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Docker – The STOPSIGNAL instruction

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The STOPSIGNAL instruction is used to set the system call signal that will be sent to the container to tell it to exit. The parameter used in the instruction can be an unsigned number, which equals a position in the kernel’s syscall table, or it can be an actual signal name in uppercase. Here is the syntax for the instruction:

# STOPSIGNAL instruction syntax
STOPSIGNAL signal

Examples of the STOPSIGNAL instruction include the following:

# Sample STOPSIGNAL instruction using a position number in the syscall table
STOPSIGNAL 9
# or using a signal name
STOPSIGNAL SIGQUIT

The parameter supplied to the STOPSIGNAL instruction is used when a docker container stop command is issued. Remember that it is vital to use the exec form of your ENTRYPOINT and/or CMD instructions so that the application is PID 1, and will receive the signals directly. Here is a link to an excellent blog post on using signals with Docker: https://medium.com/@gchudnov/trapping-signals-in-docker-containers-7a57fdda7d86. The article provides an excellent example of using a node.js app to handle the signals, complete with code and Dockerfile.

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