exploring signal handling in bash scripts, subshells and pipelines

.nvmrc

8

README.md

Signal catching test

Testing how signals are caught through wrapper scripts and how error values are returned.

Usage

docker build -t test-signals .
docker run --rm --name test-signals test-signals

From another window:

docker kill -s TERM test-signals

This will send a SIGTERM to the running docker container. Do it a few times; results might differ between runs.

chained-script.sh

#!/bin/bash

set -euo pipefail

while date ; do sleep 3 ; done | python3 signals.py one  | python3 signals.py two | cat

Dockerfile

FROM node:8-slim

RUN apt-get update && \
    apt-get install -y -q \
        python3.4 \
        && \
    curl -sSL -o /usr/local/bin/dumb-init https://github.com/Yelp/dumb-init/releases/download/v1.2.1/dumb-init_1.2.1_amd64 && \
    chmod +x /usr/local/bin/dumb-init && \
    rm -rf /var/lib/apt/lists/*

ENTRYPOINT ["dumb-init", "--"]

WORKDIR /app
COPY . /app

CMD yarn start

package.json

{
  "scripts": {
    "start": "./chained-script.sh"
  }
}

signals.py

#!/usr/bin/env python3

import signal
import sys

class App(object):
    def __init__(self, name):
        self.name = name

    def printsig(self, sig, frame):
        self.write("Received {} ({}): {}\n".format(self.signals[sig], sig, frame))

    def trapall(self):
        self.signals = {}
        for name in [x for x in dir(signal) if x.startswith("SIG")]:
            i = getattr(signal, name)
            self.signals[i] = name
            try:
                signum = getattr(signal, name)
                try:
                    signal.signal(signum, self.printsig)
                except ValueError:
                    pass
                    #self.write("Skipping {} (out of range)\n".format(name))
            except OSError as m:
                pass
                #self.write("Skipping {} (not catcheable)\n".format(name))

    def write(self, msg):
        sys.stderr.write("{}: {}".format(self.name, msg))

    def echoloop(self):
        for line in sys.stdin:
            self.write(line)
            # Pass to stdout unchanged to avoid cascading
            sys.stdout.write(line)

def main():
    name = sys.argv[1]
    app = App(name)
    app.trapall()
    app.echoloop()

if __name__ == "__main__":
    main()