarded to subprocesses; instead, only the shell process dies. With dumb-init, you can just write shell scripts with dumb-init in the shebang:

#!/usr/bin/dumb-init /bin/sh
my-web-server &  # launch a process in the background
my-other-server  # launch another process in the foreground

Ordinarily, a TERM sent to the shell would leave those processes running. With dumb-init, your subprocesses will receive the same signals your shell does.

If you'd like for signals to only be sent to the direct child, you can set the environment variable DUMB_INIT_SETSID=0 when running dumb-init. In this mode, dumb-init is completely transparent; you can even string multiple together (like dumb-init dumb-init echo 'oh, hi').

Installing inside Docker containers

You have a few options for using dumb-init:

Option 1: Installing via an internal apt server

If you have an internal apt server, uploading the .deb to your server is the recommended way to use dumb-init. In your Dockerfiles, you can simply apt-get install dumb-init and it will be available.

Debian packages are available from the GitHub Releases tab, or you can run make builddeb yourself.

Option 2: Installing the .deb package manually

If you don't have an internal apt server, you can use dpkg -i to install the .deb package. You can choose how you get the .deb onto your container (mounting a directory or wget-ing it are some options).

One possibility is with the following commands in your Dockerfile:

RUN wget https://github.com/Yelp/dumb-init/releases/download/v0.5.0/dumb-init_0.5.0_amd64.deb
RUN dpkg -i dumb-init_*.deb

Option 3: Installing from PyPI

dumb-init can be installed from PyPI using pip. Since dumb-init is written in C, you'll want to first install a C compiler (on Debian/Ubuntu, apt-get install gcc is sufficient), then just pip install dumb-init.

Usage

Once installed inside your Docker container, simply prefix your commands with dumb-init. For example:

$ docker run my_container dumb-init python -c 'while True: pass'

Running this same command without dumb-init would result in being unable to stop the container without SIGKILL, but with dumb-init, you can send it more humane signals like TERM.

See also

• Docker and the PID 1 zombie reaping problem (Phusion Blog)
• Trapping signals in Docker containers (@gchudnov)
• pgctl