Ruby Exception Handling

Ruby Exception Handling: SignalException

Next on the docket for our Ruby Exception Handling series, we’re taking a look at the SignalException class. SignalException is a subclass descendant of the Exception superclass, and occurs when a process monitored by Ruby receives a signal via the operating system.

In this post we’ll explore the SignalException class, examining where it lands within Ruby’s Exception class hierarchy, how to handle SignalException errors, and a few best practices to avoid this exception from appearing in the first place. Let’s begin our adventure!

The Technical Rundown

  • All Ruby exceptions are descendants of the Exception class, or a subclass therein.
  • SignalException is a direct descendant of the Exception class.

When Should You Use It?

As discussed earlier, the SignalException class is raised anytime Ruby encounters a received signal during code execution. Thus, to properly understand how SignalExceptions work, we must first explore a bit about signals, including what they are and what can cause them.

A signal is simply a command that can be sent directly to a running process within many operating systems. Typically, a specific signal will be a short string identifier, each with a unique purpose or intended behavior. For example, the TERM signal is short for termination, and as you might expect, is intended to terminate the running process it is sent to: Process.kill(‘TERM’, Process.pid).

The specific signals available to you will depend on your particular operating system, but we can easily gather a list of potential candidates using a simple Signal.list method within Ruby:

On Windows 7, this outputs a handful of available signals, including the TERM signal mentioned above:

Now to send a signal to a particular process, we must identify the correct process using the unique identifier, known as the process ID or pid. Virtually all operating systems use a pid system to identify running processes, so even the Interactive Ruby Shell (irb) that you might be testing simple code with will have its own pid. This can be identified using the Process.pid method call:

The output shows what the actual pid number is:

Now with that out of the way, we can tackle the raising of SignalExceptions when executing Ruby code. In this simple example below we’re using a standard begin-rescue block to catch any potential exceptions. The crux of the code is the one line where we’re calling the Process.kill method, passing the TERM signal and the currently active process pid as the arguments, and attempting to rescue the produced SignalException, before outputting the resulting exception message using the print_exception function:

Sure enough, this raises the expected SignalException and informs us of the type in question (SIGTERM):

It’s important to understand that SignalException is still a Ruby class, and thus is bound by all the rules that Ruby must abide by during execution, whereas signals and their respective processes, are at a higher level of abstraction since they only abide by the rules of the operating system. This means that, in some cases, SignalExceptions will not be raised because some particular signal types are designed not to be caught or ignored. For example, the signal KILL is similar to TERM, except it specifically terminates the process immediately without allowing the process to perform any further actions or cleanup procedures, such as raising errors:

As expected, executing the code this time with the KILL signal sent simply terminates our process immediately, without any further processing or without raising a SignalException.

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