Thursday, June 27, 2024


This is another of those things that I've always known I should just sit down and learn but never did: what's the difference between SIGINT and SIGTERM? I knew that one of them corresponded to control-c, and the other corresponded to the kill command's default signal, but I always treated them the same, so I never learned which was which.

  • SIGINT (2) - User interrupt signal, typically sent by typing control-C. The receiving program should stop performing its current operation and return as quickly as appropriate. For programs that maintain some kind of persistent state (e.g. data files), those programs should catch SIGINT and do enough cleanup to maintain consistency of state. For interactive programs, control-C might not exit the program, but instead return to the program's internal command prompt.

  • SIGTERM (15) - Graceful termination signal. For example, when the OS gracefully shuts down, it will send SIGTERM to all processes. It's also the default signal sent by the "kill" command. It is not considered an emergency and so does not expect the fastest possible exit; rather a program might allow the current operation to complete before exiting, so long as it doesn't take "too long" (whatever that is). Interactive programs should typically NOT return to their internal command prompt and should instead clean up (if necessary) and exit.

This differentiation was developed when the Unix system had many users and a system operator. If the operator initiated a shutdown, the expectation was that interactive programs would NOT just return to the command prompt, but instead would respect the convention of cleaning up and exiting.

However, I've seen that convention not followed by "personal computer" Unix systems, like MacOS. With a personal computer, you have a single user who is also the operator. If you, the user and operator, initiate a shutdown on a Mac, there can be interactive programs that will pause the shutdown and ask the user whether to save their work. It still represents a difference in behavior between SIGINT and SIGTERM - SIGINT returns to normal operation while SIGTERM usually brings up a separate dialogue box warning the user of data loss - but the old expectation of always exiting is no longer universal.

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